The ultimate beach cat, how light is possible?

How light is it possible to build a 20 feet platform that is tough enough to endure hard sailing and tough conditions(texel, worrel etc)? Some will say that the M20 at 120kgs is already pushing the limit, and might have pushed it too far as some M20s have proven to be a bit too fragile in rough conditions. Macca, who by the performance and sheer looks of his upgraded Taipan 5,7 obviously knows what he's doing, recently mentioned in the Super Taipan thread that he thought it was possible to push the weight of a 20 feet long,10 feet beam cat down to 105kgs. I know Wouter disagrees, but what about the rest of you?

5.7 meters = 18.70 feet

105 kg is very light. As soon as you go 10 foot beam, the loads increase quiet a bit and the boat needs to be beefed up.

Note the M20 is 9 foot wide also.

Not saying it can not be done, but it would be a serious accomplishment. Fun to give it a go.

Jimi,
A superlight boat would be nice if you are only going to sail it in superlight conditions and superflat water.

If a squall comes through with winds of 30-60 knots, how do you hold that boat down? It is going to go flying away like milkweed seeds on the wind.

How light is it possible to build a 20 feet platform that is tough enough to endure hard sailing and tough conditions(texel, worrel etc)? Some will say that the M20 at 120kgs is already pushing the limit, and might have pushed it too far as some M20s have proven to be a bit too fragile in rough conditions.




I don't really think it is a question of it can or it can't be done. There is a transition area (grey area) where it dependents on what is considered to be "hard sailing".

I'm sure a 20 ft x 10 ft wide platform with a sloop rig can be build at 105 kg. How it holds up under varying condition is a different matter. How it will hold up in the (grulling) Worrell 1000 is yet again a seperated issue alltogether.

All I was saying in the other post is that Marstrom for some reason decided to forget about the initial 108 kg M20 and replace it with a 120 kg one instead. Remember this boat also doesn't have a jib setup which does really load up the bows !

I think Stephen has is on the right track and I want to expand on that. Reducing the overall weight of a beach catamran gets harder which each additional kg. Say you take the 140 kg Taipan5.7 (incl 5 kg spi setup). The hulls are already pretty lightweight so not much to be gained there. You want the boat to be significantly wider, this means that the use of carbon in the beams will not result in much weightsavings relative to the 5.7 as you'll need all the material you can get to restore platform stiffness. What is left ? Carbon mast ? 5 kg gains. Lets say you win 10-15 kg by using kevlar in the hulls, carbon mast, carbon beams, boom, pole and some other things. That still leaves you at 125 kg. Cuben fibre sails. Another 3 to 4 kg. Lets say you are down at 120 kg. What is now left to win those additional 15 kg of weight ? ... Not much at all. That is why some builders have decided to loose the jib as then the bows can be build lighter again. However it is still a very long way to 105 kg. Remember that Taipan 5.7 isn't an overbuild boat like the F18's.

What we are saying here is that we want to make an additional weight savings of 35 kg when a professional builder like Boyer had to work hard to win the first 40 kg. You must come up with some very smart things to do that.

It will definately be engineering challenge.

Wouter

Take a good dump at the little boys rooom & leave all the beer on the bar the night before a regatta, take 1 paddle instead of 2, only wear a shorty wetsuite, take off all the extra gear , lines, bags, waterbottles etc and do some exercise a 2x a week.. [I do non of the above..]this all will save you easly 15~20 kg. That 15~20 kg I personally would love to have [and stay] in the structure of the boat [remember Texel 2005]

But you do want to make you boat as light as possible don't you, no matter what the conditions? Ideally, if you could have a "bulletproof" boat no matter what the weight of it, a lighter boat would be faster than a heavier boat in all conditions would it not?

Oh and Wouter, this is exactely why I'm studying to become an engineer in marine technology. Give me a few years and I will take that challenge <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I just don't get it. If I am out on the water in a LOT of wind, I for sure do NOT want to be on a super lightweight boat. Nobody seems to address this issue about how you hold the boat down and keep it from flying away, literally.

Obviously not being an expert, I can only give you a short answer. This is also why I started the thread in the first place.
I have seen a few pictures of flying A-cats, where it really did seem as a problem that the platform was too light. However, a lighter platform will require less force to drive it through the water, decreasing the all up loads on the platform and rig and therefore making it easier to control even in a big breeze. Still, when you go windsurfing and the wind is picking up you go for the smaller boards. It would be natural to conclude in the same for a beach cat. Hoping for some good answers on this one.

As much as I hate to say the following thing it is true nevertheless. A boat design can be TOO light to perform to its full potential.

I won't go into detail at this time, but suffice to say that the ratio of wetted surface area to bouyance (=overall boat weight) does play a factor, as well the ability to hold some momentum when sailing through choppy seas.

Also the first thing a good engineer needs to learn is that it is very traiterous to extrapolate (approximated) behaviour into extreme points.

But these are all topics for another time.

Wouter

"Also the first thing a good engineer needs to learn is that it is very traiterous to extrapolate (approximated) behaviour into extreme points."

Wouter, could you try to explain this in easier words for a simple, Norwegian student to understand? As I am studying to become an engineer, I will use all the advices I can get to become a good one!

Let's say you wanted to find out the relationship between the density of water and temperature. So, you measure the density of water at 10 C, 50 C and 90 C then fit a line through your data points. That's well and good, but you might run into problems using that line to predict the density of water at 101 C or -1 C.

If you stick with engineering you may hear about the concept of "inference space". Roughly speaking, that's the range of values for which we have experimental data. Using an empirical relationship outside of its inference space can lead to problems. In the example above, there are state changes just outside of the inference space which are not considered in the relationship. In other cases a variable may have a fairly constant value across the inference space (and therefore be ignored in the relationship) but not outside of that space.

I am with Mary... Heavy conditions, heavier boat. Lighter conditions, lighter boat. Obviously there is a very large overlap in the moderate conditions range but on the extreme fringes I like Mary's angle.

30 knots on the SC20 at 12' wide and well over 400 lbs was was at times a very tenuous situation, skipping on top of the water at one point then standing on the nose a second later. Without the weight and beam I am sure that we would have dmaged the boat in some fashion.

What about a hulls terminal speed?

I would hate to try and put this into a translator>>>

I believe what Wouter is trying to say is that every feature in a design has to be carefully ballanced to make a boat that can be considered good in more than 1 condition. The MacQuaire Innovation speed sailing boat is pretty fast and light but it only goes 1 way on flat water. Upwind performance must be ballanaced with downwind performance, abilty to sail in flat water has to be compromised for performance in waves or chop, no 1 design will do it all.

It is possible to build a boat significantly lighter than what is being typically made up there now. The loads on a boat during sailing are not the only consideration you need to make though. Hnadling a boat on land, on and off a trailer and through non-sailing gyrations such as righting after capsize etc. require quite a bit more structure to make something that will last. A min weight boat designed with consideration to the sailing loads only will most likely look like a golf ball after a few weeks due to heal and other body part contact, and you will probably have gone through a few masts and other rig parts as well.

From a practical consideration my opinion is that the current A cat is about as low as you would want to go design wise. Even here you only have a 1 up uni rig platform that you will only rarely see going out in winds over 20 knots, and when violently pitch poled stands a pretty good chance of requiring a new mast.

The other trade off I have not mentioned hits on the cost. Lighter is better, but below some already explored limits the advantage is questionable at best. If you realy want to save weight the biggest bang for your effort is still a diet.

<img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Matt

<img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

Had this conversation ages ago one night at the sailing club. I had a sailing mate who worked to Lola (race cars in case you did not know). His comments were that given a big budget you could get down to somewhere around 80 kg. It was really down to cost. Buy the best stuff, cook it properly and you get strong and light. BUT it will cost loads ! How much does a race car cost to build - Lots. Mucho expensive carbon !

He did a fag-packet spec, and he was saying that the problems would be the fittings. We got top the point where we were building custom carbon blocks, carbon eyes for the tramp etc. Got silly, but he suggested 80kg was possible, but expensive !

Wow, 80kg!!! That is a little lighter than my Nacra 5.2, I am sailing a battleship compared to some of the stuff out there.

Sailing a 140 kg Taipan 5.7 myself (with JIMI), I would say that Wouter's estimate of weight savings seems reasonable. Producing a reliable 20-foot cat lighter than 120 kg may not be worth the effort.

"I won't go into detail at this time, but suffice to say that the ratio of wetted surface area to bouyance (=overall boat weight) does play a factor, as well the ability to hold some momentum when sailing through choppy seas."

This I did not understand (it is not the extrapolation that is difficult). If high momentum was a priority, we would not see the light-weight building that we see in all development classes. And losing weight in the mast, bow and sterns is always desirable in order to cope with waves.

What did you mean by "ratio of wetted surface area to bouyance (=overall boat weight) does play a factor" ?
The distribution of volume/weight is of course important for the boat's behavior in different conditions. Did you mean that reducing overall weight is less important than which parts of the boat which is made lighter?

Stein

"If a squall comes through with winds of 30-60 knots, how do you hold that boat down? It is going to go flying away like milkweed seeds on the wind."

How heavy a boat do you need to be "safe"?

History of all development classes seems to show that initial problems of surviving in strong winds are overcome by improved boat-handling technique. E.g. the 49ers have moved their max-wind limits quite a bit since their debut.

As an amateur, I am more worried about breakage than being carried away (if you excuse the pun).

If you want a light boat, make it simple and clean!
* Glue the boat together, no screws or heavy fittings to transfer the load from the beam to the hull.
* Remove the jib, and make lighter hulls since the compression of the hull is reduced.
* Without the jib you can use twin forestays and get a stiff boat.
* Use lashings for the stays and use something else than stainless wire for the stays.
* Use light sails, different materials in the sails can differ several kg! For example the MAXX sails are quite heavy since they are strong in all directions compared to "normal" cloth.
* Remove fittings and trim lines. Check the latest A-class boats from geltec, they are clean!!

If you want to go the other way you can see how the M20 was downgraded by some owners by adding a jib with bridle, stainless wires, external halyard for the spi and a big fat block system for the main sheet instead of a light caskading system, no wonder it weighed 120 kg. But which version of M20 won the texel, the original M20 of course!

/håkan

There are really no issues with sailing light boats in strong winds compared to sailing heavy boats in strong winds, in fact as with everything the lighter the kit the easier it is to use.

A hobie 17 is often reckoned to be an easy boat to sail in a strong wind compared to an A cat, but is that because it is twice the weight or because it is very undercanvassed compared to the A.

To make an A go as slow as a Hobie 17 you would have to reduce the mast height by 6 feet AND the sail area by 1/3, now which boat do you think would be the easiest to sail.

Conversly to make the Hobie 17 as quick as the A class you would have to add8 feet to the mast and a whopping 42% extra sail area, whose going to hold that down in 25 knots.

My point is that it isn't the weight (or lack of it) that makes a boat hard to sail more the size of the sails compared to that weight. So in order to compare how easy one boat is to sail against another you must pick boats of similar width and speed.

A Hobie wave weighs in a lot less than a Formula 18 so does that make it harder to sail when the wind gets up.

Back to the original question (how light could a 20’ cat be built) and disregarding all the other considerations/arguments IE water, wave, wind, handling characteristics on and off the water, etc, given the available manufacturing equipment, (high pressure die moulding) the availability of the high grades of aramids and aerospace carbons, the latest high modulus high temperature resins, and an unlimited budget (wouldn’t it be nice if it wasn’t just a dream), there is no reason why a “bullet proof” 20’ cat couldn’t be built at a weight of less than 50kgs all up.

I'm calling BS on this one.

One of the very first problems, of many, you'll run into will be the lack of stiffness. Additionally nearly every component will be in constant danger of buckling.

We are talking about a 20x10 foot racing beach catamaran here. It has to go out through a surf, carry two people and carry enough sailarea and a spi to even have a remote chance of winning a race.

Sorry guys, 50 kg all up ?, we are getting into the realm of BS now.

Wouter

I am sure that was a typo Wouter........ Missed the 1 at the front <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I like what John P says. It is more about the sail area you can carry in the given wind strength. I have wondered why beach cats don't have different rigs for different wind speeds, just like the 18' skiffs and windsurfers use. That way, you could build a very light platform, use the small rig for big wind and the big rig for light air. Of course you would also need a medium rig for those medium days! <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

But it would cost 3X $ for 3 different sail packages, if you used the same mast. And you would have to make such a boat practical as Matt has said above, or nobody will buy it.

I wonder if the surviving Inter 20 boys from last weekend's Steeple Chase could add their thoughts to this? I'm thinking if they had put Hobie 16 sails and a smaller F18 spinnaker on the Inter 20 platform, they might not have been flipping so much and been "faster" to the finish line. Also, if you could put 10' beams on the Inter 20, it would be more stable. Too bad there were no Tornados to compare with this year.

You guys want lighter and lighter boats, that always turn out to be wider and wider! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

How about developing the rig more? There is an archaic rig, called a sliding gunter, that is still used on light canoes. It might offer the kind of versatility Timbo suggested.

It would require A LOT of redesign, but for engineers who are looking for a challenge. . . ! <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

If you were prepared to be really radical <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I could easily make a 20ft version of my 4 hulled boat down to 75kg, just substitute the Al beams and submasts for carbon and glue the beams to the hulls.

The biplane windsurfing rigs allow you to easily get different rig sizes just like on a windsurfer. 3 rigs 12m, 9m and 6m would give you 3 rig sizes 15m 18m and 21m, which is still quite inexpensive and lightweight. Total rig weight, mast, boom, sails, blocks, stays etc is 25kg for a 16sqm rig. How does that compare to a conventional rig?

Also you can carry a much higher sail area for a given wind strength on a biplane rig.

I could probably make a cat by having a soft central "hull" bridging the front and rear hulls and not increase the weight by much.

Gareth
www.fourhulls.com

Start with the data points that are out there

A class catamarans. Newer Australian a cats are coming in at as low as 150 lbs (68 kg). These boats have gel coat finishes and foam cores and are made using resin infusion. They are rugged and don’t break under normal racing. In Bristol, RI which is a moderately windy venue there has not been a mast break in 2 years, this translates to ~ 25 boat years sailing without a mast break. Current A cats cost 17-23K new.

You can definitely build these boats lighter if you are willing to pay for it. Change to prepreg carbon with a nomex core autoclave cured hulls, cuben fiber mainsail, use of boron fiber in the mast, all composite rigging, lighter trampoline. The problem is that you now have a very expensive boat. What would it weigh? You could reduce the weight by at least 10% if you were willing to increase the boat cost to say 30-40K$. You are in the range of diminishing returns for a 60 kg boat.

On the other end of the spectrum is a modern C class catamaran. 25 ft long 16 ft wide, double trapeze, wing sail ~ 365 lbs (new Canadian boat) 165 kg. It is a very expensive toy. I don’t even want to guess at costs.

It seems to me if you want a light 2 person 20 footer that did not cost a huge sum, what I would go for is 8’6” beam (not 10) Resin infused, foam core hulls, carbon mast and glued carbon beams. It would be a unirig with a 33 ft wing (not pear) mast and a flat 240 ft^2 spinnaker. It would weigh about 250-275 lbs. Rigging would be dyform, attached with lashings and have aramid trap lines. More importantly, it would be easy to handle on the beach and awesome to sail.
It would not be all that much faster then an inter 20 in most conditions. It would cost around 30% more then an inter 20 and very few people would buy them. Those people that did buy them can’t understand why everyone else in the world does not send their I-20’s to the dumpster and get with the program. Soon Bob Curry pronounces that this new lightweight boat is just a fad.
The builder, in order to stay in business, cuts 2 feet of the bows of the boat, adds a jib, pours in an 80 lb bag of Portland cement in each hull, paints the masts silver and calls his boat an F-18. Soon he is outselling all the 20 footers in the world and making millions of dollars.
When the price of Portland cement increase to 90$ per barrel, he sells his boatbuilding business to an Arab named Vinny Marawck Al Saud and retires to Costa Rica where he buys a 3rd rate brothel and dies of a previously undiscovered and incurable venereal disease.

This is why light weight double handers will ultimately fail, and condoms are very important.

Eric Anderson

You got it absolutely right, thanks for the laugh <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
If catsailor.com had a "post of the month" or even "post of the year", this would be the winner.


Why lashing dyform rigging instead of synthetic shrouds when you alread have gone for aramid trap lines, and what would you lash it to?

LMAO <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Interesting concept... will talk to my investors and present this as my business plan!!!

Clayton

Nice one.. you got me interested right at the end.

I think you should use carbon nanotubes. forget about carbin fibre and kevlar..that is old school.

...and Sinclair Molecule Chain for shrouds!

Eric actually point out a rather important issue. Who wants lightweight 20 footers with 10 foot beam? Why would sailors buy/build them and not I-20s or similar boats. What would the selling point be?

I think this may have been discussed a couple of times before <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Actually, F18 would be nice if they could get rid of the 80lb Portland cement...

S

Håkan,

I'm truly sorry to notify you about the following :





It was actually the original 2003 PRODUCTION version of the M20 that was finally and officially measured at 120 kg, not the modified sloop rigged M20's that can be found in NL now, those are around 137 kg ! I seem to remember that in 2003 we measure 3 or 4 M20's and took the average of the weight measurements.

I know as I was actually the round Texel 2003 rating official who personally cleared the 2003 M20 measurement right there in the Race Committee office.

However, even since then the weight of the M20 has been on the rise. Currently the Texel measurement system quotes 127 kg for what you call the original M20. Look for yourself : http://www.watersportverbond.nl/data/5Z2_numdet1_3-7-2006.pdf

Interestingly enough the 2006 Round Texel M20's were all sailing of the Texel handicap rating number for the 127 kg boat. If the Swedish boats were much lighter then that then that is cause to protest their final results on the grounds of using incorrect (too slow) handicap numbers.


Wouter

Rolf,
there is always a tradeoff between Higher tech and light weight vs cost. I don't think synthetic rigging is worth the cost and hassle vs weight savings.

You can build a moderately high tech boat (A cat like levals of technology) without going up wildly in price.

On the other hand Jim Boyer tried to import taipan 5.7's into the US and no one bought them. They were a sweet sailing, well built boat that was 170 lbs lighter then the nacra 6.0 it competed against. The problem was it was 3-4000$ more then the nacra and not that much faster. It sure was alot nicer to sail though. All that means nothing if people don't buy them.

Wouter,
are you really sorry about the M20 weight!?

I think we have the answer to the question here. If you want a 20x10 uni rigged boat with spi it is hard to get below 100kg. Marstrom started at 108 kg but 15 boats later the boat weights 120 kg. Why? The parts that have been reinforced on the M20 are rudders and centerboards.
The rest of the weight increase is a mystery to me!

/håkan

I have a true story on the concrete thingy!!!

Booth made an F18HT of an A-cat (M18) and had to add loads of weight on the boat to get it up from 75kg to 130kg of boat weight. He didn't use concrete, he used lead in the main beam. Some of the weight came from a taller mast, bigger centerboards, bigger sail and spi kit but the rest was lead.

/håkan

Hakan,




I'm sorry about the fact that I had to inform you that the 120 kg were for the bare M20 (production version) and not for the upgraded sloop rigged M20's that are now sailing in NL. I got the impression that you thought the 120 kg ready to sail weight was for the modified boats, when it really wasn't.

I do think it a pitty that the M20 couldn't remain at roughly 110 kg or so. Lets face it, that would have been a very cool thing, no matter how one looks at it.





Well, if was only my job at the time to validate the overall weight measurement so that it was far too all parties. The hard numbers were in front of my eyes, but I will not disclose them here publically. I can tell that 120 kg was a good average.

I have not researched where the increases in weight came from. I don't really know where how the boat went from 108 kg to 120 kg. Or why it went on to 127 kg since 2003. I would like to know though.

I do think that the carbon rigging was replaced by steel, stuff like that.


Best of winds,

Wouter

Eric,

Yes, the price is very important for almost all of us. Unfortunately, often the best cats have to be shipped half-way around the earth to get to us.

Maybe one should learn from some dinghy designers who have been successful in promoting their classes by letting builders on other continents take care of production.

And, yes the Taipan 5.7 is extremely nice to sail. It shows that expert design + craftmanship result in a super fast boat at a reasonable price. We found it was almost the same price as a Hobie Tiger F18 when we bought it in Europe (Holland). Maybe the US dealer did a lousy job at the time?

A problem for the 5.7 was the Australian class assoc, who did not welcome necessary developments like self-tacking jib etc.

Stein

Still looking forward to Wouter to enlighten us:

"I won't go into detail at this time, but suffice to say that the ratio of wetted surface area to bouyance (=overall boat weight) does play a factor, as well the ability to hold some momentum when sailing through choppy seas."

This I did not understand (it is not the extrapolation that is difficult). If high momentum was a priority, we would not see the light-weight building that we see in all development classes. And losing weight in the mast, bow and sterns is always desirable in order to cope with waves.

What did you mean by "ratio of wetted surface area to bouyance (=overall boat weight) does play a factor" ?
The distribution of volume/weight is of course important for the boat's behavior in different conditions. Did you mean that reducing overall weight is less important than which parts of the boat which is made lighter?

Stein

A couple of years ago in an A-cat race one of the boats just walked away from the rest of the on the fleet after the start. But when he tacked he stopped!

Why?? On the way out to the start he hit a rock and got a hole in the starboard hull. He had around 30-40 liters of water in the starbord hull at the start and that gives an indication that water ballast can help upwind speed in some conditions.

You just need to make a good system for the ballast tanks....

/håkan

I see that Marstrøms website set the M-20 to 115kgs. Question is wether this is the complete boat with sails, or sans foils or sails. What does "all up" really mean..

http://www.marstrom.com/boat.asp

Beams are bolted to the hulls on the M-20, so he could save some weight there if he glued the beams to the hulls. 9foot wide.. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

When you make a boat lighter you can reduce the bouyance of the hulls. This has a drag lowering effect by reducing the wave-making drag. Also some reductio in wetted surface area drag is achieved but the amount by which this is relatively small.

Think of the hulls as a long rectangular box. Now small reductions in the width of the box lead to large changed in overall bouyancy, but only is small reductions in surface area. However, small changes in length result in small changes in bouyancy while having relatively large effects on surface area.

If you make large reductions in overall weight, then you can make large reductions in bouyancy. If you effect this ONLY by reductions on width then the wave-making drag drops considerable but by that time it was already a rather small part of the whole. The wetted area is still only lightly reduced while it is actually a larger part of the whole drag. At that time only reducing hull length will result in significantly reductions of overall drag.

Pretty much this means that a long water line length is not always a good thing. When large reductions in bouyancy are achieved then a shortening of the hulls must be considered also otherwise the reductions of bouyance are not really effective beyond a certain level.

Therefor "ratio of wetted surface area to bouyance (=overall boat weight) does play a factor"

I still think that this is one of the pitfalls that was not carefully analysed in the M20 design. I still say that it is probably best to have 5.75 mtr long hulls like the Taipan 5.7 and then reduce overall weight of that boat to 120 kg (and no more) and have it feature a sloop rig and 2.8 mtr width. I suspect the quickest design setup to be found around those parameters while still having a good stable feel to it.

I've written many times on this forum, but people seem to forget about it, that the 5 mtr long hull on the F16 is actually a speed enhancing feature. The boat will actually have more drag at 107 kg and say 17 foot hull length. Making a F16 5.50 mtr long will also be a bad move, although it will make the boat feel more stable. But from a drag perspect it will be inferiour.

Too often I see people just "feel" that longer hulls are just faster. This I refered to by "extrapolating to extreme points". Normally a longer hull is better, but not when you also reduce the overall weight significantly at the same time.

Wouter

I did this one time on my HT. Capsized and the crew put a trap hook hole in the boat. Upwind I noticed we were smoking on one tack and the other we were almost underwater. Then I noticed the hole, we sailed back on the good hull in 30knots and we were fast. The hull was 80% full, we couldnt get the boat up the ramp. Its amazing what a hull full of water will do for your righting moment.

Water ballast would be cool if you could work out a system to dump water quickly.

About 1 kg savings, 1.5 kg if you are lucky.

Now those other 13.5 kg ! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />


Wouter

It seems to me that we see many equipment failures on the high tech low wieght boat more often than on the mainstream boats.

For a rectangular homogenous box in water, chopping off 10 % of the mass leads to the same percentage reduction of volume (boyancy) and surface area in contact with water, regardless of whether you cut the longest or the shortest (end) side.
Doesn't it?

For this discussion it seems important to disinguish between reducing weight by reducing hull volume from reducing weight but keeping hull volume (i.e. reducing specific weight of hulls).

I totally agree, however, that often statements of hull length are assumptions without data. It is quite common to think of hull length in association with sailing in displacement mode, i.e. longer hulls = faster speed. For a 20 foot non-planing keelboat (monohull) the theoretical max speed is ca 6 knots. Beach cats easily sail much faster than that, hence it is obvious that we are often not in a pure displacement mode. We do not need the long hulls to have fun.

However, going from a real short boat to 5.75 improves behavior in waves and reduces pitch-poling susceptibility immensely.

And it is of utmost relevance to the overall performance where the volume and weight is placed.

Stein

[quote
* Use light sails, different materials in the sails can differ several kg! For example the MAXX sails are quite heavy since they are strong in all directions compared to "normal" cloth.

/håkan [/quote]

The MAXX sails are considerably lighter than normal panel sails. The is achieved by less seams and more aligned load paths for the panels. The whole idea of a MAXX sail is to reduce weight!!

The MAXX Mainsail on the Taipan is 2.3kgs lighter than the same sized panel sail!

On the big boats the Medium jib on a Sydney 38 is 4.7kgs lighter than a panel sail (but a 3DL is 1.9kgs lighter than the MAXX)

So what happened with Booth's Marstrom A-cat converted to an F18? A very Tantalyzing bit there. I understand he had to add way too much weight to measure out, but what was the performance like vs F18? How about if that weight was omitted?

For the most part, cats are displacement vessels. The difference in the correlations between length of hull and theoretical max displacement hull speed is due to the fineness ratio of the hulls (length/width). The correlations that hold mostly true for mono's don't for cats because of this. How many boats outside of cats do you know that have lenghts of 20' but are only 1 1/2 feet wide?

I don't know if the additional length truly makes for a faster hull. Theoretically, it should b/c the same principals regarding length still apply, however survivability of a 20' hull feels much greater than an 18' hull. Even with more power, the I20 feels much more comfortable than the F18 in a big breeze to me.

I've sailed two different light cats (18HT and CFR20) in big breeze and felt the 20' boat even with a bigger sail plan was more controllable. It seems that as the boats get lengthened the majority of the length is out front and that helps keep the pointy end up.

I think you can get a pretty light boat if you have money enough, the boat can also be stiff enough. We have in our workshop an A-cat ready to sail of 53 kg and we sailed it in conditions upto 20 knts and there was no problem at all to control the boat or even a problem with strength and platform stiffness. The boat was build in 1994 so it was already possible some 12 years ago.
The point is only the money you are spending on the project this boat was not cheap to build and cost us a lot of workmanship to achieve this.

Regards,
Hans

Dave,

Marstrøm used to build a boat he called the M-18. The M-18 was based on his A-cat, but was wider and had a spi. I dont remember what else was different from the A. I think the boat Mitch and Herbert used in the ICCT was an M-18 de-tuned and rigged to F18HT specs. It was not very succesful as I remember, so I guess the boat was either sold or are in storage somewhere. The extra lead in the mainbeam is probably worth some money. Perhaps it is in the same storage as the 20 foot carbon Fox he has sailed in Rounde Texel <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I take exception to this rule with respect to F16's. I don't think we had any serious failure. In the last 5.5 years of the F16 existance I know of one (repaired) hull delamination issue, one daggerboard breakage (replaced) and one crack along the seam (also repaired). In this time there about 100 new F16's sold by the three builders. Most of them have been sailed in serious conditions. And several boats have been subjected to assaults by trapeze hooks and knees. My own timber boat included.

I say that this is an excellent score for a class were two designs (Stealth F16 and Blade F16) were completely new.

But then again "we" don't rush things, do more then a little modelling in the way of forces and thoroughly test sail our prototypes. Some call this slow growth, I call it being smart and being
responsible to the buyers.

I also think it is very cool that the F16's are indeed the lightest doublehanded spinnaker cats around while being totally dependable at the same time. I think we must thank the Taipan 4.9 (330 boats build) for setting us of into the right direction.

I think that my real point here is that we should spend less time gazing at the newest ultimate carbo fantasy fashion statement and see what is being achieved by simple dedicated development in the much less glamourous F16 fleet.

I think you must all be bored at this time by me telling again that boat design isn't that much about selecting ultimate material X or not, or doing autoclaving or not. It is far more about simple (non-sexy) modelling work and trying to understand what is happening and then design the components so that they are shaped optimally.

I hope that the boat part that I'm working on right now will again show the power of dedicated design work. Carbo guys watch out !

Wouter

Stein,

Do some calculations on these models before asking more questions.

You are wrong in several aspects.

Counterexample :

A cube of volume 1 has a surface area of 6
A rectangular box of the same volume but a length of 2.0 has a surface area of 6.65 = 11 % more
A rectangular box of the same volume but a length of 5.0 has a surface area of 9.35 = 56 % more
A rectangular box of the same volume but a length of 5.5 has a surface area of 9.74 = 62 % more


So no nett change in volume still results in an increase of surface area. Clearly the shape of the box itself has an effect on the surface area. Exactly the same thing happens with catamaran hulls. See for example the last two lines in the calculations.

The reduction in wave making drag better be more then the increase in wetted surface area or else the overall drag of the boat will increase. Finding out where this transition point lies is important in beach cat design.




Actually the beach cats are always in displacement mode, even at high speeds. The contradiction you underline is actually caused by the theoretical max hullspeed theory being wrong. For some reason this myth is impossible to kill. Many people, including maritime engineers, inteprete Froude's law in the wrong way and thus think that the max hull speed law has a scientific basis when it does not. These rest of the errors can be directly trashed back to this fictious law.





Also that is not a fixed law. It also ignores the role played by the width of the platform. It is not totally about the hull length. Such rules of thumb, while holding some element of truth, are often too crude to for usage in developping a superior beach cat design. Sorry.

Wouter

I would contend that on a beach cat longer is always better. Sure there is a speed at which shorter is better, for a 300kg displacement hull i.e. (150kg boat + 150kg crew flying a hull) it is above 25 knots, at 200kg it is above 20knots and at 100kg it is above 15knots. Below those speeds longer is better, above shorter is better.

So to say that an F16 is optimized at 5m is saying that it is optimized for a speed of between 20-25knots. When we are talking about faster boats in the context of a beachcat I think we are talking about faster around the course not straight line blasting, probably with an average speed of around 8-12 knots. At these speeds wave drag is very significant , for 300kg displacement (150kg boat) wave drag is the highest drag component up to about 7 knots. So long and thin is always best for low drag.

The other very important point is buoyancy distribution to enable the boat to carry the sail power. Here again longer is better.

Gareth
www.fourhulls.com

Not at all saying that what your as saying is wrong, Wouter, as a freshment marine student I really have not startet my education yet, but according to my text-book in Marin technology Intro, the maximum speed of a wessel in dispalcement mode is 1,33 * the root of its waterlength (speed given in meter per second). If you say that the waterlength for the Taipan 5,7 is 5,7 meters, that gives a theoretical top speed of 6,07 knots. This book is written by Anders Endal, a professor in Marine Technology and a legend at the department of Marine Technology. He is now 70 years old, and have been teaching for the last 30 years I think. His knowledge of hydrodynamics is great, however I do think his knowledge of beach cats is somewhat limited <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> Please understand that I'm not at all questioning your knowledge of hydrodynamics Wouter, as you are udoubtedly one of the biggest resources here on the forum concerning science and engineering and therefore I have great respect for your statements. I am simply stating that the myth as you call it of theoretical top speed of displacement wessels is something that marine engineers at the Norwegian University of Science and Technology learn in their first year as marine students...

Actually Grob, there is a very clear counter example to your thesis and as such it disproofs it.

At low speeds (below roughtly 6 knots) the wave system around any 16-20 hull is such that wave-making drag is negligiable. In effect any energy lost in the bow wave is won back at the rear of the hull. This is the scientifically correct intepretation of the Froude law, from which the (errornous) max hull speed law is derived. So at these low speeds the drag from the hulls is almost entirely created by wetted surface drag. In effect relatively long hulls are more draggy in this speed spectrum then shorter hull, that is under the assumption of equal displacement.




The F16's aren't optimized for any particular speed. They were developped with the idea of optimizing the average speeds over a wide range of (wind) speed conditions. In effect some speed in specialized conditions was traded of to improve speed under some other conditions. We know of ways to improve F16 speeds under special conditions but decided against doing that as it would have required accepting less speed in other conditions. The F16's were always intended to be good alround boats and not to be single gear race cars. This is also one of the reasons why the jib was included in the design. It allows the boat to be better over a wide range of conditions, especially races where the course is not pure upwind/downwind.





You can't look at boat design through a set of independent rules of thumb. All these factors are interacting with eachother and so for a truly good design one must look at the interactions as well. I think the Taipans and F16's have shown that less sailpower can be faster as well. Afterall the F16 upwind sailarea is LESS then that carried on a Hobie 16, still some excellent results agains F18's, I-20's and A-cats were scored on elasped time lately.

Sometimes the interaction between these factors opens up a new pathway to higher performance, one that may well go against gutt feelings and rules of thumb.

But lets forget about the F16's, this thread is not about them. We should be focussing on a 20x10 ultimate beach cat of 105 kg. The F16's aren't 20x10.

Wouter

Jimi,

I have no other option then to say in so many words :"Your text book is wrong"

Numerous counterexamples are available, ranging from full sized high speed marine fregats (ever saw such a boat planing ?) to beachcats.

Your text book is providing you with a particular intepretation of Froude's Law that really only covers a particular part of the real world. Pretty much only heavy displacement hulls of low prismatic hull ratio.

What the book should have written is that a displacement hull travelling at a speed equal to 1,33 * the root of its waterlength will experience a relatively dramatic change in its growing wave-making drag. In some displacement vessels this sharp increase is so big that it prevents the vessel from travelling through the water much faster. Other displacements vessels were the total amount of wave-making drag is only a rather small of the total (beach cats) will no be held back by this increase. The reason for this is that a doubling of a 40 % drag component is really significant while a doubling of say a 15 % component is relative small compared to the total. Beach cats and Modern Navy frigates fall in the last category.

In effect the max hull speed law is nothing more then a speudo-scientific intepretation of the Froude law which only describes what happens to wave-making drag at a certain hull speed.





I don't care much for the title of professor. In my time at the university I've seen professors making the biggest of dumb mistakes. The simple fact that the Taipan 5.7 easily travels faster then 6.07 knots disproofs the max hull speed law in the way you have worded it. If prof Anders Endal did indeed word it in the very same way then he too is wrong. No-one will be claiming that a Taipan 5.7 travelling at 10 knots is in planing mode, you yourself can confirm that. Just like at how the hulls travells through the water at this speed.




Don't worry Jimi, I'm not made of cake. I can handle my fair share of abuse (not that you are given me anything of that at all). I've had discussions with many people were the discussion have gotten alot more heated. Also I'm not unfallable either, I fully expect to be put in my place when I say something that is scientifically wrong. However in this particular case I'm certainly not wrong.





That may be his reputation, I don't know. However, are you sure that you inteprete his writings correctly ? If his writings are exactly as you presented them then I fail to see the thruth in his reputation.





I understand, hell even my own universities textbooks were less then thruthful on this subject. I had maritime courses during my studies. Later on it turned out that they knew better but thought it too much to burden the young students with the truthful situation. I think this to be on the border of what is acceptable. It would have been alot better to describe the law not as a law but as an intepretation that was only valid on a limited numbers of cases. How difficult would that have been for young students to understand ? We must give the youngster some credit, they are smarter then we
often think they are.


Jimi, if you ever see a discussion about the jib slot effect then you can again expect to be taught a myth, much like this max hull speed law.

If you really want to learn then read Marchajs book or Frank Bethwaite book or the writings of Arvel Gentry. These writers have gotten it right on max hull speed and jib slot effect.


Good luck,

Wouter

I used to think this too. I now do not believe it to be true. There is some very good drag prediction software called michlet, specifically for long slender hulls like catamaran hulls. It appears to be well validated, and I have done a little experimental validation on it myself. I am happy with its predicted results. It is by using this sotware that I have come to those conclusions.

If you know of a better resource for drag prediction of slender hulls I’d like to hear about it, or better still experimental data.

So I will stick to my guns on this one when I say that on beachcats longer and lighter is always better for low drag at the speeds that count and that at 6 knots the wave drag is far from negligible.

In addition when you say that for a 107kg boat 5m is better than 5.5m I would say that that only holds true for speeds above 20knots, below 20 knots 5.5m is better. Although to be honest there is bugger all in it.

Gareth
www.fourhulls.com

I guess I just have to ask the guy himself, prof. Anders Endal, on what he thinks about the subject. No matter what, if we acutally are tought things that are wrong simply dued to the impression that we(the students) cannot handle the science behind the truth, then that truly is f.... up. I will ask him the next time I see him.
As to the book written by Frank Bethwaite, my dad (Stein) has read it and also thought it was great.
Thanks Wouter!

Well, this discussion has the potential to explode into a very large validation research. But such a thing will get beside the point.






The best experimental data I have is Bethwaite and the Miss Nylex C-class design article. Especially the last quantifies the different drag components. Here the wave-making drag (form drag)for the conditions in which the C-class races was given as 15 % while the wetted surface area drag (skin drag) was given as 22 % of the total drag of the whole boat.

To put things in perspective, the drag related to the rig itself was given as 30 %. The daggerboards at 21 %

I still value the C-class designers as the most scientificatlly oriented catamaran designers. It also squares nicely with what is known from other sources like Bethwaite and the design of mono-hulls. The model I described neatly fits all the experiences over a wide range of vessels.

Added to this the F16 experience itself. The F16 concept was developped fully based on the model I presented earlier. As of yet the F16 is doing exactly what it is predicted to be doing by the model and so in that experience I find no counterproof as of yet.

I don't know what kind of model your software is using. I would be interested in finding out what drag it predicts for a long flat plate dragged through the water. Good predictions are to be found for this situation to check the software itself.


Having said all this I spot a contradiction in your statements. You wrote :




First you say it is far from negligible but later you say "there is bugger all in it", implying that indeed it is negligiable !

Please clearify.


Regards,

Wouter


p.s. I'm afraid that I won't be able to discuss everything in detail, I simply don't have the time for that.

From experience, synthetic shrouds stretch too much and if you wiggle carbon back and forth too many times it breaks. I like dyform

Also have the rigger configure your wires for minimum weight where the wires connect to the mast.

You say that “ at these low speeds the drag from the hulls is almost entirely created by wetted surface drag” wheras the Miss Nylex data you present shows that there is a 60/40 split. I don’t know off the top of my head at what speed or Froude number this data is taken but I bet a cold beer its higher than 6knots. At 6 knots the split would be the other way around 40/60 with wave drag taking the higher percentage of drag. Surely the Miss Nylex numbers back up what I am saying.

That is why I say that wave drag isn’t negligible. When I say there is bugger all in it between a 5.5m and a 5m hull. I am saying that the wave drag has a big influence on total hull drag but changing from 5m to 5.5m length only is not enough of a change to affect the wave drag by much. A thinner longer hull is always better but reducing displacement is the real key.

If you want to find out more about Michlet it can be downloaded for free at http://www.cyberiad.net/michlet.htm

Here the wave-making drag (form drag)for the conditions in which the C-class races




Correct, that is why I wrote in the preceding sentence :"... for the conditions in which the C-class races ..." These speeds are above the Froude law based max. hulls speed number. The article names speeds around 10 knots (6 - 14 knots). This speed is the one that is most encountered along the track even when higher speeds are achieved on smaller sections.




No Grob, this is completely wrong and most fundamentally flawed scientifically. Wave-making drag INCREASES when the hull speed is lowered ? While the wetted surface drag decreases ?

I'm not going to explain away all nonsense arguments that are put forward. I'm truly sorry.




Now were are back at the "always" rules again. Ahh well, I've explained my side. Whether other people believe or not is up to them.

Best of luck Grob,

Wouter

Wouter,

If following a weight reduction we only reduce width or only reduce length or only reduce height, the boat will result slower. The right procedure would be to scale down the entire hull in proportion to the weight reduction, so that the original shape is maintained. You proved this point well, but forgot to define some necessary conditions for this to hold true:

Firstly, we need that the initial hull shape is already "ideal" for its weight. If it isn't, reducing only one dimension might actually improve the shape.

Secondly, the crew weight should be reduced in the same proportion as the boat weight, so that the original hull shape remains "ideal". If it isn't, there will exist a new "ideal" shape and a single dimension modification could work well (although different reductions in each dimension would work best to adjust the shape)

Lastly, the sail area should be scaled in the right proportion (not linearly, of course), so that the original hull shape remains "ideal".

Bill Roberts likes to use this type of rationale and posted something along this same line of thought in this forum some time ago.

Cheers,
Luiz

<img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" /> You are saying I am completely wrong and then agreeing with me, I do wonder about you sometimes.

Wouter it cracks me up when you get fired up but it appears youve thrown some good info my way, so before I accept all this writings of Arvel for gospell are there any of his conclusions to be wary of. ( Sorry to be off topic )
thanks in advance

Luiz,

You are mostly correct. Actually this is the basis under the F16 design in relation to the F18's. More info found at http://www.geocities.com/f16hpclass/F16HP_to_F18_analysis.html

I may write some things differently in the article if I had to rewrite it as my knowlegde has advanced somewhat over the last 5 years, but the general line of thought is still the same.

Bill Roberts often misapplies this method to components that are not really accurately described by these laws. Example; he uses them to calculate boatweight by scaling, but this is wrong as several aspects are already at their minimum wall thickness etc in the bigger boats. You can't scale such components to smaller dimensions. But displacement calcs etc are accurately described by these laws.

Also the is no need for the crew weight to be adjusted, this can be done by reducing the boat weight by an extra amount and adjusting width to compensate for differences in righting moment. Again this is what F16's have done. It is one of the reasons why the F16 width is 2.5 mtr instead of 2.6 mtr.

Have to go now.

Wouter

I guess my lack in mastering the english language in combo with my typing skills is to blame here.

the phrase :

No Grob, this is completely wrong and most fundamentally flawed scientifically. Wave-making drag INCREASES when the hull speed is lowered ? ...


Would have been better if written down as.


No Grob, this is completely wrong and most fundamentally flawed scientifically. It is illogical to claim Wave-making drag INCREASES when the hull speed is lowered. ...


Extra explanation

This would either mean that wave-making drag is high when the boat is motionless, which is to weird to consider seriously. Or for some weird reason is is zero when motionless then increases with speed only to decrease again after Froude's law max hull speed and possibly increase again with additional higher speed. This would be weird as that that would predict heavy monohulls to NOT be held back at the max hull speed rule of thumb as we know they are. Either way you run into a contradiction with well documented real life phenomena.


Also I've lost sight of my own "red line" now.

Basically my model is :

-1- Both wave-making drag and wetted surface drag increase with increasing boat speed.
-2- Wetted surface drag is a significantly larger component at low speeds (below Froude's speed)
-3- At higher speeds wave-making drag starts to become a significant factor when compared to wetted surface drag. The transition to this situation is to be found around the Froude speed. (and is what the Froude law actually says)
-4- Overall wave-making drag on cats doesn't seems to be a very large component of the total drag.
-5- Wave-making drag is not very dependent on hull length beyond a certain min prismatic hull ratio.
-6- Wetted surface drag is always strongly dependent on the total wetted surface area.


Wouter

You should never consider anybodies writings as gospel and that certainly includes mine as well. As I wrote earlier I've even seen professors make huge dumb mistakes. I don't consider myself to be something that even the greatest minds are not.

I can't name problem issues with Gentry so far but I haven't read all of his stuff yet. Always use your own mind and intellect, not forgetting common sense, to check anybodies writings.

Maybe one extra note that is interesting in the larger discussion is the fact that Texel measurement handicap system EFFECTIVELY uses a 2.5 power relationship between speed and hull length. The relationship between wetted surface drag and speed is a quadratic one (2 power). This means the wave-making drag component is approximated by 0.5 power relationship on top of the wetted surface drag component. Both are continiously increasing functions for increasing speeds. Texel handicap seems quite accurate in its speed predictions relative to speed. It is not a big argument but I do think it is another element supportive of the model as presented earlier by me.

Wouter

Why would you want anything lighter than an M20?
On the beach these boats have sandbags on the front beam to stop them from blowing away!
I talked to a top M20 sailors at Texel this year and he told me that the M20 was very scary in high winds, something about the cat controlling them instead of the other way around.
And how much performance does weight reduction affect performance in practice? If you see this year's Round texel results, very little. (F18 six minutes behind the first M20, and 1.5 minutes on the first Tornado spi (all sailed by crews with equal skills).

I think I have to go back to the initial question in the thread-starter to answer this one. First of all, as wind picks up, the advantage of a low weight boat decreases quite rapidly. I do not recall how the conditions were during the last texel, but were they not around 10 to 12 knots or more? And I think it is fair to say that low weight boats have their biggest advantage in lighter winds than 10 knots, take the A-cat for example.
Second, I initially asked the question more like a "what is theoretically possible question" more than a statement that low weight always is best for a beach cat. However, one do have to keep in mind that all high performance development classes use pretty much of their time (and money)to find new ways of saving weight. And as time goes by and new techniques and technologies are discovered, maybe there will be ways of making the mast for instance a couple of kilos lighter AND 500 dollars cheaper. And that is exactly why one should keep focus on, and ask these questions on catsailor. First because it stirrs people up and causes discussions, second it "provoces"(sorry, only word I could think of) the home builders and engineers on this forum to think of new ways of imroving beach cats. I don't know about the rest of you, but I myself am extremely keen on trying to find new ways to make the cat go faster <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

We sail the Super Taipan in 30kts with no problems at all. The only small issue is the fact that the kite is pretty big and the hulls lack bouyancy to drive it hard downhill. but we just go a bit deeper and get there almost as quick.
The advantages are very clear in the lighter wind ranges where the power to weight really comes into its own and the boat is super quick particularly downwind.

With a more bouyant (ie longer) hull and slightly less weight i would expect the boat to be quicker in all winds and in particular downwind in a blow.

The light weight is only a problem if you chicken out and start sailing too high upwind, as soon as you do that the boat slows up a lot and then the breeze starts to pick the tramp up and then you are in a world of hurt....

We keep pressing and drive the boat at its proper angles and it gets upwind very well. Some of this is due to having a jib which moves the COE of the sail plan lower than if it was mainsail only like a M20.

should read :

-5- Wave-making drag on cats, in an absolute sense, is not very dependent on hull length beyond a certain min prismatic hull ratio.

In this case the wetted surface drag is by far the most dominant factor and it is this aspects that will favour shorter extreme lightweight hulls over longer lightweight hulls.


Sorry,

Wouter

Drag consists of
1.Skin friction drag, which depends on the surface area in contact with water.
2. Form drag (or pressure drag), which depends on the shape of the moving object. Boats moving in the water surface and planes at transsonic and supersonic speeds create waves which create wave drag.

1. Skin friction drag:
Wouter, you are correct that reducing weight by reducing length reduces surface area more than reducing the same amount of weight by reducing hull width. Reducing weight of a rectangular box floating on the water surface by 10% in a 6 m boat by reducing length, reduces wet surface area by 9.37%. Reducing weight 10% by reducing width, reduces wet surface area by 3.7%.

2. Form drag (pressure drag)
Cross-section area of the hull and hull shape are of importance.

2.2. Wave-making drag is a component of form drag.
For displacement ships, the power needed increase speed above the (root of water-length * 1.34) increases abruptly. This rule is theoretically founded on the fact that max wave-speed depends on wavelength and gravity.

When increasing speed over theoretical max speed, you need to climb the bow wave, and this demands a lot of power.

One way around this limitation is the semi-planing mode. With a vertical flat stern the water detaches from the stern at a certain speed. Thereby, the stern wave is tricked into reattaching far behind the boat. Hence, the wave-making boat length is much longer than the water line.
The International 14 class guys use horizontal wings below the water surface to trick the water to behave as if the waterline length is much longer. (In addition, these wings prevent pitch-poling.)

Most of our beloved catamarans sail in the semi-planing mode.

(Some has maintained that cats like the M20 can sail in planing-mode, but there are different definitions of planing.)

Another way around the displacement speed, is to make the hulls so narrow that they only create very small waves.


By shortening the hull, you may as Wouter, has pointed out, reduce friction drag. However, increasing hull cross-sectional area to maintain volume, increases form drag. One part of form drag is wave drag.


Most sailboat classes have a maximum length.
HAVE YOU EVER SEEN SUCCESSFUL BOATS THAT ARE SIGNIFICANTLY SHORTER THAN THE ALLOWED CLASS LENGTH?

Stein

Stein,

I always considered the "semi-planing mode" to be akin to "a little bit pregnant"

In aerodynamics and hydrodynamics lengthening the object by adding a virtual object is possible, the aerospike rocket engine is using this principle very effectively, but it isn't happening with cat hulls. Look up aerospike rocket for more info.

For such a virtual hull to work it needs to excert a raised pressure onto the vertical stern itself. As in real life the sterns are only experiencing ambient airpressure it results that no vitual hull is effectively present. The theory of virtual hulls applied to cat hulls is now nothing more then fitting the errornous formula to "explain away" conflicting real life data.

I'm sorry.





I'm of the opinion that beach catamaran hulls are not of the "semi-planing mode" type if I could bring myself to ever accept such a concept, but rather that they are of the long narrow type with only relatively small wave-systems. I can actually SEE the truth of the last statement when I sail my own F16 while there are absolutely no indications of any planing.





When the reduction in wetted surface drag is bigger then the increase in form drag then going for a shorter fatter hull is more attractive from a drag point of view. Clearly this is not always the case, but just as clearly we can conclude that this isn;t an impossibility either.




To be really honest, I know of no boats that are shorter then their maximally allowed class lengths. In effect I have no data points of this kind. I put this down to nearly everybody believing that longer is always better.

The first real recognision of the fact that shorter make be less draggy, that I personally know of, was made in the F16 class. That is with one exception as Phill Brander once told me that such a thing was tried by a homebuild A-cat at one time. Not further data is know to me about this experiment.

But also my point is worded a little bit differently. Is wrote that when you really reduce the overall weight (boat+crew) for a given setup that it is best to also shorten the hull length by some amount over ONLY reducing the width of the hull. Most boatshowever are far to heavy to even consider such a move. Thereis of course a transition point somewhere. Also the only class that ever made a weight reduction big enough to have encoutered such a phenomenon are the A-cats. The F16's are still relatively young although the level racing with the F18's despite being shorter with a smaller rig does hint at the theory being right. They can only be as fast despite these "disadvantages" is the overall drag of the boat was reduced by at least the same amount. Now either the hull drag component is too small to matter in relation to other drag factors like the rig or the hull drag was reduced by at least 15 % as well.

Wouter

Here is the drag of a 5m 300kg (150kg boat+150kg crew on one hull) catamaran hull. Predicted using Michlet.


Blue Rw is wave drag, Red Rv is Wetted surface drag, speed is in m/s so double it to get knots. Drag is in kN.

As you can clearly see wave drag is far from insignificant, and is in fact larger than WS drag up to around 9 knots.

The next plot is the total drag (wave + WS) of a 5m hull compared to a 6m hull, both at 300kg and both optimised for minimum wetted surface area for there length.


The Blue hull1 is the 5m hull and the green hull2 is the 6m hull. Again you can clearly see
that there is no speed which favours the shorter extreme lightweight hull up to 20knots.

When I talk of wave drag at low speed I was talking about under 10knots not under the hull speed perhaps this is what is confusing you.

Gareth
www.fourhulls.com

Semi-planing mode seems to be accepted by all other boat designers. And it has nothing to do with pregnancy (I know, I have a medical degree).
The assumed "needs to excert a raised pressure onto the vertical stern itself" is irrelevant. Why should it?

Ask yourself: Why did the Int 14 sailors move the horizontal rudder wing to right below the water surface?

I think comparing the F16 with the F18 is a little bit unfair. The F18 is a very, very heavy boat, it is 70 % heavier than the F16 (180 kg vs 105 kg). The comparison proves that weight reduction IS important.
And that brings us back to the main theme of this thread....

However, I really do think F16 is a great class, Wouter.
Although I personally would have liked more sailors could enjoy the 19 foot, 140 kg Taipan 5.7.

Stein

All the application of formula and theory is fine (and necessary) when applying it to a basic hull shape, but as we all know (or should know) sailing craft are always operating in a continually varying compromise situation between the actions of hydrodynamics, aerodynamics, and their interaction at the boundary layer as well as the effects of movable ballast, its relative position, total ballast weight, sail control, and the overall combined weight of the craft and of ballast.
So many variables, such difficulty in calculating and/or predicting mathematically or practically the combined overall effect in all/most sailing situations.
By the application of JUST the appropriate, two dimensional
Hydrodynamic formulae in the hull design without due consideration to a global formulation of all the other interacting effects that are always present when actually sailing, will generally not produce a very efficient craft. One noticeable example of a compromise design trend seen more commonly in recent years is of building more and more cats with greatly pronounced “tumble“ home in their bows combined with longer finer entry’s BUT still carrying more buoyancy low down in the bow area. Theoretically this should actually reduce their resistance to forward motion when the bows are driven deeper into the water rather than increase the “tripping” effect that bows that become wider at the deck display. This “trend” is not solely the application of hydrodynamics for the “theoretical” best hull shape as seen in a test tank or theoretical formulae applied situation, but mostly it is a practical application response to an unwanted effect when actually sailing. You may “back calculate” the effects of the tumble home in these designs to show why this is so, but why were their predictable effects “overlooked” for so long in the theoretical design of so many catamaran hulls?
My point being that to take in isolation just the theoretical “best” hydrodynamic hull form without compromise to all the other variables applicable to practical sailing, will invariably produce a craft that is generally “a dog”, or at the least no where near it’s optimum.

I lengthened a cat by 9 feet by adding to both ends with foam and very thin plywood. Carbon was used in key areas for strength and stiffeners. The result is that the main platform is strong; the ends are fragile; the length is 26 feet; it has 350 sq. of sailarea; and weighs 375lbs. I feel that it is safe in that the ends could get destroyed and I'd still get home OK. It sails great. So I've concluded that focusing on making a strong platform to stay and then making ultra-light ends delivers a good boat that is pretty light.

The amount of rocker can also affect hull speed. Generally the greater the rocker, the slower the boat. http://www.stewartriver.com/special.html

PERFORMANCE: As the amount of rocker increases, the cross sectional area increases, but wave drag is reduced. This sounds backwards. Pushing a larger area through the water should result in more drag (paddles work that way, right?). Wrong. Wave drag on a canoe hull is more sensitive to how "sharp" the waterlines are than it is to cross sectional area. Even though the midships cross sectional area of #4 is 42% more than #1, its predicted drag is 16 % LESS. Canoes #1&2 push the water apart quickly, and pay a drag penalty. Rocker takes volume out of the ends of the boat, which tends to sharpen the waterlines. Sharp, fine ends are common in racing designs, but it's interesting that even a slight amount, like #3, results in 11% less drag than #1, and 9% less than #2. This amount of reduced drag would be very noticeable after a few hours of hard paddling. http://www.johnsboatstuff.com/Articles/canoe.htm

A lighter catamaran requires less rocker?

Gareth.

Those are very interesting plots. For now, I accept for arguments sake the Michlet software produces dependable results. This does not mean that I accept it as such at a later time as well. I've found that results produced by engineering software products can easily be wrong, while the the basic components of the software can be proven to be right in themselfs. I won't go into detail.

So the plots. Actually I don't see much that contradicts the model that I presented. I even see the miss Nylex data reflected in the plots; 15% and 22% drag (of total drag) due to wave-making drag and wetted surface drag at about 10 knots.

The local bulb in the blue plot between 2.5 m/s = 5 knots and 3.75 m/s = 7.5 knots does correspond to the Froude's law prediction that around speed = 1.33 * hull_length^2 the wave-making drag increases significantly.

The wetted surface drag look like power relationship with 1.8 as the factor. I expected a factor very close to 2 and 1.8 is not to far away at all. Also I had to measure the coordinates of the screen.


But I do have some issues with the plots are presented.

-1- We are discussing 105 kg overall sailing weight platforms and not 150 kg platforms. I don't really think that a 150 kg 5 mtr (16 foot) platform qualifies as a lightweight catamaran. It certainly does not qualify as a "shorter extreme lightweight hull"

Can we do this excersize again but now for 105 kg platforms ?

-2- The drag from the daggerboards, a large factor compared to the other two, is not included in the overal drag plots. The immediate effect of including this will be that the green and blue lines will lay significantly closer together in a relative sense.

-3- I never wrote that ANY shorter hull would be less draggy then longer hull with the same displacement. In the past I mentioned that there would be a transition point, thus implying that for one given set of hulls going shorter was better while for another set of hulls going longer is better. The only way to discover this relationship is to plot the drag plots for more then 2 examples. At least 5 examples (different) hull length is needed to go a somewhat go feel for the curved nature of this behaviour.

With only two example it is possible that the lowest drag hull lies between the 5 mtr and the 6 mtr hull for example. Without more then 2 data points one can not tell at all whether the relationship is linear or curved with a possible optimal length.

Is it possible that you work out the plots for 105 kg boats having hull length; 6.5 ; 6 ; 5.5 ; 5 and 4.5 ?



-4- You wrote :"As you can clearly see wave drag is far from insignificant, and is in fact larger than WS drag up to around 9 knots"

However I clearly see the wave-making drag to be smaller then wetted surface drag in the speed ranges : 0 to 3 m/s (= 0 to 6 knots) and 4 to 10 m/s (= 8 to 20 knots). Additionally I never claimed to be insignificant. I said it was much less important that wetted surface drag in these regions. This is also clearly visible in the plots. In the high speed range the blue is significantly lower then the red line; so here it is obvious. But even in the low speed range the difference appears to be noticeable. The lines lay close together that is true but proportionally the red line is significantly higher then the blue line and that ratio is important. It is important because a high ratio says that hull drag is mostly made up out of wetted surface drag. Each component may be rather small in an absolute sense, but RELATIVELY the wetted surface drag is noticeably bigger. By about 20 to 100 %


-5- Also can you produce plots where the individual wetted surface drag is plotted for the 5 different lengths. The same for the wave-making drag.


Thank you.

Wouter

Gareth,

The fact that your simulation finds that wave drag levels off above 12 knots means that the boat is in semi-planing mode, doesn't it?

What was the cross-section area you entred into the simulation?

Does the program take different bottom- and transom-shapes into account?

Stein

Can somebody define "semi-planing" in more detail ?

Wouter

That was quick, Wouter!

Even the definition of 'planing' is controversible.
From "a boat travelling faster than its stern waves",
via "lifted by dynamic forces that are larger than static forces (bouyancy)"
to "lifted to the water surface by dynamic forces".

Most planing boats produce some waves, hence even for planing, there may be some wave drag.

My impression being a non-expert trying to interpret what I find in books and on the web, an ultrabrief version is that 'semi-planing' is "static lift being larger than dynamic lift, with separation of water from the stern/transom" or "the transition between displacement mode and planing mode".

See this thread
http://www.boatdesign.net/forum/archive/index.php/t-2347.html
which contains referances.

If you search "semi-planing" on Google, you will find many ads and stories on vessels and military ships using the term semi-planing.
E.g.:
http://www.globalsecurity.org/military/systems/ship/semi-planing.htm

If 'planing' is imprecise, then it is not possible to produce an exact definition of 'semi-planing'.

Stein

Stein,

There is no dynamic lift element in the drag prediction used by Michlet, so there can be no account of planing, or "semi-planing". I usually post on Boatdesign.net but have stayed well away from that thread!

Wouter,

I intend to answer your questions more fully but do not have the time to do all the runs requested just yet. In the mean time here is the total drag plot for hull1 5m hull2 5.5m and hull3 6m hulls at 260kg (110kg boat + 150kg crew).



Gareth
www.foruhulls.com

"Semi" in my vocabulary equals something akin to "partial"

To me it appears that "semi" in semi-planing is not really intepreted as "partial" but more like "sub" or "pre" (full) planing.

To me is occurs as a catch all definition. I find it personally unsatisfactory and even unnecessarily confusing.

I followed the links and even there the concensus seems to be that planing and semi-planing are not sufficiently defined.

Several definitions even assume that there is a planing mode for a given hull. That assumption in itself seems errornous to me. It is probably the result of people focussing too much to on standard monohulls. As a result I really do find the concept of "semi-planing" useless when looking a beach catamarans.

Wouter

Gareth,

Please don't feel rushed, of course this is not a life or death matter. I can wait a few weeks if that would be better for you, I'm sure the other can so too.




I'm assuming that Blue line = 5 mtr boat, green = 5.5 mtr hull and red = 6.0 mtr. hull. is that correct ?



I also see that you've extended the drag plots to 12 m/s = 24 knots. Something interesting is happening at the far right of the graph as you have mentioned yourself in earlier posts. In the far left the same can be seen. Here the hull with less overall wetted surface area sees less drag. For larger regions at these extremes the differences in drag are to small to favour one hull over another. That is if the colour coding is the one as I described above. But having said all this, it is only found in the extremes.

I'm looking forward to the individual wetted surface drag and individual wave-making drag plots. From the plot for the overall drag is almost appears that Michlet use a base drag level for a given displacement and then adds a wave-drag component.

Is it also possible Gareth that you specify the hull shapes that you have used in producing the plots ? Are the scaled versions of eachother or do they share bow wedge angles. I'm thinking along the lines where the I-20 front of the hull is identical in shape to that of the shorter I-18. Stuff like that.

The difference in hull drag (wave+wetted surface but without daggerboard drag) between 5 mtr hull and 6 mtr hull appears to be 14 % at 10 knots. Including the daggerboard drag (based on miss Nylex data) will reduced the hull drag difference at 10 knots to just shy of 10 % which would be roughly a difference of 5% on the overall drag of the whole boat (based on miss Nylex data).

A 5 % drag difference at this spot would roughly translate into a speed difference of 2.5 % neglecting other limiting factors like minimal angle of attack considerations. A measurement based handicap system like Texel predicts a speed difference of 5 % in such a case, as a direct result of a longer hull line length.

This still seems consistant (in the rough) with real life experiences. It is my personal opinion that the long hulled lightweigt boats are unable to consistantly sail to their Texel handicap numbers. Surely their are more causes for this but this hull drag factor may indeed be one of those.

Assuming Michlet produces dependendable results, these plots show that relatively short hulled lightweight boats don't suffer much at all at very low speeds (light winds) and high speed, but do have more to make up for in the medium range speeds. That is indeed my own experience with my own F16 as well, as I've stated quite a few times in the past.

As a side note. I also suggests how adding a spin to the beach cats helped them to close the gap between short and long hulled boats. By increasing the overall speed of all boats, hull length is significantly less important factor in determining the overall drag of the boat. Maybe this equalizing in overal does also help equalize performance between crews of different weights but with the same skill level. All things that many of us have experienced in reality to be true.

Very interesting, Gareth, thanks for your efforts.

Wouter

It seems to me that in order to create dynamic lift, a boat has to actually be in the water. If the boat is not in contact with the water, you won't get any lift from it (obviously). So, even a fully planing boat displaces a bit of water and so has some component of static lift, so planing cannot be an absolute condition, but rather an arbitrary decision about whether enough of the lift is dynamic.

I think it's quite telling that most of the hits for "semi-planing" that Google turns up are boat ads. This suggests a phrase coined by builders not scientists, and I assume is intended to mean a boat that sits somewhere in between planing-as-in-skiff and displacement-as-in-car-ferry. i.e. there is a significant component of dynamic lift, but not so much that you'd call it planing.

As for cats "semi-planing": I've never really believed it. I find it hard to believe that something that shape is going to produce any significant fraction of the 2.5kN (like 255kg) of lift needed to keep my 2-up Blade afloat. Calling cats semi-planing strikes me as an attempt at explaining why cats go as fast as they do by people who don't understand what Froude's Law actually means. But this is just speculation: I only made it to one lecture on fluid dynamics , and it all seemed much too difficult <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Paul

Good points Paul,

Here some additional comments :

Scientifically speaking a boat is suspended at the water surface by nothing more then a pressure difference between the keel and the decks. It doesn't matter what causes this pressure difference to be present, by displacement (hydro-static pressure difference) or by planing (hydro-dynamic pressure difference). This first force is the result of the water pressure increasing when you dive deeper below the water surface and thus the ambient air pressure as present there. Planing is the result of a pressure build up under the keel line as a result of the onrushing water being slowed down or bend away from its initial direction. Example; as the hull moves through the water to particals are hitting the planing surface and are pushed out of the way to the side or downward. This creates a (dynamic) pressure zone under the keel line that is added the pressures created by displacement. This excess of pressure results in the hull being lifted higher out of the water till this system finds a new stable balance between the forces. But as this example makes clear, there is absolutely a significant amount of water displacement present, if only by the water that is being squirted out to the sides as (white foamed) water. So planing is not a concept that reduces wave-making drag, because it really isn't. We all know that wave system created by planing are very significant. The idea behind planing is to reduce the wetted surface area and its related wetted surface drag. In Gareth plots you can see this potential in the plots. his Michlet plots show that for higher speeds the amount of wetted surface area is much more important in determining hull drag then wave-making drag is.


Wouter

I totally agree with Paul.

The Froude's law at 1.0 just simply states when boat is going faster than the wave it generates, it does not state anything about planing or wave drag amount.

I hope that this doesn't spread as much as at one Finnish forum, there was a 345 post discussion about Froude, planing and sport boat definition. At least let's drop Froude and planing relation with beach cats out of this super beach cat conversation <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

--
Valtteri

Wow. And there I was, thinking that my boat actually just floated. Now I`m too damn scared to go sailing for fear of it being suspended somewhere between the air and the ground, with only a mixture of hydrogen, oxygen and algae holding it all together. You guys need to go sailing a bit more, winter over there, eh ?

Hey, some guys actually do enjoy the science behind sailing as well, it adds up as a additional aspect to sailing. And it is a pretty fun way of learning about hydrodynamics compared to some of the prof.s I have( I'm studying marine technology). But yeah, it's winter.. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

The latin prefix "semi" literally means "half", not partly.
Semi-planing literally means "half-planning", like semi-annual coupon means a coupon every six months (exactly every half year, not sometime in the 1 to 364 day range).

Regardless the definition of planning and displacement, "semi-planing" is a rare event for the simple reason that it is rather difficult to sail at the exact speed that divides in two the speed range between displacement and planing speeds. "Pre-planing" is a more adequate expression, but that includes displacement speeds too.

A correct indication of the regime when a boat is sailing faster then displacement speed (whatever its definition) but slower then planing speed (idem) would be "post-displacement pre-planing regime".

Take care with those names, though: the existence of a precise name for something transmits a false sensation of scientific accuracy. A precise name lends no precision or accuracy to the phenomena itself. It also doesn't assure that it actually happens.

You do find the term 'semi-planing' on designers' websites and in official military sites.

An example of semi-planing boats: Along the long Scandinavian coast people developed a vessel type looking like short merchant viking ships with fairly deep keel and rather fat round bow and stern. Fishermen used 20-30 foot boats doing the hull speed: 6-9 knots with ca 20 hp engines.

When glass-fibre reinforced polyesther enabled production of such boats for recreational use, the possibility of modifying hull shape to increase speed immediately become important. The designers changed the aft 1/3 and introduced a horizontal flat bottom section with sharp edge between bottom and sides. Keeping the canoe-shape or cruiser-type stern. The deep keel was kept to preserve sea-worthiness. With double engine power, speed increased by ca 50 %.

Hence creating lift of the stern to prevent the sucktion down at hull speed + a sharp termination of the wet surface to enable the water slipping from the stern, this boat type surpassed its hull speed. The boat is not lifted out of the water more than very few %, bouyancy is than main contributon to floating, but there is dynamic lift of the stern.
This type of boat may be considered in a transition between displacement mode and planing mode. Or one may consider this a system in which part of the boat is planing.
In other words: 'semi-planing'.

Consider this:
Almost all high-performance cats have a transom stern to allow water to detach from the stern. Why?

High-performance cats' sterns are not sucked down when surpassing hull speed. Why?

Stein

Wouter,

You argue for advantages of shorter hulls:
Wetted-surface decreases, hence surface drag decreases.
However, to preserve volume (bouyancy), you need to increase cross-sectional area. This leads to increase in form drag (pressure-drag). Even if your length/width ratio still is low enough to keep wave drag low, there is still increasing form drag.

Hence, reducing length is not automatically a optimal solution.

The consequence of your arguments for shorter length cats, Wouter, would be to cut off a feet or so from the stern of your excellent Taipan 4.9. Or switch to a 12-14 foot class.

But this thread is not about boat length. Let us return to the original question!

What is possible today?
Are the Super Taipan or the M20 the fastest cats around?

Stein

“Planing” lift occurs when an upward force is generated on an object due to it’s velocity through/on the water and that velocity lift becomes additional to the objects displacement buoyancy, with those two combined influences acting on the object it will move with less of its volume in the water therefore less drag, more potential speed, IE a stone skipped across the water at speed will truly “plane” as it lifts completely out of the water after every early contact with the water, until that speed reduces to the point that for the stone to stay above the water surface it has to displace more weight of water than it’s own weight otherwise it will sink. Semi planing is when an object is under the combined influence of both dynamic velocity lift and buoyancy lift (so in effect all boats, whether considered on full plane or only partial on plane are in reality "semi" planing? if they were truly fully planing with no buoyancy/displacement factor involved wouldn't they be airborne?).
Given enough velocity most objects will “plane”.
Take a snow mobile, at speed it can and does travel across the water surface, when it's speed reduces to a certain point it will sink due to it not being able to displace more weight of water than it's own weight. Displacement (or buoyancy) is always a factor for all “floating” vessels, even the fastest at plane, if it wasn’t then it wouldn’t be in/on the water, it would be flying and therefore airborne instead. My contention is that there is no such thing as a truly "planing" boat and that instead there are only boats that semi plane to one degree or another.

It is cheaper and easier to build that way, plus it allows easy fitting of the rudder pintles. Ever tried to allign rudder pintles on a curved catamaran stern, I did !

My point here is of course that such shapes can also be the result of pretty non-spectacular considerations. A while back we covered the reason for the rakes back bows and that too was pretty mondain.

But I have no desire to endlessly discuss any virtual hull theories, they are all simply wrong. The reason for it that the pressure at the stern plane is still very much the ambient airpressure. As a result there is no significant difference whether the hull has a virtual extension or not. Again I refer to aerospike rockets engines to see a real example of how a virtual lengthening should look like. Here the zone between the exhaust jets is a (gass) spike of much increased pressure pushing against the flat between the rocket engines. In effect the rocket base is entlarged with a spike made up from compressed gasses. Theoretically this spike can also be made of metal but this works just as well. Actually it works better as you have less weight in the engine and less trouble preventing the metal from melting. This is a true example of a virtual extension. Water cleanly seperating from a stern is really not. I may have some secondary drag reducing effects but nothing major. It certainly doens't involve Froude'a law much. I leave it at that.




How do you know that they are not ?

Additionally, maybe the dynamic pressure zones around a cat hull are too low to result in significantly lifting or suction forces. Afterall the hulls are very long and slender and water is only accellerated out of the way relatively gently. This can also be seen in Gareths plots. Overall wave-making drag is not a very big factor. If you make plots like that for mono-hulls you'll see the wave-making drag to be alot more dominant. Rememder Miss Nylex crew found a wave-making drag to be only 15 % of the total boat drag, the pressure forces will be accordingly small. Besides on a sail boat you also have a mast and sail trying to push the boat over its bows with a leverage that requires two people to be at the very back of the hull. If anything, the suction downward at the stern is helpfull !

later !

Wouter

Wouter

earlier in this thread you quote drag due to centreboards as 21%, is this the figure an average for upwind and downwind or is it the upwind drag.

Seems to me that this drag must be mainly caused by the leeway a boat going upwind makes so that the boards are slanting through the water, I would have thought that the downwind component induced by boards must be far less than the upwind since the board is travelling parallel to it's centreline(pretty much).

This post intents to show that there exist an optimal hull length for a given displacement. This optimal point is defined in the way of overall drag ; wave-making and wetted surface (c.q. form drag and frictionous drag)

If such an optimimal point is found then the immediate result will be that some boats will be better off with shorter hulls and other with longer hulls when only looking at overall drag.

Do this mind experiment.

Stretch a flat plate to such a length that its volume is equal to the required displacement. Its crossection area is extremely small while its hull length is extremely long and so the wave-making drag (form-drag) will be negligiable. Hull speed laws (Froude's law) will predict very high hull speeds. But the wetted surface area will be inmensely large and so to the wetted surface drag. It will be even be much much larger then the overall drag of say a normall sized catamaran hull of the same displacement but with much less wetted area.

As a counter experiment place the same plate perpendicular to the movement. Now we have an extremely small hull length with a inmensely large crosssectional area. The wave-making drag (form-drag) will now go through the roof while wetted surface drag in the direction of the movement will be infinisemal small. Again the overall drag is much much larger then that of a normal catamaran hull. Try to pull this plate through the water and you'll experience the truth of the last statement.

Clearly both extremes are more draggy then a normal hull, these three points can never be linked up by a straight line, ergo the connecting line is a curve of some shape and will have at least one minimum (drag) value. This point corresponds to a single hull length where the overall hull drag of the boat will the smallest. This will then be the optimimal hull length from an overall drag perspective.

This is also the direct counterproof against the rule : "Longer hulls always have less hull drag (and are faster)"

The same proofs that "shorter hulls always have less drag" is wrong as well.

Simply put the use of "always" is wrong.

For us it will be interesting to find where this optimal hull length is located. Is it inside the 6.5 - 4.5 mtr hull length spectrum of beach cats or out side of this range ?

That is why I asked Gareth to produce the plots for several hull lengths.

It is my opinion that for some bouyancy levels the optimal hull length is to be found inside this normal hull length range. Prime candidates are the 105 kg bouyance (M20, eagle 20, F16) and the 75-100 kg singlehanded bouyancy boats as with A-cats and F16's. Boats being significantly heavier like
that will have optimal hull length laying beyond 6.5 mtr and are therefor always better longer hulled. That is when limiting our hulls to the range 6.5 - 4.5 mtr.


I hope this is clear enough for all

Best of enjoyment on your steps into boat designing science !

Wouter

Stern,




Actually I haven't yet, I've only pointed to the fact that drag graphs are curves and that therefor an optimal point can be found. This directly implies that a longer hull isn't always better from a drag point of view. Of course the same can be said for a shorter hull. The trick for a boat designer, among others, is to find this point and balance it against other considerations.

The true advantages of shorter hulls are in fact :

-1- it is easier to build a lighter hull
-2- it is easier to build a stronger hull
-3- it is easier to build a stiffer hull
-4- the boat feels more responsive to steerage
-5- can be build light, strong and cheap with plain materials.

Actually points 1 to 3 are strongly dependent on length of the hull and the size of the crosssectional area. The fact that the M20 despite Marstrom best (carbon autoclave) efforts can't hold its weight down to 108 kg while homebuiders can made dependable 108 kg F16's from ply shows this in undeniable terms.

There are of course also drawbacks. But others covered those already.





That is not entirely true. The overall drag will ONLY increase when the increase in wave-making drag (absolute sense) is more then the decrease in wetted surface drag. If it does not then the overall drag will actually reduce when making the hulls shorter. But because of the curved nature of these dependencies you will find that reducing hull length beyond at certain hulls length will increase drag again.

Also you can see both things happen at different boat speeds, go take a look at Gareths plots again. You'll see the 5 mtr hull having less overall drag at low and high speeds when the 6 mtr has less overall drag in the mid range of hull speeds.

By now we have enough data to proof that any rule using the word "always" is wrong. Therefor a longer hulls is not *always* faster because it may not always have less overall drag.






I never said it was automatically an optimally solution, that is only the way you intepreted it.






No it is not, you are trying to insert the word *always* again.





No, the Eagle 20 carbon is, same weight as the M20, same hull length but it has a jib as as such it is faster over a significant spectrum of windspeeds. The only exception being the light winds as here the jib is not of much help.

Wouter

This data comes from miss Nylex C-class design article. The text with the table says :

Approximate drag values for the following conditions :

Boat speed - 12 mph
40 degrees from true wind
true wind speed 15 mph
apparent windspeed 25 mph

They have more or less optimized the Miss Nylex cat for these conditions as they found the C-class boats on the C-class course to spend at least 43 % of the time on such a leg. Therefor most gains were to be had with improvements here.

Also remember that Miss Nylex had a solid wing sail and those rigs produce more thrust for a given drag and side force. Therefor you can expect the dominance of the rig to be larger on soft cloth beach cats this means that the drag numbers given for hulls drag etc (precent wise) probably can be considered as upper limits for those found in beach cats.





I expect as much as well.

But also note that boats under spinnaker achieve higher speeds then when going upwind. Wave-making drag becomes increasingly less important relatively to wetted surface area drag with increasing speeds. As such downwind legs can actually favour shorter hulled boats more then upwind legs, meaning that when a shorter hulled boat is found to be favoured on the upwind (which is not a certainty) then it is logical that under spinnaker it will be favoured even more.

I hope this helps.

All very interesting stuff really.

Wouter

What speed range would we want to optimise a beach cat for, As I said in an earlier post at 100kg displacement shorter cats are only better above around 15knots (according to drag prediction from Michlet). My guess is that the average speed of a cat around a race course would be in the 8-12knot range.

Also it should be noted that Michlet does not take induced drag (leeway) into account. And the induced drag effects the hull drag as well as the rudder/centreboard drag. So I would expect a different story going upwind, but I am not sure how to quantify it.



In simple terms you can look up the drag bucket plots for the foil section and estimate it that way.

But in broad terms upwind has higher drag than down wind for lots of reasons.

Gareth

It seems as these arguments tend to forget that form drag consists of two components, and wave drag is one of them. Even submerged submarines and airplanes at lower speeds are subjected to form drag even when no waves are produced. Hence, increasing cross-sectional area increases form drag even when wave drag is (almost) constant.


I have been trying to interpret your arguments that the shorter 16-foot hull should produce less drag than a 19-20 foot boat (that this thread was about).
I cannot agree that introducing the observation that sailboats almost always tend to be at maximum class length, or mentioning 'planing' or 'semi-planing' (either defined as transition between modes or as part of the boat being dynamically lifted), can be brushed away as simplistic absolutes: "always" or "never".
Rather to me it seems that absolete denial of the possibility that beach cats are subjected to some dynamic lift, or that 'semi-planing mode' does not exist, classifies as absolutes.

I am not at all an expert in boat design. Furthermore, the texts that I read tell me that several different mathematical models/algorithms are being employed to model different phenomena of hydrodynamics and boat design.

Hence, I fully agree that we as amateurs should be humble and careful when trying to discuss these matters.

This is a great forum, and the possibility learn is great because people like Wouter try hard to help and explain. We have to live wih disagreements and uncertainties of what is really correct until somebody comes along and brings us hard empirical data.

Stein

Quote [If anything, the suction downward at the stern is helpful!]

To "drag" your transoms on any point of sail is always the slow way. There is always a better compromise. (Besides, "suction", doesn't exist as such, it is a layman’s descriptive term for the effect (in most cases) of a head of air exerting a positive equalising pressure)

Quote [But also note that boats under spinnaker achieve higher speeds then when going upwind. Wave-making drag becomes increasingly less important relatively to wetted surface area drag with increasing speeds. As such downwind legs can actually favour shorter hulled boats more then upwind legs, meaning that when a shorter hulled boat is found to be favoured on the upwind (which is not a certainty) then it is logical that under spinnaker it will be favoured even more.]

Personally I would have thought that the fact that boats going downwind go faster than they do upwind was due more to the fact that they are carrying much more sail (with their spinnaker), rather than any alteration in the effects of any drag inducements or hull lengths. IE drag coefficients become much less important to overall speed than actual sail area/efficiency (assuming that the hulls are relatively efficient to begin with) Or perhaps I missed your point?

so a smaller vessel must absorb a lot more energy to create speed then a larger one ie: sardine=tuna. The sardine is capable of the same speed as the tuna but cannot travel the distance the tuna can when the tuna is not in a can.Peregrine= chicken ,if the chicken is not protected it will probably be hit like a "Pinata".(this is a poor example). thank you ..the Zissou society.

BUT put a spinnker on the sardine then------

You missed my point

Wouter

That is one of the reasons why I don't the identifier "form-drag".

I'm very strict in the wording of definitions if it is ambiquous then I consider it useless.



All hulls have aerodynamic drag as well and we haven't included that one yet either. Gareth mentioned also excluding induced drag (because of the sideways slipping). We can think of a bunch of other factors not included, this still doesn't mean that they are large enough compared to wave-making drag and wetted surface drag to really factor in in the overall picture. Any scientist will have to simplify his models at some level, in this thread we implicetly decided to only look at the wave-making drag and wetted surface drag factors as they are the largest ones.

But still my original statement applies even for these simplified models. If the wetted surface drag decreases by a larger amount then the wave-making drag increase then the overall drag of our (simplified) hull will decrease.

The question we are trying to find out in this thread is whether this is found to happen in the hull length and weight ranges that are associated with beach catamarans. I'm on record stating that I think that for some lightweight craft this may be the case.




I did not really state it that way. I said that I thought that a really lightweight (105 kg) 20 footer may be better off with shorter hulls like for example 5.7 mtr long hull. I later refered to the F16 design to show how a smaller boat with less sail area can still run with the larger F18's by having been made both lighter and shorter. It does so over a wide range of conditions. This was achieved, in my opinion, by both reducing wave making drag and wetted surface drag by equal percentages, in this shortening the hull was a way to achieve this.




I'm not sure whether I did that but I do know that I brush away descriptions like "semi-planing" because they are not well defined and so allow everybody to read something else into it. As such these "definitions" are useless and can even be confusing.




I did not deny that. And I argued that some hull shapes will not have a planing mode no matter how fast they travel. Therefor the definition of semi-planing is increasingly useless as it implicetly assumes that all hulls plane at some speed. Submarines are great counter examples, so two are ships with fully submerged floats where only the skirts penetrate the watersurface.




Correct.



True

Stein

Quote
[I'm not sure whether I did that but I do know that I brush away descriptions like "semi-planing" because they are not well defined and so allow everybody to read something else into it. As such these "definitions" are useless and can even be confusing.]

Please outline your "description/definition" and source of the term planing as applied to waterborne craft.

It would seem that "semi planing” is the more accurate description, as although all/most sailers feel that they know what planing is, that term when applied to boats is, in reality, a misnomer. No vessel under horizontal motion and in contact with the water is ever FULLY planing it is instead in a state of "semi plane". The only thing that varies between what is called, when in motion, "full displacement" and what sailers vaguely interpret incorrectly as "fully planing" is the degree/percentage to which the hull is affected by the "lift" generated by their forward motion. If a boat were “FULLY planing” it would be, as the name implies, flying (as per a ‘plane, aeroplane, aircraft) I fully realise that most people commonly apply the term planing to waterborne craft, but that does not necessarily make it correct, especially if/when precise descriptive analogies are being asserted.
Mind you if the terms aquaplane or hydroplane were used instead the meaning would be a little better defined, but that still doesn’t alter the application of the terminology “semi plane” as being the most accurate.

Quote
[I did not deny that. And I argued that some hull shapes will not have a planing mode no matter how fast they travel. Therefor the definition of semi-planing is increasingly useless as it implicetly assumes that all hulls plane at some speed. Submarines are great counter examples, so two are ships with fully submerged floats where only the skirts penetrate the watersurface.]

EVERY and ALL shape(s) of object/boat WILL plane (by your definition of planing) IF its velocity is adequate (even a submarine). To say otherwise is to deny a fundamental physical truth.

“When "lift" generated due to the velocity of any object when in contact with a fluid is greater than the “weight” of that object, the object will then (aqua) plane”
(Plane by your definition of plane)

There are too many examples of this principle to fully list I.E just a few being. An automobile is not a very hydrodynamic shape yet it still commonly “hydroplanes” across a puddle of water on the road when travelling too fast for the prevailing conditions (same principle). A spacecraft re-entering the earths atmosphere will skip (plane) across the outer earths atmosphere before it’s velocity is reduced enough to safely re enter (same principle). Large bombs were “skipped” across the water to destroy German dams during the second world war, calculated to lose enough of their velocity (and their “lift” due to their velocity) so that they would sink at the base of the dams before they exploded (same principle), etc ad fin idem.

One of, if not the key foundation stones of mechanical engineering have to be precision and accuracy, particularly with definitions and principles, otherwise conclusions are nothing better than subjectivity

Back to the original question...

I have personally seen some very light boats forced to shore because of "fragility" (if that is a real word)thin carbon parts snapping, dismasting, etc....

At this point in time, are the carbon 20's smokin' the T boat?

nope.

w,
Can you enlighten me on how heavy the big 18teens were?
Boats like the last colour 7 before they were banned?

S