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