How fast can a beachcat go?

How fast are beachcat hulls ACTUALLY CAPABLE of going with lots of motor power on the back?

Supposedly beachcats are displacement boats and therefore have a maximum hull speed, since they are not capable of really planing.

We have a Hobie 18 that has been converted into a powerboat (no mast) on it. We put a fiberglass "pod" on it that is fastened to the main and aft beams. The outboard motor mounts on the aft end of the "pod," and the front end of the pod has a console with steering wheel and controls for the motor.

Originally, we had an 18-horse motor on it, and the boat would go just under 25 mph (verified by radar gun). That motor got stolen, so now we have a 30-horse motor on the back, and Rick thinks the boat goes about 30 mph (on flat water).

This is including the "pod," which weighs about 150 pounds, and the motor, which probably weighs over 100 pounds, plus a 200-pound person on the boat.

So what I am curious about is whether this means the hulls are capable of going that fast (even with at least 450 pounds of "crew" weight) and, therefore, they could potentially go that fast under sail if they had enough sail area, and with the center of effort positioned correctly.

Obviously, with a heavy outboard motor on the back, the sterns squat down, and the bows are up and there is no tendency to pitchpole.

I just think that if you can prove the potential hull speed of a sailboat by using a motor, you can then figure out how to design a sail plan with enough equivalent power to get it to that hull speed without pitchpoling. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Am I wrong?

No disrespect, but I think you are, unless you are designing for a very narrow wind range. It's a fact of life that if you want to be powered up in light air, you'll be struggling in high wind, and vice versa...

I've been clocked by the Michigan State police going 90 pulling a beach cat. Very expensive time trial! I believe I have additional undocumented runs around 95.

Obviously, I am not explaining this well.

There have been past threads on this forum, sometimes hotly debated, about how fast a beach cat can go. Some have said that the top potential speed is slightly under 25 mph (or maybe it was knots, I can't remember). There have been anecdotal reports about going as fast as 30 mph. People have been trying for a long time to get gps readings that confirm speeds into the mid and high 20's, but most are just brief spurts, like when surfing on a wave.

Some have said that the limiting factor is that most catamarans have displacement (rather than planing) hulls.

So the question is, what is the upper limit of the potential hull speed?

In almost all cases of beach cats going very fast, the thing that ultimately stops them is the bows digging in and causing a pitchpole.

Therefore, based on speeds with outboard engines, it seems obvious to me that, even under sail alone, if you can get enough weight at the back of the boat, keeping the bows up out of the water, you would be able to achieve much higher speeds than have so far been recorded.

Like, when it is blowing about 25-30 knots, strap some 100-pound bags of sand on the back of the boat, rake the mast back, put up a spinnaker (to lift the bows) and see how much faster you can go.

Also, I wonder how much faster it could go if you leave the mast off and use a kitesail. <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

1997 Bermuda World Tornado Championship, during an event called the Bacardi Blast sprint. A timed run between gates that were 500 feet apart.

The Record stands at 27 MPH or 23.5 knots average over the 500 foot course. Completed in 12.53 seconds.

I'm sure you have all seen it, but here is the short video of the event.

http://www.youtube.com/watch?v=do7nWUgijZs

Yes, that is one of the events I was trying to remember. Do you know what the wind was for that?

I have never heard what the wind speed was, but by judging the water in the background and considering it is a protected section of water, I would venture a guess of 18-20 knots. This could be a good discussion on its own (how much wind is in this picture?) That would be good practice for goobers like me to judge conditions. I know that my normal stomping grounds will start to white cap at around 12-13 knots if the wind is against the tide. But I have seen it blowing over 20 knots with no white caps and light chop when going with the tide. I have almost put my behind in a sling a couple of times trying to figure out the conditions. Other than erring on the side of safety or knowing the conditions will get worse, I could have had an unscheduled appointment with rescuers. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

modern hull designs will plane.

Some of the older ones would as well (see warbird's Tigershark project.)

Let me caveat this message with the fact that I am not any kind of engineering expert. I just read a little... plus I stayed at a Holiday Inn Express once.


I would assume that most catamarans plane since if they were displacement, they would be tied down by their theoretical hull speed. According to the Annapolis Book of Seamanship, hull speed is calculated as:

Hull speed = 1.34X (square root of waterline length in feet)

That would put the hull speed of a 20 foot beachcat at right around 6 knots. So anytime you are sailing over 6 knots, you are probably planing to some degree. Right?

-Rob

No; Cats, because they have long slender hulls do not obey of the hull speed rule.

Is this really part of the "Test"?

Okay, Pete, since you jumped in on this: If you put a 30-horse outboard on the back of your Tiki, how fast could she go? Or a 50-horse? Or a 70-horse?

This formula is derived from Froude's law. Froude's law is derived from how the wave created by the bow interacts with the wave created by the stern, as well as how the waves are related to the boat speed. If these two waves interfere contructively, you will get a big increase in wave drag.

The Froude's number is given by:

Fn= Boat velocity/ "the root of" (g*length in waterline)

Typical Froude's numbers:

Type of boat------------length(m)------speed (kts)------Fn

Oil carrier-------------300------------16,5-------------0,16
Catamaran(transporter)--35-------------35---------------0,97

As one can see the catamaran has a relatively very high Froudes number. These wessels are designed to carry people fast and safely between islands etc, and so that requires high speed, but also a not too big vessel that easily can be handled in port. The answer: Long, slender hulls in the water line. These hulls creates alot less wave drag as they "pierce" through the water. The slenderness however means loss of stability, hence why one have to have more than one hull.

When asked of what the max speed for a high performance beach cat was, my hydrodynamic prof. simply said: "there's no such thing as a theoretical max speed. If you have enough power, you can always go faster."

That been said, personally, I think the Tornado blast contest already mentioned here gives a pretty good estimate. Tornados are wide and have alot of power. In addition, the record set in the speed contest, was done by a highly skilled crew.

Expecting Wouter to fill in here as well.

There is always a limit at some point.

Why don't you post this question on boatdesign.net to see what answers that you get there as well?

It wouldn't matter. Max for that boat is around 12 knots, after that the rooster tail would just get bigger.

I'll bet posting that on Sailing Anarchy, with someone like Wouter to keep the pot stirred, would be quite entertaining.
:::Wicked Evil Grin::::: <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

Here is an image of the first run of the boat. Tiger Shark 18 foot Ron GIven design.
I sailed for two hours just checking everything that was wrong with the rig and boat (which was plenty).
The boat sailed faster than my Nacra 12sq and NEVER tried to pitch. The Nacra would have been a hand full at that stage and I would have been constantly altering trim with the bow an inch or two from the surface. My Hydra 16 would also have been pitching.
When I got onto this boat I stepped over the bows slopping sand spots on both hulls. The sand was still there on return to the beach.
The boat planed up wind and down.
most noticeable difference in driving the boat is a much lighter and more stable feel.

My reasoning is very simple here.
The TS was designed on the Paper Tiger basic platform and stretched to be a
30 foot twin trap rig.
WHile the hulls my want to pane thay cannot in the twin man guise as there is not enough boyancy to get it going as the power drives the hulls down into the water.
I have cut the rig down by more than 5 feet. I have square topped the old sail and taken some amount off the foot length. I have raked the rig back a lot.
I am at present one man up and so the power quickly pushes the hulls up and out of the water.

I have ben working getting my H17 together this last month as it is my daily ride and can now go back to this project.
Whirlwind are making new sails for it and I have to get into heavier weather.
Again, my stated goal is 25 knot runs over the distance the Tornadoes were running.

I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions.

If you provide progressively higher power and manage to keep the boat stable and in one piece (vibration, ressonance, etc.), the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed.

If this problem is solved, maybe with ceramic foils, about the same thing will happen at a higher speed but with hull and rig friction with air. I believe this is the actual limit for a space shuttle. Now we are talking a few thousand knots.

Sailboats are far from both limits.

Warbird,
The photo didn't fully upload.
-Rob

my under standing of hull speed is just that there is an energy "hill" to climb when the bow and stern waves meet and interact

traditionally most boats lacked the power to climb that hill and get to the other side

big engines on light boats changed that by lifting them into a plane

beachcats can also get over that hill with their huge sail area but they don't have to plane to do so

some experimentation with rocket bottles on an old hobie 16 in dead flat water would no doubt lead to boat speeds of
approaching 100knots
would it mean much to us sailors?

probably not

Speed is only limited by power.

Maximum velocity is acheived when power = drag

In general, drag increases as the square of the velocity. To double your velocity, you need to quintuple the power. This is true for ships, however, small boats behave differently.

Drag on the hull can be segregated into three components:
Skin Friction, Form and Wave-making.

At low speeds, skin friction and form drag dominate. At moderate speeds, depending on the fineness of the form, wave-making drag begins to dominate. As velocity approches "hull speed" (approximately 1.2* sqrt(waterline length) - varies depending on the fineness of the hull), wave-making drag is the dominant force. This is the point just prior to a hull planing and is analogous to the sound barrier in air.

Once a hull "breaks the sound barrier" and begins to plane, wave-making drag drops away and becomes almost negligible. Most motor boats have steps and strakes in the hull to facilitate separation of the water from the hull and achieve planing at a lower speed and reduce skin friction at higher speeds.

As speed increases, air resistance begins to exert more and more influence and becomes a major component of total drag (hydroplanes flipping over).

Like the old question – “How much wood would a woodchuck chuck, -----“
It’s like the fighter pilots analogy – “given enough thrust, a house brick will fly like an F16 (plane that is, not cat)”

Sorry, the net is on the blink here.
I think that as wee set weight out to the side for righting moment we push the wave piercing hull further into the water and create more stress.
The Tiger Shark as I have it leaves almost no hull in the water once up to about 12 knots and from then if I can keep the hull down it just keeps accelerating and planing higher and creating less drag.

I am including this shot also.
Note the halyard at ring of bridle as a speed indicator.
At this speed my Nacra would be creating an arc of spray from the bow to the stern. The Tiger is cruising at no fuss.
The bow is of absolutely no consequence, only trim for speed and I need to pray for more wind, set the trap up and use it.

That boat is NOT (fully) planing. There is none of the telltail spray pushed out to the sides of the hull. There maybe some hydrodynamic forces in the vertical direction but the wake does not look any difference then from any other lightweigth fast beachcat.

There is also alot of errornous comments presented in this thread, but I'm not going to correct them.

Wouter

That is very unusual of you.

I just fell off my chair. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I think practicality takes over before theory applies. In the case of my Tiki, the decking isn't strong enough to take the weight of a large motor. Also, fuel consumption goes up very quickly. At 4.5 knots, the Tiki will get about 30 mpg (the noise is also more tolerable) at 6 knots the fuel efficiency is less and the racket will make you crazy.

I don't see that testing sailboat hulls under power demonstrates anything useful.

>>>"I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions"

I agree, I think that the structural integrity of the hull will be the biggest limiting factor of speed for your average Hobie 18. You can continue to add more power (well, assuming the boat has enough flotation to support the motor), but eventually the forces generated by the flowing water will crush the hull. Just like with wind, the force of the flowing water increases as a function of velocity squared, so if you double the velocity, you quadruple the forces on the hull.


>>"the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed."

I seriously doubt the hull is going to catch fire/burn as a result of hydraulic frictional forces. Fiberglass hulls regularly travel through water at high speed- the water acts as a coolant. Again, the hull collapsing will probably be the main issue. Just a guess here, but I would think you will start to have structural issues around 50knots, maybe even sooner- especially in a turning situation.

sm

This is all very well studied already (using a Hobie 16 and rocket propulsion):


Results in a peer reviewed engineering publication and some more technical details in
scientific paper

Mary

To answer your question exactly, it depends.

To actually answer in a simple fashion that is probably wrong but gives you an answer that you can actually use:

Cats, for the most part, follow the same rules as other displacment hull boats. The very simplified way to calculate maximum speed is:

Maximum speed(in knots) = square root (waterline length(in feet)) times a constant

The constant is related to the ratio of the waterline length and the waterline width. For big heavy boats the constant is fairly easy to calculate, it's 1.2-1.5. For little boats it's not so easy. Little boats move a lot with respect to the water and lift from the hulls and sails come into play. My guess is the constant for TheMightyHobie18 is ~5.5. Which means at 25 kts, your boat is trying to plane.

I would be careful at speeds above 22 kts. On many cats, the stern trys to plane first and the bow digs in. With all the weight you have on the stern, I don't really know what will happen.

So what I am wondering is: If you have enough wind to potentially go over 25 knots on a beach cat, say a Tornado, but you are limited because of the boat's tendency to pitchpole at a certain point, can you break that speed barrier by putting a lot more weight on the back of the boat to keep the bows up? Obviously, two crew members on the back is not enough to do that.

Please... I'm no scientist or anything close, but it's not "Maximim" speed. It's hull speed or wave speed or some other term, but not MAXIMUM speed. Hull speed is not a synonym for maximum speed. Saying maximum speed just confuses these threads. Hull speed may be the max speed for the current configuration or the realistic max speed, but it's certainly far from the actual maximum speed.

That is all. Carry on <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I think (and thats the operative here!) that the answer is NO ONE KNOWS. Like Newtons theory, with poetic license, However fast a cat can go in theory, someone will have another theory as to why it won't. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Just My Own Theory,

Clayton

Thank you for clarifying that. I always thought that "hull speed" was a speed beyond which that hull could not possibly go, even with more power added.

Has anyone ever strapped a rocket onto a cat and checked that speed?

This guy is ready!!!!

Adding weight presents the problem of needing to add "horsepower" to get the additional weight up to speed.
Then of course you have the problem with the additional weight causing the boat to sit lower in the water so you have to add even more sail area.... its a viscious circle that results in something that looks a lot like....................... Playstation.

It seem like I heard that the initial design of Playstation had a tendancy to pitchople, so they added like 25 feet to the bows to add bouyancy. Soooooo, My guess is that rather than adding weight to the back of the boat the more effective solution is to add bouyancy to the front of the boat. which lengthens the waterline which changes the calculations of the "hull speed limit"
O Gawd, my head is beginning to hurt. <img src="http://www.catsailor.com/forums/images/graemlins/crazy.gif" alt="" />

Stephen

Well, I don't believe any of this. I am going to test my theory this summer on my Wave. I figure if I tie about 12 gallon jugs of water to my aft beam, I should be able to radically increase my speed on the beam reaches and deep reaches in heavy air. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I sure can't eat enough to gain 100 pounds myself. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Fill them with Beer, Bourbon, Whisky Scotch, Pina Colada Mix, Marguerita Mix and Rum and Tequila and you will be the most popular girl on the race course. For a whole new reason besided your stunning good looks and your sterling personality. Heck, I'd probably follow you around, even if you Don't laugh at my jokes.

Speaking of Jokes......

An older man approached an attractive younger woman at a
shopping mall.

"Excuse me; I can't seem to find my wife. Can you talk to me for a couple of minutes?"

The woman, feeling a bit of compassion for the old fellow, said, "Of course, sir. Do you know where your wife might be?"

"I have no idea, but every time I talk to a woman with a chest like yours, she seems to appear out of nowhere."

I have devised a special diet to help with your goals… 1 pizza for breakfast, 1 pizza for lunch and an unsensible meal for dinner (like 12 bigmac’s, or 12 whoopers)

Thanks for the advice.
Breakfast and lunch sound okay, but I don't think I have ever been able to eat more than five Big Macs at one meal.

LOL!!!!

what's a "whooper" is that like a yooper from Michigan's Upper Peninsula?...or is it a whopper from BK? <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

Wouter, this is VERY specially for you.

The boat is only sailing at about 12/15 knots in these images. So no, it is not throwing spray out the side. It did on occasion on that day but no photo. I include a photo of my Nacra sailing in less wind and going slower as you can see by my being inboard. Check the lee hulls of the two boats.
I have been checking my theories out with a proven boat designer and Olympic sailor. Oddly enough I take his opinion far more seriously than yours.

I make this pledge to you.

This boat will travel comfortably at 20 knots and will be tracked at 25knots over a measured distance or I will crash and burn trying.

I do not care what you think about what is erroneous and what is not.
There is a point at which all of your computer "virtual" world becomes nothing more than the twittering of a "knowitall".
I have made my own less than scientific observations and will put my actions where my mouth is.
You however will wait and watch for the opportunity to pull someone down.

This is one of those impossible to kill falsehoods that is debunked about once of twice a year. We've been over this topic about 10 times the last 7 years so do a search and learn what is really true.

Don't believe 80% of the sailing books out there. They all got it all wrong.

Wouter

You could be talking to Glenn Ashby for all I care. That boat is not planing in those pictures in the scientific sense and that is final.

And I don't care what you think of me.

And why does everybody convince himself that I'm a computer sailor ? Hell, I have 2 sailboats (1 homemade), 6 landyachts (partly homemade), 1 kite buggy, a quiver of stunt kites (home-made) and 1 surfboard. And I've been sailing since I'm 12 and have worked as a sailing instructor for 4 years. Even got a engineering grade in this field.

If you don't like me, then that is fine, but don't convinced yourself that I'm some knucklehead who doesn't know what he is talking about. You only make a fool of yourself.

Wouter

to delay pitchpoling at high speeds, (where the sail is trying to go faster than the hulls and the cat trips over the bows), you could add weight to the rear of the hulls

BUT, because of the problems mentioned already it would be better to simply cant the rig back as speeds increase, thereby increasing the distance between the bows and the sail's center of effort

which is exactly what windsurfers do already...

Ain't it the truth. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

THE ANSWER IS 42 (Not 42 knots or 42 MPH or 42 KMS just 42) everyone SHOULD know that!!! - it is the answer to everything - (watch out for the mice!)

Can't wait for your book, when do you plan to publish?

Can we have a catsailor poll/vote for the name?

We'll need potential titles for a poll/vote. I'll go first.

"Steve Fossett and Larry Ellison Can Eat My Shorts"

"The Wouter Identity"

"True Lies II"

"Born to be Riled"

"Wouter Wonka and the Reality Factory"

"Mien Truth"

I pick #1 <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

In my case, the answer is not very damn fast until it warms up.

AERO-HYDRODYNAMICS OF SAILING, C.A. Marchaj;
SEAWORTHINESS: THE FORGOTTEN FACTOR, C.A. Marchaj;

HIGH PERFORMANCE SAILING, Frank Bethwaite

www.arvelgentry.com Arvel Gentry's website (Marchaj references him.)

Once you understand the principles, you can work it out for yourself.

Ah yes.... But the fun really starts once you MISunderstand the principles and work it out for everyone else <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

The head over heels moment is a product of the height of the center of effort(pushing point) above the drag (of the hulls) of the sails and the force of the sails. As such, the position of the sails fore and aft is irrelevant.
Windsurfers tilt the rig for various other reasons:
Their arms arn't long enough (the can only get their weight aft by pulling the sail with them)
They would rather have lift than de-power, so canting it over makes sense.
Canting it over lowers the center of effort

Wel let's see... Last year in the Hogs Breath over a measured 82 actual miles on long running tacks before jibeing to short running tacks we had a Marstrom 20 complete the course in 4 hrs 38 Mins and 6 seconds. Add 15%, my guess would be more, to the distance for the jibes and we see it traveled 94 miles in 278 minutes and 6 seonds. That means the average speed was over 20 miles per hour. This year if the forecast holds, we will see similar conditions with the winds at 13 - 14 mph out of the east. I will look for the first boat on Smathers Beach, Key West, around 3PM. If you do not like the 15% choose your own, In any way it is fast. Without the fudge factor, it means over 17 mph, average, for the same trip. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" /> Some say they saw over 25 at times on the GPS.
Have fun with this, The Nacras will be close by and maybe ahead.

The heavy-ish mast I have raked back sits back past the rear beam at the tip. This does provide a little lift and a lower centre of effort which is not so irrelevant but it pitches weight back meaning I have to get out less to provide righting moment and so makes the boat more stable at speed.
Cutting the rig down by 5 feet ( much lower center of effort) also means I have to drag far less useless metal and rag through the air as if you check the boats such as Tornadoes trying for these speeds it is clear they have far too much power and are too busy dumping in gusts to settle to boat properly.

Darryl,
I saw this thread once and concluded SEP.

How could you see the thread and recognize it as such then? The math used here is very powerful though.

If you needed more weight while getting up to speed you could have a scoop to fill a bladder on the tramp just by putting a pipe facing forward which would force water into the bladder then you could let it out when you slowed down. You'd need good baffles in the bladder or the cat would roll over when you turned.
regards

Warbird: Do you have any GPS tracks from any of these 20-25kts sessions?
I am really interested in how those runs where built up because 25kts is a lot of speed (especially without a spi),
I have done 28kts on my windsurfer and cant image doing that on a "simple" beachcat.


(btw. I found 1 of your bookname suggestions a little disturbing, and its spelled "Mein" not "Mien". http://www.learntospell.com

Those books and articles are part of the 20% that got it (mostly) right.


Wouter

Forget about adding weight to the rear of the boats or any piping boys. Just add T-foils to your rudders and give then a downward angle of attack and watch those baby's provide many kg of downforce much like a F1 race car when the boat speed picks up.

But what the hell do I know right ?

Wouter

The boat has only been out twice since I changed the rig etc for speed runs.
It has not been out in more than 15 knot winds and has not travelled faster than that in its new guise.
I have sailed the boat at twenty with its high rig and they are known as an extremely fast reaching cat. Well faster than an H 16 for example.
The spinny is not my option at these speeds nor is it needed. There is heaps of power enough in 25/30 knot winds in this rig and indeed it may not need the jib.
I am happy with the results so far and so I have just ordered new sails for it from Whirlwind and as I set the boat up and run it I will prvide GPS tracks etc. The 25 knots over a measured distance is my aim. It has not been achieved so I may well fall flat on my face with these ideas.

But, obvously, I do not think so.

I understand that cats are racing up and down wind courses with no reaching.
Something about reaching being a parade.
Perhaps if we got them to go fast enough on a reach it might be worth it again.

Nice rig change on the PT. PTs go like stink off the breeze when it is fresh, especially for a 14 footer. I would watch those loads on that tiny front beam though.

Build a 18 or 20 foot version with 10 foot beams (cut and shut 2 old PTs for some fun even), reduce the rig size and dont be affraid to run it flat and I am sure you will get some impresive speed out of it.

As for no kite, a cat will sail the quickest on a 2 sail reach without kite due to less drag. Kite does not make you go faster..... Just increases VMG down wind.

Your eyes deceive you. That'a a Tiger Shark and it's already 18'.

Going faster than hull speed without planing is called "forced mode" and cats are doing that any time they have their transoms depressed but not submerged, indicating that they are making a longer hole in the water than their hull length, and they are sailing 'up' out of this hole. Conventional lead mines don't have the power:weight to start climbing out of the hole the way cats do, and 'hull speed' is the point at which one enters forced mode.

--Glenn

Spot on Glenn !

Wouter

As stated this is a foam core 18' version of the Pater Tiger and it is already a fast boat...I am not widening the boat as this will induce pitch not lessen it.
It would give me more righting moment with two "weights" strung out the side but this would push the lee hull down into the water more creating drag and the whole idea is to eliminate as much drag as possible by having the boat plane rather than be 'force moded".
The boat feels like a far less stressed platform than say the Taipan or the Nacra and that has a direct affect on my attitude in that it "feels" a lot safer at speed.

Who says that the drag associated to "forced mode displacement" is higher then the drag incurred by planing ?

Or if you will, are Tornado catamarans faster or slower then 18 foot skiffs while carrying less sail area ?

Without some dependable data we're all (me incl.) just guessing here and that isn't science.

Wouter

I am going to go out on a limb here Wouter and say that a hull of equal dimension, shape etc being forced through water has more drag than that same hull planing over the top of the water.

That is why that hull traveling through water needs more power to make it travel....hence the word "force" in the description "force moded".

In regards to the T traveling faster than the skiff it may be that the huge rig and wings and extra crew etc of the skiff create MORE DRAG via windage than the T.

Really, is that why we always rake the I20 crazy back for distance races and when we thing we're going to have big weather? Hu?... I never knew

[sarcasm off]

moving the weight and the CE aft helps survivability in big air.

Sorry Warbird but subjectively "observed reasoning" without empirical testing will invariably lead a person to incorrect conclusions, particularly when applying “low” speed hydrodynamics with aerodynamics interacting on an object at the boundary layer between the two – it is just not as simple as it “may” seem without applying the correct maths AND actual objective testing any/all theoretical results.
(Slightly off subject but question for Wouter – are you by any chance familiar with the “theory” and the maths that is being applied to research being conducted into “hyper-cavitation” for submerged “missiles” (formally called torpedoes) at present by the military?)

what about the 1 where the torpedo forces out steam? from it's skin so that it effectively eliminates almost all of the skin drag! kind of like air hockey i guess

cut from the bbc webpage
"The new missile, called Hoot, or Whale, could be deployed on Iranian ships in the oil-rich area, which is home to the US 5th Fleet.

No warship can escape from this missile because of its high speed

It is said to travel at 360km/h (233mph), three to four times faster than most conventional torpedos."

Don't be sorry Darryl, I understand what you are saying but we are not talking "missile science" here as I think (without checking google) the weapons you're are talking about are trying to create a "cushion" of air around the submerged projectile so the hydro drag does not exist and the missiles can travel mach 2 under water.
This science has been worked on by the Russians for two decades and they "may or may not" have it.
Like the scram jet engine it might finally come to fruition but not in an obvious way for the beach cat back yard.
So a truly erroneous avenue for what I am doing which is simply setting up a beach cat that planes easily and is predictable sailing above 20 knots.
In that area, as I suggest to Wouter I think a planing hull will go faster , easier than a submerged, wave piercing hull. I think that because I think the planing hull wil have less drag than the semi-submerged hull.
I am happy to be wrong about that and I will provide results here.

I spent a lot of time around science when working in the Auckland Museum. What I observed was that science bends and scientists are often have to back track. When they do they cover their butts by pretending they really knew what the problem was all along and just had to find some "empirical proof".
Science has its drawbacks as sometimes it is about what you don't know you don't know like Einstein not believing in the expanding universe theory and fighting until he died against it even the genius can be wrong.

You never got back to me about the foiled rudders by the way.

Not that it helps when you try to apply the results back to sailing, but since an engines thrust is likely below the center of drag you shouldn't be speed limited by the bows pitching down.

It would be better to have Jacob's Ladder problem, controlling lift.
http://www.cobrakite.com/jaclad.html

John

I'm aware of it and have read some articles on it but I'm not familiar enough with it to be knowledgeable about the math and physics applied.

But I always did feel that it bore strong resemblence with a ship hull design of the earlier 20th century. Here a normal but rather box like cargo ship hull had wide trenche all along its keelline that was filled with air and kept under pressure. This meant that a very large portion of the "wetted area" wias in fact between this aircushion and the water which lead to a lower drag overall. This idea was tested by never fully persued.

Wouter

Isn't that the same principle that makes a hovercraft and Erkanoplan work (although less extreme).

As an premise for further development and research that is a tenable position even though it is not the obvious one to take.





This is just nonsense. A planing hull also needs more power in order to travel faster, in fact you totally misapply the word power here. It is physically possible (and actually does happen in some cases) were the driving force descreases when travelling faster but the required power rises. This is because "POWER" is the product of "driving force" and "speed". If the speed increases more then driving force decreases then you get into this situation. Several dinghies that transition from displacement sailing to planing actually go through such a situation. So I propose that we'll only using the driving force and no longer the concept of power.

With respect to "forced mode" We must be careful to not read more into a given word then there is too it.




Or is may just be that long slender hulls on a lightweight platform in forced mode lead to lower drag at 20-30 knots speeds then a planing hull of similar length.


Wouter

this thread all seems a bit pointless ,

why not look at powerboat catamarans that allready exist and find the answer, feking fast !!!!, surely these big bouys and smaller ones can give an idea of what can be done if you strap enough hp onto two hulls ,(the thread started with engines) yon funny looking sailing cat above hulls dont look too unsimilar to those on the smaller powerboats .

its intresting to note that as far as i know all catamaran HP racing (motor )cats use planing hulls . and are probably more akin to low flying air craft using forced air under the bridge deck to provide lift .

Nigel Irens has developped high speed wave piercing muultihulls for long distance stuff (trying to keep up with orma 60s who are under sail ) haven't heard of any on smaller fomula racing cats though , would by struggle at corners <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />


PS I always find stuffing the luff exstension down the mast after hoisting effective and helps sail shape at the bottom of sail , though I don't think I'm alone in this tinking <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I don't think so and I'll explain why.

With the Hovercraft and Ekranoplan the air cushion has a higher pressure as the surrounding medium. As such it is more akin to planing. This earlier 20th century CARGO ship has a air pocket that was at the same pressure as its surroundings. Basically the first derive lift from a dynamically created difference in pressure above and below the craft while this cargo ship was still fully in displacement mode. In fact that cargo ship design would also work if the trench was fully closed off and there were no pumps. The pumps were only there to keep the trench filled when air is lost due to rolling of the ship and other leaks.

Basically the Hovercraft and Ekranoplan completely dependent on the aircushion in order to remain levitated while this cargo ship could even work when the trench was filled with water. In the latter case it would just have more hull drug. So with the cargoship it was just a means to lower skin friction drag and nothing more. And that is why it could carry lots of cargo.

But yes, there are also strong similarities, all four (incl. planing) aim to put as little surface area into contact with the much denser water as possible in order to lower overall drag.

Wouter

That is just the question isn't it.

Powerboat cats are all designed the plane

Sailboat cats (like ORMA 60's) are designed to sail in forced displacement mode.

Only exception the latter rule is the Yves Parlier "seaplane" catamaran "Médiatis Région Aquitaine"

This cat has been pushed hard for 4 years now and not proven to be a winner.

Also of interest is this little bit of info as contained in a press release :




Source : http://www.ybw.com/auto/newsdesk/20040104160707ywnews.html

So indeed it is not a black and white situation. With each design you need to look at the specs and operating point to determine which of the two (or three) paths available is the best. For racing sail powered catamaran the paths most favoured are forced mode displacement and indeed foiling (L'hydropthere, longshot, Techiques Avancee).

A while ago I had discussed planing hulls with some A-cat designer and he replied that they had tried during the 80's to make it work, but failed to beat the normal hull designs. One issue was to keep the hull planing smoothly. Typically the boat would drop off the plane when heeling or pitching due to a gust. This could well be one of the more important reasons why powerboats are all planing designs were sailing cats aren't. It is easy to keep a powerboat "flat" or tilted at an optimal angle.

Wouter

An interesting thing about our Hobie 18 powerboat is that because it is NOT a planing boat, it is able to accelerate to its top speed very rapidly. Makes it better for pulling water skiers, for instance, because you don't have to wait for the boat to get on a plane. (Obviously, the ultimate top speed will not be as high as for a planing boat with the same amount of power.)

Maybe the same thing is true of planing multihull sailboats that it takes more power (and wind) to get them up on a plane in the first place.

Like Richard Roake's Hardcore 16, which seemed to be a dog in light air (compared to displacement cats), but very fast when the wind picked up and it could get up on a plane. Sort of like a sailboard, I guess.

Has anyone ever tried injecting air around the hull(s) of a catamaran when approaching top speed to create cavitation? What would be the probable result?

Power based boats are generally using the tunnel to create lift and less friction. Again, I think this is not realistic regarding sailing beach cats.

Well this is exactly what I am interested in Mary.
My experiment is in the size of the boat to the crew weight also.
My boat is almost as fast as the full rigged two up boat in light so is not exactly a dog but it is aimed at heavy air.
Are there any photos of the Hurricane as I can find none on google.

Are you being serious?, That is a phenomenal boat. It is the fastest catamaran ever sailed solo in 24 hours, no other solo catamaran even comes close to that boat.

Gareth

I think this is what some animals do to allow them to swim faster, a while ago I saw a piece on Discovery about a Blue Marlin who ejected some type of oily fluid to lessen waterresistance.
Also there is a powerboat that is based on dolphin skin.
More info here: http://www.physorg.com/news68812337.html

A while ago I had discussed planing hulls with some A-cat designer and he replied that they had tried during the 80's to make it work, but failed to beat the normal hull designs. One issue was to keep the hull planing smoothly. Typically the boat would drop off the plane when heeling or pitching due to a gust. This could well be one of the more important reasons why powerboats are all planing designs were sailing cats aren't. It is easy to keep a powerboat "flat" or tilted at an optimal angle.

Wouter [/quote]

I have certainly experienced this affect. I have presumed it was because I have cut the rig down to almost a masthead and I was overreacting to hull lift. It has helped to make much smaller adjustments in mainsheet. But when it does drop it feels like time has stopped.
Ideally I am trying to keep both hulls in the water and this is a learning curve also as I only want outright speed and am spending no time on general performance. The boat is so far much easier to keep on an even keel than say my Hydra which has less sail area and is heavier.

Warbird: From where are you doing your speed sessions?
I have spent some time in the BoI (Opua/Pahia area) but dont remember are places with flat enough water or high enough winds (then again, it was a few years ago).

I forgot another exception. Yellow pages and Macquary Innovation. These are planing contraptions for an all out speed record. But they are very different to a normal catamaran and mostly in the sense that they are very wide and don't have long hulls at all. Yellow pages and the other have three hulls which are all pretty small and the leeward ones are in line with a significant gap of water between them. These will remain "flat" very well due to this overall configuration, but makes it very different to a normal cat like the Tiger Shark.

Wouter

We must not forgot another things as well. The way a hull rides the waves and chop. Typically wind powered craft need alot of wind to propel them to high speeds and such conditions have a very strong correlation to a pretty uneven watersurface.

It would not be the first time that a sailing catamaran outpaces a powerboat in a blow because planing in such conditions is very violant. This also leads to the question of how the rig will behaved when it is shaken about very violantly because of the planing surfaces trying to follow confused seas.

Actually I have this experience with landyachts. Sometimes the beach is not flat but has small wave sand structure. It is very hard to keep the sail drawing in such conditions. Even so much that it pays alot to just take a much longer route around such an area, simply because of the ability to reach much higher speeds when the craft and rig operating in a smooth manner due to traveling on a smooth surface.

Of course waterskiing stops with even a very little wave like disturbance of the water surface.

Wouter

And still, it doesn't hold the outright record for 24 hour solo run.

Actually there are 3 forced mode displacement boats and skippers that in front of it in the record listing. One of these being a 60ft tri. Note how "Médiatis Région Aquitaine" is a 60 ft cat build to the same rules as the 60 foot tris.

Basically, you statement is only "true" because you choose to include the condition that any challenger must be a "solo catamaran". If you had used ""solo multihull" then your statement would not even have been in the least bit "truthful".

Situation is very much the same for fully crewed 24 hour records even in the 60ft or shorter class.

Nice try, but definately a smoke and mirrors kind of counter argument.


Wouter

If you meant to refer to the Hardcore, I cannot find any pictures of it, either.

The only time we ever saw it was when I think they had two of them at a Red Lobster Regatta in Sanford, FL back in the late 1980's or early 1990's(?) Excellent sailors were sailing both of them (one an Australian, and I can't remember his name, I think maybe it was Brett Dryland?)

Anyway, they did not do well, and after that debut, nothing was heard about them. The Hardcore is still listed in the Portsmouth ratings for old boats, but it obviously had a provisional rating, since it never raced enough to get any good data.

Randy Smyth won the Worrell 1000 one year on another Roake planing design, but he said it was a pretty rough ride. Rick saw him from shore and said the boat was skipping across the waves like a sailboard, but perfectly level, while the other boats were going up and down over the waves.

Warbird,

Overall width may really help you here also.

It could be an idea to just remove the trapeze and make the boat wider and only hike off it.

If you are really after planing then have the luff hull take part of the planing. This will make the platform very rigid in its side to side attitude and allowing you to keep optimal planing angle better.

Again this assumes that you can get proper planing to work at all.

Wouter

after almost 6 months of winter snow out at the lake this week i finally got my nacra 5.2 back in the (cold) water, surrounding volcanoes still covered in snow

today had the most breeze and as i had the gps on board i tried to better my previous top speed of 16knots, set the end of last year

still a noob and without a proper trap harness i've been making do with an old windsurfer harness and just sliding my butt off the side of the hull. all winter i've been reading "catamaran racing for the 90's" so was sitting just behind the dagger to reduce transom drag

winds picked up, aimed for the reach and off we went, played the main to keep the windward hull just kissing, wind blew a little more, speed picked up another level, leeward hull starts kicking up big spray and suddenly the whole platform starts tipping forward! leeward nose almost completely under and i slide back to the rear beam as fast as i can

everything pretty stable in that position and a couple of minutes we are across the lake and out of the main breeze

gps says top speed 15.5knots and later trackpoint data downloaded to show the points either side

23/04/2008 12:46:07 0.2 nm 0:00:41 14 kt 98° true
23/04/2008 12:46:48 0.1 nm 0:00:29 15 kt 109° true
23/04/2008 12:47:17 0.2 nm 0:01:01 13 kt 113° true

looks like it isn't going to be easy for me to get much quicker. going to need a proper trap harness, foot straps, nonslip tape, more time on the water and slightly bigger balls;o)

another plug for GPSaction replay, very cool cranking the speed up and watching the boat zip around the track with each section colored according to speed, pic attached

I hadn't heard of the Hardcore before and also haven't found anything on google. If RetiredGeek is around maybe he can share some history.

However while poking around on google, I did find the following, which might also be of interest (as a historical note).

First of all, my earlier post was more or less a brief summary of a few pages in my Textbook about Hydrodynamics on foils and ships (I'm a student at the Department of Marine Technology here in Norway, civ.ing degree), so that text is not false.

Erice, I wouldn't be too suprised of your top speed not being higher. Or, put in other words, 16+ knots on a Nacra 5.2 is fast! My dad and I sail a Taipan 5.7, which really is a fast cat (rated faster than F18s for instance), and we have never logged a better top speed than 19.7 knots. We are both certain that that particular blast isn't our fastest ever, so we consider our top speed as being just above 20 knots. This was done when a big puff(I'm guessing around 10 m/s) hit the boat, me out on the trap, dad inside on the helm, no spi up. No doubt, a better and more experienced crew could probably yank a couple of knots more out of the boat, but not a whole lot.

As for the 18ft skiffs, according to Frank Beathwait those monsters topped out at more than 30 knots in big winds and sailing with small rigs. There's a spectacular picture of a skiff screaming along in crazy conditions in his book. Around a course a Tornado would outsail them, as for sheer top speed however, I think the skiffs are way faster.

I assume you mean the Hurricane 6.5? What a monster!

I've seen them, sailed around with them, but never sailed one!

A quick google for "Hurricane 6.5 catamaran" came up with:

this

We even have a gallery on Catsailor here

This one is Reg White helming the rack boat:



This one is Rob White helming the ladder boat:



Here is a pic of a yellow one:



Some old stuff in german I think

I think he was referring to the Hardcore Hurricane, which was apparently a 16' boat (listed here), but there may be a connection to the Hurricane 6.5... Here's an article that indicates that the Hurricane 6.5 was aka the Roake 21 (and links to those photos here at catsailor). I guess this is the one Mary was referring to.

Edit: Indeed, one of the other images confirms this connection...

http://www.catsailor.com/gallery/displayimage.php?album=81&pos=8

Wouter,

My argument is not "smoke and mirrors", I am merely pointing out that "Mediatis-Region Aquitane" is a very succesful boat, contrary to what you seem to be trying to indicate in your post.

Regardless of the fact that there are three trimarans that have since beaten Yves Parlier's 24 hr solo record, I still can't see how you can maintain that this boat is not a winner, unless you are so desperate to never admit that you have made a stupid statement.

For people who are not familiar with this 60ft catamaran and who may take Wouter's statement at face value:-

As far as the 24 hr record is concerned, the boat is the fastest solo catmaran ever, it is the fastest fully crewed catamaran under 100ft and has proved itself to be up there with the best of the trimarans regardless of size. I will leave it to each individual to decide wether that makes it a "winner" or not.

Gareth

Ok, much more important!!!

What... is the Airspeed Velocity of an Unladen Swallow?

Well the Laden Swallow airspeed can be calculated by using the pi r round theory, thus the Unladen Swallow is calculated by subtracting the square root of the volume of recently ingested food times 1.23174 divided by its inverse factor to get EXACTLY what we think it will acheive (optimally). <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Of course this assumes the bird hasn't had sex in a while which could make it much faster given its looking for a mate!

JMO,

Clayton
BS Engineer (and we know what the BS stands for)

African or European?

Is that one of those "Like you or love you....swallow vs spit" arguements?
Airspeed of her spit of an unladen swallow? Sorry for the triple XXX but Andrew brought it up. <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

Dave, swallow (the bird, not the act of)



and now for something completely different

and now for a different, something completely different

The Hardcore 16 was the Hurricane 500, but I'm not sure if it received the hull(skeg enlargement and reduced rocker) and rig changes of the 500. Great boat but too much volume low down for singlehanding in light winds. It was not a 'dog' as often quoted but did not have enough 'Bite' in the water without either a crew of two to load up the leeward hull or enough wind to generate lift from the skegs . Using a hurricane sport jib I had to slow down once to let the safety boat catch up. I had previously clocked the safety boat at 25.8 mph on gps and my hurricane 5.9 at 22.9 mph around the bouys. Pity it was not built with daggers or given a proper rig. John Pierce apparently tried to buy the moulds, but white formula wouldn't sell. So JP made the stealth.

Cheshirecatman

We must be talking about different boats entirely.

Now you are putting up a characture of my post that you can then knock down.

I never wrote that "Mediatis-Region Aquitane" was a failure. I wrote that it wasn't a winner and it is exactly that. In direct comparison with other design in its class, the forced mode displacement trimarans of the ORMA 60 rule it failed to show a clear advantage. And it is currently second in its 24 hour speed record (in all categories) to such a forced mode displacement tri.

I refer to this example as to show that by simply going to a planing design one must not expect to automatically have superior speed. There are many counter examples that argue against such a oversimplification. This includes "Mediatis-Region Aquitane".

Sory mate, you have to read more carefully.

As do we as your argument totally dependent on the fact that no other (singlehanded) true racing catamaran has been build in the 50-100ft length range. By being the only one in this length range it is indeed the fastest of IS KIND. Big deal. Of course you top the range off at 100 ft or else you have to included craft like Orange 2. That is the smoke and mirrors part my friend. Afterall we all know that the Hobie 16 is the fastest boardsless, spinnakerless and strict One Design beachcat between 16 and 16.5 foot length.

So what are trying to do here ? Taking a piss at my personally and confusing everybody else.

Wouter

I think this thread needs some Doug Lord.

Is forced mode faster than fifth mode?

Apart from the personal attacks, I think this thread has some technical merit.

I'm no expert, but doesn't a trimaran mostly operate with a planing center hull and a forced displacement outrigger at the same time? If so, that would seem to complicate matters...

I must say, this is all quite mind-boggling... 42 is looking better and better all the time, I need to enlarge my hatches so I can take my towel along when sailing. One never can tell...

I have really appreciated any sensible criticism or feedback as I have discovered several threads of thought that will help, a man who reports he was happy sailing a beach cat at 25 knots, the fact that very credible modern designs do not and while my efforts might not mean much they are efforts I appreciate help with.
I want to be clear I am not trying to create a buoy racing cat. I just want to successfully, without huge changes make my old hard chined cat do 25 knots.
I have however had interesting side results such as not losing much overall speed in any area and that the boat is well faster and much easier to helm solo than My old Nacra 14sq. So much so I sold the Nacra as it was not worth pulling the sail up on what with the TS being lighter on the beach as well.

As to where I sail in the Bay. Anywhere but mostly Te Rawhiti way.
You must have seen the Te Puna inlet which is a fabulous low chop high wind area.

By the way, to the swallow guys as the bird which flew out in front of my car yesterday, taking up residence in the grill, forced mode can reach 75 miles an hour while passing.

was he african or european?

Forget all of this theoretical BS that only about 1/4 of us can understand or care to understand. Let's see some GPS screen shots at speed. Describe the boat and conditions. Step out of the theoretical and into the real world.

Lat and Long would equal off the coast of Yemen, pretty tough shore break

How right you are! Right in the Gulf of Arden (I think thats right...)

Clayton

I don't think so. Reg increased the skeg size from the prototype (see attached) and lowered the transom 1" to remove rocker from the planing section. Modifications were also made to the rear beam pylons and front beam. I think the annapolis tooling was given these modifications as it delayed UK production. The beam sheeted jib was changed to a much larger one that sheeted off the trampoline and the mainsail gained an extra 4" on the leach.

Cheshirecatman

warbird

some comments on your quest and zoomed pic attached

those pics of the tigershark are very interesting to a noob on many levels

some questions

1. if the TS is a scaled up PT are the beams and dolphin striker any beefier?
you mention that it is a lot lighter than your old nacra14sq. as far as i can work out the nacra 14sq is a shortened nacra16, which was a shortened nacra 5.2. which was already over engineered. so i imagine the nacra 14sq has a very solid/heavy beam and dolphin striker setup. way heavier than needed for the reduced hull volume?

the PT was almost the A class of it's day, so very lightly built. if the TS is uses the same beam and dolphin striker arrangement is it going to be strong enough to withstand the forces generated by those longer hulls at 25knot? not really an issue i guess as this is really only a 1 shot project

2. noob question here, noticed the boom has 3 hangers for the main block. seen similar on boomless mains but not boomed. when would you use the other hangers?

3. guess you'll put the lower batten in on your new sail?

4. does your TS have nacra rudders?

typos are a fact of life with keyboards and most any english speaker can read through them, but number typos are very hard for noob's

"The boat sailed faster than my Nacra 12sq....
The TS was designed on the Paper Tiger basic platform and stretched to be a 30 foot twin trap rig."

I took the second number there as a reference to the mast length, although my recollection (can't say I ever actually measured mine) is that the TS mast was 28'. Still, in the ballpark of what warbird quotes.

Little late to stick this in, but Yellow Pages has tiny hydrofoils, so not purely a planing boat. But please, no Lording it around here, plenty of that at SA

Yes, the second number was mast length and the boats are 18'. My first TS was 28 feet but this was is 9.2. The rig seems to have been made bigger as several I have heard of have the bigger rig. Some money had been spent on the up-date.
Mine came with two sails, the larger and the smaller. The mast had a section added to the top and was sleeved.
I cut the small sail down to square top and less foot and raked the mast and shortened a spare mast I had.
The boat seems to sail very well. Is well faster than my friends Hydra even with the cut down and rake.
I am pleased enough to have ordered two new sails from Whirlwind with a roller furler jib off a foil and sheeting back to the beam instead of the tramp.

Yes, Nacra 14 sq
But 30 foot is real.

The nacra is well over engineered and short in the bow. Far heavier to move around the beach but I do not know how heavy. Not as heavy as a Hydra.

But the 14sq was a great boat out at sea and very right-able and trustworthy in what is often an area where you have to look after yourself out there.

Yes, the Tiger Shark does have much bigger beams but it is just stretched as the freeboard is no great amount bigger. The hulls are well wider though. Also, it has been engineered for two big Kiwi lads and 30 knot winds so I do not expect it to fall apart.

No, they are the original TS rudders which seemed to end up pretty well generic.
Note the dagger boards which are cut away and very comfortable around the deck.

I will take Chips advice on the lower batten but I do like the lower batten on the Hydra as it makes shaping down wind a dream.

The three yokes on the boom are for separate pulleys.

Yes, I think it is all about actual results and not so much theory. But I have to get my act together in a sequence that does not leave me broken, wet and by myself.
Soon I will care about nothing but GPS and speed results.

Because that is all I actually care about. some speed in my life.

photo shows the boat to be stretched rather than simply bloated if you check against a PT.
The mast is now raked a little further and that is a fully battened Buffalo sail I thought I would try

Speaking of Paper Tigers (the 14’ variety), I think that everyone here realises that one of the most critical components governing the maximum or potential “speed” of any sailing craft, is weight, and to give an “anecdotal” example of just what kind of effect weight can make. In 1980 I was patrolling the course around the buoys of a local race at a local club in a powerboat in conditions, which had become marginal for racing in. The wind had picked up from approx’ 15 to 18 knots to a fairly constant 25 to 28 knots. A lot of cats and dinghies had already headed back to the beach but the fleet of paper tigers (about 15 of them) had continued to race. I was tracking behind the tail enders of these tigers when as one of them tacked the skipper slid across the tramp and straight into the water. I assisted and successfully had him aboard and we turned to take him back to his cat. Now when he tacked he had inadvertently cleated on his mainsheet with the sail set, as it would be for a loose reach. Well with the 150hp outboard flat out and a speed of around 40mph we never could catch his cat. It just took off, sailed flat on both hulls and outpaced us until we were forced to turn back after about 10 miles of trying to overhaul it. The owner actually got his PT back about a week later as it had been found by a fisherman on the other side of the gulf about 90 miles away from where we last saw it. Apart from a few scratches where it had apparently run up the beach and tipped over onto its side it was fine and back sailing a couple of weeks later. I have no idea just how fast that cat sailed but it was certainly going faster than us and certainly MUCH faster than it could possibly go with a sailer onboard.

[quote[
Little late to stick this in, but Yellow Pages has tiny hydrofoils, so not purely a planing boat. But please, no Lording it around here, plenty of that at SA
[/quote]

Aren't those cavitation/venting gates ?

Wouter

I have performed towing tests with some of my hulls, to correlate some drag predictions with real measurements, and have found that when there was only the weight of the hulls, ie. not ballasted in any way even round keels (U shaped as opposed to flat sterns like a paper tiger) will plane at relatively low speeds (about 8-10mph).

Gareth

So I guess this means if we really want our boats to go fast, we should stay on shore and operate them by remote control? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

oh, and i forgot my favorite sign of "you are approaching max speed" sign

around 13knots the whole rig starts singing a mournful siren's song, have generally been a little too busy on position, sheet and rudder to spare it much time, maybe later

the big orma tris must really be humming

"Erice, I wouldn't be too suprised of your top speed not being higher. Or, put in other words, 16+ knots on a Nacra 5.2 is fast! My dad and I sail a Taipan 5.7, which really is a fast cat (rated faster than F18s for instance), and we have never logged a better top speed than 19.7 knots."

I find this quite surprising, as I believe 16knots is really easy for a spinnaker-equipped beachcat in the right conditions. I don`t know the Nacra 5.2, but I believe the Taipan 5.7 should do 20knots easily in 15-18knots of wind with a spinnaker up. In defence of my theory, I have sailed my Mosquito 16ft with spinnaker at 16.4knots on GPS in 12knots of wind with crew to leeward, so I think 18knots should be acchievable on a 4,9metre cat, a 5,2m cat should go a bit faster than that, and as the cat gets bigger, it should get faster, if Playstation is anything to go by.
What I do know from windsurfing with a GPS though, is that when you approach the speed at which your craft is "maxed out" you will have to work extremely hard and possibly do a lot of modifications to extract another knot out of it. I sat on 27knots as my ultimate top speed with the board I had for two years, and could reach it almost every time I sailed, but just couldn`t beat 27.4knots, in fact I did this exact speed so consistently that I was beginning to think my GPS had a fault. I then got a faster board and broke 30knots the first time I sailed it, now I`m reaching 32 on a regular basis, but once again I am approaching the limit of my equipment.
What I also know about the sensation of sailing at 25knots is that it is going to be extremely difficult to acchieve this on any beachcat design, regardless of any marketing brochure claims. I wish Warbird lots of luck and success with his efforts though, but I think some design modifications might be necessary to get there, and possibly some large ones. I would look at T-foil rudders, and possibly adding bruce-foil type daggerboards in front of the main beam to assist in anti-dive. These could even be laminated onto the outer skin of the hull on the chined surface, but the internal hull framing would have to be beefed up to support them. The biggest modification I would look at though is to mount the rig on the leeward hull, this would get the boat sailing flatter as warbird suggests, and get his weight and the weight of the weather hull further from the rig. He could then also experiment with canting it to weather, in much the same way as sailrocket is doing (www.sailrocket.com) Perhaps the scientists on this site will have a good laugh, but I think that would get more speed out of the boat. Of course that might also fall outside of what warbird is trying to acchieve, as it won`t be a general use sailing craft anymore, in that you would battle to sail it back upwind to the start of a speed course. Either way, I`ll be watching the progress of this rather interesting project, so please keep posting your findings and results.

I just saw them described as foils in a couple of different articles, so no first hand knowledge...

"Yellow Pages Endeavour (YPE) is a Lindsay Cunningham tri-pod design. YPE could loosely be described as a proa, because it sails only on starboard tack.It rides on three hulls or pods located at the ends of three wing-like arms. The vertical wing mast is mounted on the hub where the arms meet, and is stayed by wires. The craft has eight or nine foils under the pods - three on the forward pod, three on the aft, and two or three on the crew pod. The three forward foils rotate to steer the craft."

The picture I saw showed small foils, like a T foil but oval shaped...

FYI neither the Nacra 5.2 nor the Taipan are originally equipped with a spinnaker.

Or skip remote control all together:
http://www.hackaday.com/2008/04/20/autonomous-catamaran/

I just came in from sailing my Blade with my Garmin ETrex Legend taped to the boom. I had it set for MPH, I didn't realize until after, but it showed 19.3 mph (anyone know what that is in Knots?) was my max speed. I was alone and running deep downwind in some 20 knot gusts with spinnaker pulling bows up, sitting on the tramp running on both hulls, not out on the wire on one hull. I think it might have been trying to plane. I think I have gone faster on the boat but that was all the wind I had to work with.

19.3mph = 16.77knots

Thanks Karl. I think I've gone faster but I usually don't bring the GPS unless I need it for a distance race.

Isn't it funny how SLOW 20 mph is when you are in your car crawling through a school zone and how FAST 20 mph is on a sailboat? <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

Yeah, in a car 20 mph is torture! But, if you were to grab hold of a car from the outside, and try to RUN with it at 20 mph, you wouldn't make it to far before you were being dragged! <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

I can't imagine what it's like on one of the big cats (120') going 40 knots down an 80 foot wave face in the southern ocean!! Must be a real wild ride.

to convert between knots/mph/kph with a gps is very quick. and all stored data will also switch to the new units

on an etrex go to main menu/setup/units and then change to whatever units you want to display

on your computer if you are using garmin's map management software, mapsource, you can do the same to tracks saved years ago by going to edit/preferences/units and doing the same

guess i'm a geek but i try to take my gps along on every sail. the track files are tiny and download to the computer easily.

even tracks made years ago can now be opened in the latest version of mapsource through google earth. view/ view google earth

and if converted to .gpx files, (open in mapsource and resave as .gpx) the free version of GPSaction replay will display the track files as a color coded series of speed legs with an animated triangle racing around the course

the track i made when first seeing how high the hull would fly is quite instructional to watch animated as you see the boat speed along on a reach and then start slowing without changing angle much as the hull gets higher and higher and the rig dumps air. then when it has almost stopped you see the boat do the final little twist to windward to dump the hull back in the water

when my little lake community starts it's sailing races in the summer i will put my old etrex onto a friends laser and the new one on the nacra and we will get a better idea of how our boats sail the same course in the same winds

cheap gps and free software, best $100 to spend on your boat for safety, entertainment, racing and boat understanding

Why don't I just send you my ETrex and you can dowload it all and send it back along with an eamil of my track... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I'm pretty sure a little bit of poop comes out the first time you are placed in that scenario. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I was out on this FULLY CARBON 56 foot beast the other day and cruising at 16/17 knots through the islands. Maybe a hundred meters off one I sail a lot.
It was smooth and just felt like rolling along at a good clip.
I went to the side of the boat and just tried to gauge the speed if I was on one of my beach cats. I was happy that I often hit that speed but that it feels WAY faster low down and wet. However the island slips by at the same speed.
I had also sailed an older Alan Write 42 at 16 knots but that felt stressed and fully extended in 40 knots of gusts. Isis was in 12/16 knots that day and was almost always traveling faster than the wind.
One can only imagine what 40 knots feels like in those crazy cats and tris.
Unstoppable, scary and wonderful come to mind.

I love the mast set up on Isis.
It is a carbon, rotating wing section.
No spreaders.
The lowers are set lose but as the mast rotates the windward mast connection moves further way from the chain pate and automatically tensions the lower mast. The connection at mast also rotates to lower stress at that point.

if you've got the old monochrome legend you need the "special" garmin cable to download through a serial port:o(

the newer garmins use a standard mini-usb cable as found on many digital cameras, mp3 players, card readers etc.

but as long as you have some kind of cable for it all you need is free software like easygps

http://www.easygps.com/download.asp

to download it's data on to your computer and then you can send it to anyone as an email attachment if you want

pm me if interested and i'll see what i can do;o)

to download the data from it

Thanks, I installed it on my laptop then afterwards found I don't have a port to connect the gps! I will try it on my desk top later...

this is a video sailing 19kts on a nacra 5.8

don't remember wind speed, but the ocean was very flat that day

bernd

http://www.youtube.com/watch?v=B0U5T9W_pKI

here are the stats from that day

sorry here it is

bernd

good deal on a gps
http://tinyurl.com/6offwk

19.8 knots over 1/2 mile during Miami Key Largo
very gratifying <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Was that on the Tornado Classic?
I just came in from another run with my gps, got it set for knots now, with not as much wind as yesterday, got it up to 17.2 kts. under spinnaker on one good run. Need more wind! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

The definition of 'planing' is controversible. Either:
1. A boat travelling faster than its stern waves,
2. Lifted by dynamic forces that are larger than static forces (bouyancy),
3. Lifted to the water surface by dynamic forces.

A displacement boat with a vertical sharply cut stern may allow the water to detach from the stern and "fool" the stern wave into assuming a much longer waterline length. This is called "forced mode" and allows the high speeds with huge engine power in naval ships.

With a catamaran sailboat the heeling forces at higher wind strengths submerge the leeward hull, hence there will be wave drag. A planing hull may produce enough dynamic lift to counteract heeling. The net effect may be less wetted surface and lower wave drag. I.e. lower resistance, faster boat.

To my knowledge, planing hulls have not been competitive in lower wind speeds, and with high resistance during transition to planing, they seem uncompetitive in round-the-buoys races. However, several modern monohull dinghys have a chine (e.g. 49er).

Tornados beat planing 18-foot skiffs around the buoys, but the 18-foot skiffs seem to exhibit the highest absolute absolute top speeds.

Hovercraft catamarans, sidewall hovercraft, SES, use an pressurized cushion to lift the boat. This reduces the wetted surface of the hulls substantially.

Ecranolets reduce resistance by flying near the ground, thereby reducing the wingtip vortices that produce drag (and reduce lift).

Our catamarans should be very fast with small engines, hence very efficient compared to most other vessels. But for really high top speed, we probably need planing hulls.

Go for it Warbird!

Stein

1 mile/hr = 1608 meters/hr
1 knot = 1852 meters/hr

Stein

At what point does waterline come into play. Is the terminal speed higher for an 18' cat higher than a 16' cat considering the same amount of thrust and drag?

Stein,

Your comments are correct to the extent of what I've learned over time but I would like to make one comment :




This is a typical example of a statement that is so tortured because the original source wants to cling onto an orginal concept that is so obviously wrong (or misunderstood). In this case that the max hull speed myth.


There is no "fooling" or "assuming of longer waterline length". You can't "fool" a law of physics and neither does such a law "assume" anything. It is not an object with a information processing unit like a brain that can be "fooled".

The foundation of all this torture logic is the misunderstanding that Froude's law says anything about a hull. And indeed, I've have encountered many well educated persons who make this mistake as well while the way out of this situation is sooo obvious.



And now too all :

Again, Froude's law doesn't say anything about a given hull, it ONLY says something about the speed a given wave length travells across the watersurface for a given set of temperature, pressure and water density. That is all.

So why is the Froude formula referred to so often ? Because the pressure distribution around a hull travelling through the water is dependent on the wave system around it and therefor dependent on the wavelength of a wave system that is travelling along the hull at the SAME speed as the hull is travelling.

Obviously, a wave system with multiple crests along the hull will result in a rather symmetric pressure distribution. Such a situation leads to the lost energy (drag) incurred on the front part of the hull to be won back (drive) at the rear part. OFTEN, this leads to lower hull drag. When the hull starts to move faster then there will be fewer and fewer crests along the hull till the last crest moves behind the stern, resulting in a much more a-symmetric pressure situation along the hull where basically there are only pressures working against the movement of the hull (drag) left and there are no more positive pressured (drive) that recouperate some of the loses.

For very heavy boats like frighters and ballasted keel yachts the hull drag (excluding wetted surface drag) makes up a very large portion of the total drag, so when that component increases rapidly then it will quickly use up all available power (drive) from an engine or sails and thus stop accellerating. It will then have reached max. speed. Again for heavy craft this point happens to coincide often with a distanced travelled per second (=boatspeed) that is a fraction longer then the wavelength of the wave travelling alongside the hull (as caused by the bows and sterns). This leads to the misapplying of Froude's law as a formula to calculate max. (hull) speed for any given hull length. Of course for some situations (heavy boats - low power) this works rather well but for many others it is simply wrong. As in the past we only had "heavy-boats with lower power", people began to believe the errornous intepretation of Froude's law.

So what happens on a design like a beach cat ? Exactly the same as descibed above, with only one very important exception. The overal fraction of the hull drag in the total drag of a beach cat is MUCH smaller. For wave-inducing drag it is something like 10-15% of the total where it could be over 50% for a heavy yacht. Now, you can easily double 15% and have the beach cat hardly notice much difference while you can't double 50% or more and expect the yacht to keep travelling at that speed. It will need (alot) more drive then the sails can produce. On a beach cat the increase in boatspeed leads to INCREASES in saildrive that are LARGER then the increases in hull drag when acquiring higher hull speeds. I know this sounds weird but that is exactly what is happening. On a heavy craft this situation is reversed. Here an increase in hull speed increases hull drag more then it increases saildrive. Therefor when surfing off a wave it will pick up speed beyond Froude's inpired max hull speed but then slow down later. A cat however will keep accellerating even without the help of a wave till a point were the aerodynamic limits of the rig stop any further accelleration (increases in sail drive have topped out). And this is why so many forced displacement catamarans and planing dighnies are so close in overall speed. Their real obstacle is aerodynamic performance of their rigs, not so much the drag of the hulls. Just an example, on a beach cat aerodynamically related drag is 25-30% if the total and as good as twice as large as the wave-making drag component of the hull. (Miss Nylex C-class data that using a highly drag efficient wingsail).


So summerizing :

Froude's law equates wave-travelling speed to wave-length for a typical wave on earth on the surface of a body of water with density around 100kg/m3. Nothing more, nothing less. It is certainly not explicetly saying anything about a hull. The fact that Froude discovered this law by looking at ships doesn't change this fact.

Froude's REAL law is : Wave-speed = 1.34 * square root of wave-length

The coefficient 1.54 that is used often is nothing more then designers trick when designing a "Heavy Weight - Low Power" Boat. Remember the earlier comment on how a hull of such a design would typically travell a tad bit faster then its surrounding wave system. The difference between 1.34 and 1.54 is that tad bit. Basically these designers made Froude's law fit their experience with such boats instead of the other way around where the experiences fit the formula. Some people like to use a coefficient of 5 for beach cats but that is just more of the same tweaking of the formula to fit a particular set real life experience. And for each new design you have to redo this, meaning the formula is useless in such roles as it will only reflect what you already know and not predict what you don't already know. And why use any formula when you already know what the result is going to be ?

For only a portion of hull design (heavy craft - low power) this Froude formula coincides closely to the maximally attained speed in real life because in these situations the accompanying wave system (that DOES follow Froude's law in all instances) is so important in the total amount of drag.

In a very large portion of other designs Froude's law still applies to the accompanying wave-system but as its related drag describes only such a small part of the total system that it is of very limited practical use. Beach cats fall into that category.

Wouter

Yes. pintop main, a jib made in the early 80's, 1 trap 8:1 mainsheet(too much work)

WHAT IF...you were to build a catamaran out of two windsurfer type boards-hulls, and some how put a regular cat wingmast and sail (or even Ben Hall's wing) on it, so you would definately be on a plane when the wind was right? How fast could that go?? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Faster then 45 knots - just split the leeward board in two, increase the distance between the bow and stern parts and add tiny foils here and there.

Its name is "Macquarie Innovation", formerly "Yellow Pages Endeavour".

I love the "pod" the driver sits in!

However, I was thinking more along the lines of a Hobie 16 tramp type platform where you pull the Hobie hulls off, and replace them with windsurfer type "hulls" that would act like giant water skis, then wait for the right wind, power it up and away you go!

Wouter,

My post did not at all mention max hull speed. Please, read the posts carefully before criticizing!

I assume that almost all readers of this forum understand quote marks in the sentence:
“….”fool” the stern wave into assuming a much longer waterline length”.

Froude number: Froude developed his formula from boat-model experiments. The Froude number says something about the wave generated by a hull moving in water.

There are several versions of the formula (dimensionless, densimetric, etc), so stating that “Froude’s REAL law is …” is a simplification at best.
For a ship, the Froude number is defined as: Fr = V/square root of(g*L)
where V is the velocity of the ship, g is the acceleration due to gravity, and L is the length of the ship.
See JIMI’s previous post.

The Froude number relates to the fact that wave velocity is related to wave length.

A displacement boat generates waves. A displacement boat travelling faster than the velocity of the waves it generates, is subjected to large resistance against its movement (wave drag). Hence, a large amount of power is needed to increase the speed further.

With very slender hulls, wave drag is a relatively small component of total drag.

There are ways of altering the stern of a boat to move the stern wave to behind the boat (“fool” the stern wave):
- the vertical sharply cut stern which allow the water to detach from the stern,
- the horizontal wings of the International 14s just beneath the water surface,
- the appendix of the New Zealand’s boat for America’s cup 2003 (the Hula).

Almost all small cats have vertical sharply cut sterns that allow the water to detach from the stern.

Stein

Peter Ifju (Prof., Aeronautical Engineering Dept, U. Florida)
sails very wide light windsurfers of his own design. It planes at 6 knots: add 2 under a Hobie tramp 14 structure, add proper Carbon mast, modern sail with foam battens and what else?

Use a light board to replace the windward ama and two Formula boards to replace the leeward ama. Leave a big gap between the two leeward boards. It will be faster than the Hobie in straight line sprints over flat water, but certainly not worth the trouble.

And you could trap off the side while wearing water skis! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I wasn't critizing you, it was more a post to the general public that I had place somewhere in the thread. This turned out to be you post as you gave a typical tortured reasoning that is often expressed by persons who keep clinging on to Froude's law as a predictor for maximum speed.

Some reply to your new post.

Froude's NUMBER is not the same as Froude's LAW or FORMULA. I did not discuss or refer to Froude's NUMBER which I personally feel is a pretty uninteresting quotient anyway. Someone described it ones to me as the water surface based Mach number which is indeed it most similar cousin.





No it is isn't. Changing the symbols of othering in a formula doesn't change the formula it is still one and the same. Meaning

V = 1.34 * sqrt(L)

is exactly the same formula as.

sqrt(L) = V/1.34

As both describe exactly the same behaviour of a wave.

CHANGING the formula to V = 1.54*sqrt(L) is intentionally altering the formula to make an error and as such is a totally different formula and can not be called Froude's formula. This fast and loose actions are what caused the eternal "maximum hull speed" myth; which is pretty unscientific.

Compare it to this. NO-ONE changes newtons most famous law F=M*A to have it cover different freefalling objects like lead and feathers in the atmosphere better either. We are expected to improved our model and include other effects like aerodynamic forces to be able to use THE ONE AND ONLY ORIGINAL FORMULA.

Only ship designers are so scientifically challenged that they have to change Froude's formula in order to avoid the need to let go of a mental model that is far too simple. I'm sorry but that is just bad science.






Now you make the same mistake like many other people again. The statement :

A displacement boat travelling faster than the velocity of the waves it generates, is subjected to large resistance against its movement

IS SIMPLY NOT TRUE for a large portion of hull designs. ONLY FOR RELATIVELY HEAVY ship designs. For example a beach catamaran does indeed see a (non-linear) increase in its wave drag component when passing the wave-velocity but that still does NOT mean that the related increase in drag is a LARGE component in the total amount of incurred drag. In fact it isn't. I glarified it as such in my earlier post so you should have spotted that. And indeed you yourself argue again this very statement at the end of your posting.


My comment to ALL readers here is to stop using Froude's law (NOT Froude's number) in ways that are inconsistant with what it described and tell any professors who say otherwise to stop mystifying the situation.

Froude's law (NOT Froude number) does not say anything about any hull design. It only relates the wavelength of a wave on the watersurface above deep water on earth to its travelling speed. In some cases this travelling speed happens to coincide closely with the maximum speed a certain limited group of boats are experienced to achieve in real life.

An analogue example : The fact that some sports car can sustain an accelleration of 1 G in real life does not mean that Newtons law F=M*A = M*G describes the achieved accelleration of all automobiles cars ! Not even when we include some (fictional) Newton Numbers to "Correct" the formula for different automobile designs like. F = Newton number * M*G

So why are ship builders doing to Froude's law ?

Wouter

Really waiting for that book of yours Wouter to dispel the myths and set us all right.

Wouter, I appreciate your restrained responses on this subject.

Your explanation really strikes a chord from my landsailing and drag from the rig being the primarily limiter of speed. In beachcats I was sure it was hull drag, although I knew it wasn't Froude's thing. Also experiments of others have shown that while increasing max speed, planing hulls aren't the answer for general sailing beach cats. At least until highly sophisticated, very robust cheap systems become available.

So, for just an approximate understanding of what's happening on more recent beach cat hull designs when the crew sense 'planing' are the hulls just experiencing 'forced mode' in different (from, say, a Tornado, Tiger or P19) way?

That book is not coming. To choice is up to you lot to believe my comments or not. Either you use them to your advantage or you prefer to believe that you can sail to the rim and fall off.

Either way, I not losing any sleep over it.

Wouter

There be dragons!

Ever considering posting what you know w/out the attitude?
Good info, tho...

max speed is always the result of the play between ALL drag components and ALL drive components, this does always included hull drag. The question that is valueable from engineering viewpoint (improving speed) is to what extent each component is important. It makes no sense in spending resources on lowering a drag factor that only comes 6th in line with respect to importance. If oen does then it will most likely means spending lots of resources on very marginal performance increases. At this time the whole rig (and others) are above wave-making drag on the listing. Actually wetted surface drag (surface area hull) is higher in the listing as well. And that is why planing works on some sailboat design. Planing reduced wetted surface drag while accepting increases in wave-making drag. This is a very important principle to understand by all who want to make sense of this whole thread.

My point to you is that I was argueing that we should not totally ignore wave-making drag but give it the correct placing on the listing of importance (magnitude) and Froude's law intepreted as max hull speed law doesn't do that.




That is my understanding for 99% of catamaran designs.

Wouter

Ahh, this is an interesting philosofical question.

Can it be because of this attitude that I ever undertook the effort to learn these things ?

Would I still have known them if I ever had been a nice, charming, happy to follow others kind of guy ?

I see, ignore my charming self and make maximum use of the information provided. Nobody has to like me in order to max use of what I provide. I guess I've accepted being my own biggest demon ! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Wouter

You know, what we should be talking about here is how fast can you make your catamaran go...(the greater You, as in all of us, not just the Wouter in you.) <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I'm going out back now and see how fast I can make mine go. See ya! <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

No.



Yes.




Bingo.

Maximum use? Not sure how much actual boat design will come from any of this, but the intellectual pursuit is interesting.

17.5 knots this after noon showed on the gps while under spinaker, uni rig, on one hull with the boards half way up, in about 15 knots of wind. Every time I tried to heat it up more, I would stuff the bow and slow down to 15 knots. I think in 20 knots of wind, I could get it to plane if on both hulls boards up. I'll just have to wait for more wind. Off to Bombay tomorrow. I should be passing over Zandvoort about sunrise Tuesday moring. I hope it's clear so I can see ya! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

You put so much time and effort (with and without attitude) posting here all this kind of stuff. All you would have to do is copy and paste your numerous posts and you would have a book! But would need an editor to adjust the tone in some instances.

How far back on the boat were you? I imagine you obviously had your foot in the strap... but were you leaning back also? How much do you weigh? I have hit the same number several times but it is hard to maintain it with any significant chop (more nose diving).

I was sitting on the hull at the aft beam, forward foot up against the half raised board, other foot under the foot strap. I have tried trapping but it only nose dives more. I was trying instead to get it up on a plane by quickly bearing off onto both hulls when a good gust would come along, but there just wasn't enough wind for that, so on several runs I just sailed it on one hull to see how fast it would go. 17.5 in that light-ish air was good, but I want more wind! I was trying to keep the bow knuckle up out of the water which seemed to work well in a good gust.

Please go ahead and be my editor !

Wouter

Higher top speeds as quoted by Bethwaite..... Even the Skiffies take what he says with a grain of salt. I believe he also quoted a rediculous max speed for a Tasar once (over 20 knots from memory).

I wounder why the 18s have never done any form of measured speed runs....... Hard to BS timed runs.

I have no understanding of what you are saying most of time. Eyes glazing over kind of stuff. Editing the mountainous amount of data/posts would put me to slumber for many a year.

Perhaps Doug Lord may be a better choice of Editor.

Well I must be one of the few who can actually believe his Tasar claim. I have had several 16/18/20ft cats but my last mono (20 years ago) was an early lightweight (kevlar) tasar. I can recall launching it one wild day when it needed to be held down on its trolley with its sails down. Once on the water it was amazing. Real crash 'n' burn stuff but incredible to see the rate at which the waves were being overtaken downwind. I have never heard my wife swear so much in her life! When I returned to shore I was immediately accosted by several spectators who had been impressed by what they had seen the boat do. I am not saying it could perform like this regularly, but put the right ingredients in the mix and it is surprising what can happen.

Cheshirecatman

Thats very interesting. You have raced the 18s on several occations, haven't you? From your experience, how do the 18s compare downwind? Also interesting that even the skiff sailors have a hard time believing this. As already posted, I think Bethwaite claims a top speed for the skiffs well above 30 knots.. That been said, if I'm not terribly mistaken, I do believe that the official Tornado site claimed the top speed of the Tornado to be around 30 knots as well. To me that seems a bit high. Though I did believe the skiffies would achieve speeds in excess of (at least) 25 knots..?

Now that would be a match made in heaven. The revolution combined with technical facts

This discussion relates to speed limitations of small catsailor hulls.

We all agree that wave drag is one of several components of total resistance. In very light and slender catamaran dinghys, it constitutes a small component at low speeds. However, wave drag does increase significantly when the boat travels faster than the velocity of the waves it generates.

Wouter stated that
“wave-inducing drag it is something like 10-15% of the total”.
He proposed a doubling of wave drag with increasing speed over the wave velocity (“Now, you can easily double 15% and have the beach cat hardly notice much difference”).
Do we really know this number? We do not know whether it actually increases by a factor of 5 at 20 knots http://www.boatdesign.net/forums/attachment.php?attachmentid=18865&d=1203073097

It has been proposed that wave drag is of similar magnitude as viscous drag in kayaks already at 7 knots!
http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm


I tend to believe (and agree with Wouter) that surface drag is more important than wave drag in our cat dinghys. But a major question is whether wave drag of displacement hulls constitutes one (of several) significant limiting factor in attaining high speeds.

My experience has been that it is nose diving that has stopped us from sailing faster than 20 knots. Like Wouter, I think T-foil rudders are promising (next X-mas, Santa?)


Attitude:
Wouter, I really do appreciate your contributions to this forum. Many of your posts are helpful.

But why do you always seem to respond in a condescending way?
One example among many:
Quote: “as you gave a typical tortured reasoning that is often expressed by persons who keep clinging on to Froude's law as a predictor for maximum speed”.
This was to a post that did not even mention maximum speed, and to a person who has no interest in defending Froude or any other of his time.

You tend refer to science. This is good, we look for state-of-the-art knowledge. However, a typical characteristic of almost all scientists who publish in leading international peer-reviewed journals, is that they never exhibit condescending or arrogant behavior during scientific discussions.
A respectful approach to colleagues is also very productive in developing better equipment and skills.

When it comes to hydrodynamics of hulls and foils, a lot is empirically derived knowledge rather than formulas.
We need more empirical data.

Stein

Hi Stein,



We don't know this number, because we can measure only the total drag, but we can calculate a drag break down. Wave drag and viscous drag are plotted in this two diagrams:
http://www.catsailor.com/gallery/thumbnails.php?album=lastupby&uid=230
Please note, that the speed is in m/s which is about 0.5m/s = 1kts. I selected a scale to unrealistic high speeds, just to show the behaviour at high speeds. Also note that only wave and viscous drag is shown, but no induced drag. And the displacement, pitch and heeling attitude is constant over speed in this calculation. The method used is called "michlet" and is free available.
You can see, the higher the speed, the lower the wave drag ratio. That is because after a certain speed, the wave drag is more or less constant, while the viscous (friction) drag raises and raises.


There is nothing mystical about hydrodynamics. A lot of knowledge is available for free. However your skills has to be trained to use it, it is sometimes difficult to understand.

Finally even if you can predict the drag of the boat, it is a different job to optimise a boat, which requires interdisciplinary skills. A homebuilder can still hit the right design to achieve high speeds and give boat development a new momentum, without any CFD calculation and experts. However the probability...

Your catamarans speed is limited by drag and sail power at light winds, by capsizing at strong winds and up wind courses and by pitchpoling at strong winds on the other courses. So yes, foils or ballast tanks may help on certain courses and conditions.

Cheers,

Klaus

My buddies Pontoon boat will do 35kts using a 115hp outboard. It's larger than a 18' cat and uses, what to me looks like 55gal drums with a pointy front. It's definitely not planing, and I bet it's above hull speed. I've seen pontoon boats on Lake Norman here in NC that goes a lot faster than that, people even wake-board behind them here. I guess with enough engine you can make anything move.

Yep, raced them quiet a few times now on the F18. The F18 smashed them upwind and had the edge on the downwinds with the 18 Skiffs performing pest on the downwinds in relation to the F18 when it was 10 knots or under. The Tornado always had the edge on the F18.
The Tornado class claims a top speed in excess of 33 knots. I think they are just as optimistic as Bethweight.
Once again the Tornado class ran speed trials and whilst it is surley not the quickest run on a Tornado, they would be up there. Results were 27 MPH or 23.5 knots average over the 500 foot course. Completed in 12.53 seconds.

A far cry from 33 knots, however this was an average over 500 feet. The 18s have never done these types of runs...... Perhaps because the Bethwaite claims may be exposed for what they really are (along with many of his other wild claims)..... BS.

Like I said before NO ONE KNOWS!!!!! Actually does anyone remember the actual question? <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Clayton

The highest speed I have known the Tornado to go was at CORK '75.
A 40kn storm hit, when the the fleet got to the reaching leg.
The peak of the storm must have hit, when the leaders got to
the jibe mark. At this mark a couple boats broke their side stays,
one of them was Reg White who released the other stays, so as not
to damage the boat from the mast.
Notary was there on "Salt Water Wine", Zutec on "Twister" & Smyth.
Anyway a Canadian destroyer radared the fleet at 28kn average.
That's 33mph average, at that speed the boat was NO longer in
the water it went like a skipping stone from crest to crest 9'(3m).
It was amazing the Tornado could skip the crest, and soar over the trough.
It was sheer madness, with some poet license here, it seemed like
we zoomed over 5 crests in ten seconds, and the wind would
calm down to 2 or 3 crests in 10 seconds a la bucking bronco,
and then accelerate again, & fly over the trough.
I will never forget the thrill, not to mention wondering will
the wind ever slow up. I recall a Tornado to windward of us,
and counting it cartwheeling/rotating about it's mast 3 1/2 times, and
the crew later righted the boat, unassisted?
Anyway, I would call it somewhere between sailing/flying & oh ____.
Great wind & waves,
Chris

Did you guys see that "thing" on the front page of SA?
They got rid of hull drag by removing the hull!
http://www2.jundt.ch/blog/?page_id=35
Video of the thing sailing...

Klaus,

Thank you! I know of the Michlet code, but have not used it yet.

There is one question I wish someone could answer:

What is the reason that different sources report totally different speed-wave drag associations for seemingly similar hulls shapes?

Compare the curves you are presenting with the curves for the kayaks
http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm
- totally different relationships.
Does the sharply cut vertical stern of beach cats make the difference?

Stein

If Wouter's statement that aerodynamic drag is a major speed-limiting factor is correct, this will not be a success in terms of max speed. I think they will need to cover the tubing spaghetti with some kind of skin.
Stein

Hi Stein,



Actually, they are not different. The kayak data have a range from 0 to 7kts, which is 0 to 3.5m/s. While the scale in my diagrams ranges from 0 to 15m/s (0 to 30kts). Until 7kts the curves a quite comparable. Sorry, I don't have a zoomed view of my diagram. After 8 kts the wave drag stays more or less constant, and I would expect the same for the kayaks. There is always a risk of taking wrong conclusions if data of limited range are extrapolated, in other words: If we step beyond our knowledge.

As long as the flow is not attached at the vertical stern, a boat behaves as a slightly longer one, but with slightly reduced friction drag. If the flow 'attaches' around the stern, you get a different source of drag, called form drag. If the sterns of your cat are 'sucking', you should sit more forward to avoid this additional form drag.

Cheers,

Klaus

A bit more heresay from me.

Went out yesterday on the TS. Gusty and hard to settle the boat.
In waves it was a seriously rough ride. The faster it went over the waves though the smoother it got.
I am having trouble setting the boat so as not to lift at the reach I want.
I had one big, sustained gust headed further off the wind in less chop. The boat compressed onto both hulls and into the water, certainly NOT planing.

This was my fastest run but also by far the most scary.
I routinely sail cats in 25/30 knots and I have never been afraid of crashing. I also have a strange "slo-mo' sensation of not really fast.
I was clear yesterday that I was going faster than I would like to crash at but sadly..no planing.

A good GPS is on the way for me and yes, I am aware I cannot take those results as fact.

I did however watch the Tornado Worlds off Orakei in Auckland about ten years ago. Small rigs.
The day I watched was held in close to 30 and 35 knots.
They were absolutely traveling on the last foot of the lee hull and one of the most impressive sailing memories I have. So impressive were these TOrnados that I heard about them on my car radio and diverted to watch the spectacle spoken of. I knew then I had to sail cats.

As far as the foiler thing going fast. Why is it I am not interested in what such an oddball, PITA to set up and use sort of boat does?

Surely we all deal in a boat that has a pleasure/race capability?

And by the way. There were several people (not one group) waiting for me on the beach when I came in wanting to know how the boat cold go so fast.
One guy saying "the water coming off your boat looked like smoke" over and over. So cats can interest the public. They just have to be seen.

Klaus,

Thank you!

So for slender beachcat hulls and kayaks the wave drag vs speed relationship seems to follow a sigmoidal curve (After 8 knots wave drag stays more or less constant).
While for conventional displacement monohulls, the wave drag vs speed curve looks like there is a quadratic trend.

Is there a critical width/length ratio or critical width which determines the transition between these two very different relationships?

Or maybe the weight of the hull comes into play (the g in Froude number)?

I have seen people mention width/length ratios, but I have never seen anyone give a good reason why a certain ratio should alter the wave drag vs speed relationship dramatically.

Stein

Hi Stein,

I never did a calculation with monohulls, so I can only guess.


The first thing I would do is to make a calculation to very high speeds again, much higher than the boat would be able to go, so I can be sure to cover the whole range.
For sure the beam/length or slenderness ratio will have an influence on wave drag. But I have no idea, if it scales the curve only or changes the whole nature of the curve. For this somebody should run a lot of different hull shapes in the michlet code. However I wouldn't expect any dramitical change.
A low wavedrag non-planing monohull may be to slender to have suffient roll stability. That's maybe the reason why they go the planing configuration.

But again, I guess rather than I know for monos.

Cheers,

Klaus

I used to tow my daughter behind our Taipan 4.9 on two skies.