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.