Flat or round bottom for a foiler "board"

A proper answer would require a fair bit of theory. In short, stability is attained when the centre of buoyancy moves in the direction of inclination and the centre of mass stays put (or moves further away from the direction of heel) which creates a righting couple or lever arm effect.
If there is no beam at the waterline this doesn't occur. Even a rectangular section which is completely immersed will keep the same COB, as none of the volume is lifted above the water surface. (Lots of reading is available, just Google Ship stability theory)
Try rotating a round vs. rectangular bottle on the water surface first then fill it with water and repeat under water to prove it to yourself.

Moths have not had much form stability for years, I doubt that the underwater profile makes a hell of a lot of difference. The squared off version will float higher for a given waterline beam, so its likely to be about performance as much as balance.

The other aspect with an immersed torpedo is that it will be incredibly weight sensitive. Any increase or decrease in weight will have massive changes in immersion due to the tiny water-plane area. SWATHs do use this principle, but are a specialised design area with fairly specific applications of use.

 

these assertions aren't really accurate. all hulls start generating some lift was upwards as soon as they start moving. however the more rocker (prismatic) you have the more that is offset by the Bernoulli effect pulling you downwards.

a flat bottomed board will have less immersed lateral surface and thus be easier to turn at lower speeds than a narrower curved hull. And as is pointed out it will have a lot more stability. JRD does a good job of explaining why.

The stability of a square moth to some extent also comes from the chine effect. As soon as there is some heel on a squared hull you essentially have a sharp keelson. The old Windsurfer OD's used this by rolling the leeward rail a bit into the water (though you could get in trouble that way if the breeze was up) and lots of boats from the Star to the Thunderbird use hard chines for tracking stability

But basically both foil and board will start to generate lift as soon as they start moving. But because a flat board has more lifting surface, even though its a less efficient 'foil' (its still a foil from the perspective of how water flow works on it), at low speeds it will generate more lift than a round hull.

This could be seen in the "old days" when you had "D2" boards and windsurfer ODs.... The D2s would beat me to the weather mark, but I'd pass them back again off wind and reaching.


Another thing to think about is "touch down".... since buoyancy is a function of immersed volume, if you pop off the foil at speed (for whatever reason), a flat board gives you more initial buoyancy more rapidly. add in that a flat board requires displaced water to accelerate a greater distance - you get a combination where a flatter board is a lot less likely to go UBoat on you if you fall off your foil

 

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JHD, the concept of hull form stability on rectangular shape with an L/B of 11/1 or a semicircular shape of 6.9/1 is nonsensical-"form stability" shouldn't even be considered-assume none.
The OP ,as I understood him, was trying to understand why a narrow ,skinny
Moth hull was faster than a lower wetted surface semicircular hull of the same displacement and length at pre-foiling speeds.
You're right that a lot of theory can be gone into for the answer-researching some of Leo Lazauskas'(member here) work can provide that. But the short answer is that it has been proven conclusively in the Moth class that at pre-foiling speeds(below 7 knots) the skinny squarish hull trumps the semicircular low wetted surface hull because it has less total resistance(wavemaking resistance is lower because of the narrow hull).
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It's plain nonsense to bring the flat part of the hull into lift calculations since at prefoiling speeds the lift from a flat bottom on a nearly double ended, high L/B hull will be a very small fraction of that available from foil lift.
The hull being flat is a result of designing the lowest resistance hull shape which is determined by the L/B ratio not wetted surface on a Moth type hull.
The hull is useless for much lift in the most frequent cause of a Moth coming off foils-a dive caused by the wand losing contact with the water and swinging all the way forward which causes the mainfoil flap to go up resulting in huge downforce and a crash.
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On any new development concentrating on foil design and control with the lowest resistance hull that you can have at pre-foiling speeds will pay the proven highest divdends.

 

Proven Conclusively? Doug how much time have you spent designing or sailing these older "low riders"?

As for lift from the board vs the foil - at 3 knots, the board is displacing quite a bit of water. that displacement results in upwards lift. You clearly have not spent enough time sailing a flat board.

 

My question was not about stability. It was purely abstract question regarding resistance ant its relation to waterline L/B only.
Imagine we add stability some other way, by internal ballast, or by magnetic field, or by black magic, whatever.
Will be that underwater hull have less resistance just because its waterline L/B is extremely large?

Or another even simpler example. Will a rectangle cross-section that has depth 2m and beam 0.5m have less resistance than cross-section 1x1m?

Try to attach a one foot ruler to a bottle, round or rectangle, and then try to rotate it. You will feel why flying moths are so stable. And hull profile makes little difference for stability.

Again, my question was not about practicality of that profile, just about its resistance and its relation to waterline L/B.

 

Doug, another simple example.
Will a rectangle cross-section that has depth 2m and beam 0.5m have less resistance than cross-section 1x1m? Both are 10m length.
Does it make sense in this case to make a catamaran with two very harrow, but quite deep hulls?

Then how to explain that quite long and narrow water ski push out a rider being underwater (with some trim) during initial acceleration, when speed is not that high yet?

That is very true. That is why I am here asking my dumb questions.

 

Empirically the Gougeon brothers documented that the ideal L/B ratio for a hull was roughly 22:1 for minimal drag. since then I believe the math has shown this to also be the case. And if you look at racing multi--hulls, that's pretty much the hull shape of the ama that is intended to be immersed. Now for monohulls that are intended to plane, the equation changes, hence why boards are different shapes, but for a buoyant hull in sub--planning conditions, that applies.

<removed jab towards other member>

 

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I'm not interested in exploring the what if's of this fact: a high L/B ratio Moth hull is always faster than a low L/B ratio semi circular Moth hull in pre-foiling conditions with both having the same weight and same power. Further exploration of the theory as applied to other shapes could be gained by contacting Leo. But I don't think they are relevant to what you said you were doing.
The waterski example is irrelevant since it is not working with foils: I suggested you do not try to bring what residual hull lift there may be into foil calculations and I think that is the best way to proceed with a high L/B hull of the general shape of a Moth. To get an angle of attack where the flat portion of the hull would develop substantial lift ,at 7 knots or under, would require an angle so high the the foils would likely stall.
L/B of cat hulls under 20' tends to be around 16/1- talked to Dave Carlson (A Class racer and foil designer) and he told me the average L/B on an A Cat is about 16/1. As multihulls get larger the L/B ratio can increase, but under 20' the dominant factor in determining L/B ratio is the weight it has to carry--and not on both hulls-on the lee hull with the windward hull just flying.

 

Doug you know this how? I don't believe you have ever raced or designed moths and I doubt you know many moth sailors.

As to the waterski example it is completely relevant because until the foil generates flying lift, the question is "what hull shape" aids in acceleration and liftoff the most.

The reason a water ski is the shape it is, iis because its tuned for top speed not takeoff. on takeoff you are generating excess lift by having a higher AoA on the ski to generate more lift.

The water ski analogy here would actually be that of "getting up" on 2 skis and then dropping one ski to ride only on one ski (reducing surface area and drag by going only to the lift from the foil).

So Doug it is relevant... if you think about these things from something other than a pedantic insistence on being right

 

Oh good grief. Do you have to stalk Lord in every damn forum in the universe. Worse than he is...

 

Who's stalking? I have a shelf full of Windsurfer OD trophies and we are talking about windsurfers. A newbie asks questions and is pedantically addressed, I call Doug on it.

Where's the stalking?

 

Live and let live

 

please stop derailing the thread

 

The U sectioned Moth hull was largely a response to the problem of getting enough buoyancy into a short and incredibly narrow boat. This has been confirmed to me specifically by Moth gurus like Andy Paterson (creator of the narrow Moth) and Mark Thorpe (creator of the last and fastest seahuggers). If one is not constrained by Moth rules one may want a different shape but the only "giant Moth" does not perform as well as a standard one, indicating the complexities of the issue.

IIRC, some people who should know reckoned that in dead light stuff the wider Moths such as old Duflos and Magnums can actually be faster than the narrow Moths. The issue was that no serious sailor would sail one against a narrow boat because of the enormous speed difference in anything of a breeze.

Years ago I did a few races on a round-bottomed Olympic D2 Lechner A390 windsurfer against the best two Moths in the world at the time. This was before foils. I only had the 7.3m sail off an IMCO (not the best rig for the Lechner) and while I used to be OK in that class at national level (including a very close 2nd in the Heavyweight division against a multiple world champ) it has never been my main class, I was years out of practice, and was well outclassed in terms of skill.

The board was as fast in the light (under about 6 knots) although the Moths may have had a touch more pace reaching. Considering the various tradeoffs and skill difference, I felt that the rounder board showed a lot of potential against the boxy Moths. However the board had the advantage that when the wind came up it could be "railed" to reduce waterline beam and therefore did not suffer the drag that a similarly-shaped Moth would have; from about 8 knots it was clearly quicker than the Moths. It was a classic case of different shapes for different craft.

I have thought about putting foils on a D2 as they are clearly much, much faster than short fat Formula type boards in most winds that are too light for planing. There are significant issues and I have never been able to find cheap foils.

Given the issues with foiling boards and the high speed of a Formula board once planing, I tend to feel that a foiling FW board may not be any advance over a standard FW board or a Moth. A foiling longboard, which could be more versatile than a foiling FW board and may have some advantages over a Moth, could be a fascinating beast but would involve too much hassle for me to create.

Two guys who have been in the top 3 (?) of the foiling Moth worlds are former top-class Windsurfer One Design sailors. One of them (Andy McDougal) is the creator of the top Moths and top windsurfer rigs. He would have this stuff pretty damn well worked out and if anyone knows how the concept would work, he would.

Pic below is of a Division 2 board; one of the magnificent Brazilian/Greek creations; to give an idea of the shape. I was offered one for a song after the '85 worlds but couldn't get it home. Luckily I ended up with a collector's D2 in the form of the board that won the bronze medal in Barcelona.

Pic is from Rainer Frohböse; we've both been involved a lot in longboard administration and promotion so I assume he won't mind me using it.

 

A bit wider in front than my old Crit D2... but damn that's a familiar shape..

Pretty too