Foil Board Hull Design

I am hoping to tap into the knowledge base of this forum in order to optimize the hull design of a foiling board (wing board, kiteboard, SUP board, eFoil). For the purposes of this thought exercise, the board is simply a means by which to get onto foil, and other than the acceleration period, the board bottom will rarely contact the water surface (in theory). The following parameters are prioritized:

• Acceleration / Efficiency - ability to reach speed required to engage the foil, with minimal power input. Most foils are going to engage around 10-13 kph
• Easy release from the surface
• Practically speaking, the board needs to be at least 18" wide for stability (20" would be ideal), and as short as possible, while realizing the length is a major parameter in acceleration and efficiency. Im thinking 6' is a good and practical compromise. The short length is important to maximize nimbleness once the board is on foil.
• Stability - the shape needs to provide some stability. This is partially addressed in the width, but it isn't practical to have a deep V for the entire length (although that may be viable in the bow and stern with a flat feature in the middle).
• Less of a priority, but the board should have some rebound feature to bounce back onto foil if the hull touches down.

The downwind boards that are currently optimizing for this design have hard chines leading to a "V" shaped hull feature in the tail. This makes some sense for release of flow in the stern, but Im wondering whether the hard chines might actually slow the board down and would be better with a softer chine displacement hull feature. See below for examples of the hard chines:

Dagger - Freedom Foil Boards https://www.freedomfoilboards.com/freedom-ecom/foil-boards/dagger/5637991442.p
Downwind SUP Foil Board | The Sultan | Amos Shapes https://amosshapes.com/shop/new-boards/downwind-sup-foilboard-sultan/
E3 Wing / Prone / Downwind | Kalama Performance Foil, SUP & Surf boards https://kalamaperformance.com/product/e3-wing-prone-downwind/

Any critique of these designs?

Im completely green in this field, but excited to learn. Any thoughts would be appreciated.

 

Deep vee will be the least stable of the several bottom configurations that you might select. The most stable is likely to be a flat bottom or nearly so.

Consider the required displacement of the board. Lets say that you weigh.....pick a number.........170 pounds. The board will weigh something, but ignore that for a moment. If the board is 72 inches long and 20 inches wide, and sort of boat shape, we can make a wild guess about the bottom area. Maybe about six and a half square feet. ....... Wetted surface is a factor here.

Lets look at it from another direction. If you and the whole boat weigh 170 pounds, then you will need 4,700 cubic inches of displacement to hold up your weight. 64 pounds per cubic foot/12 x 12 x 12 = 0.0361 pounds per cubic inch. Let's say your boat is a bit boxy. That is to say, it is rectangular in cross section. The prismatic coefficient (Cp) might be something like 0.60 divide 4700 cubic inches by the length, 72 inches, and we get about 65 square inches.....Use the Pc to determine the prospective mid section area....65/0.60 = 108. That is the SWAG estimate method of testing your design. The midsection needs 108 square inches of area....divide by 20, the beam of the boat, and we get 108/20 = 5.4 inches of draft. Remember it is a simple rectangular cross sectional shape.

Where am I going with this you say? Your boat needs to be both longer and wider so that it will not sink so deep into the water. If you are heavier than the 170 that we started with, boat needs to be proportionately larger in both directions.

The bottom line here is that 72 x 20 is not large enough to have much hope for success.

Why not start with a design similar to proven paddle boards ? Then add some foils.

I have no intention of raining on your parade, but your boat may need to be much bigger. The design and size will depend on your weight and the weight of the finished boat. . Perhaps other members will weigh in with more detailed advice.

 

It's really just about take-off speed (foil design) and weight (the two are related).
As a "thought exercise", it would simply be restating the theory, which is already known.

 

Thank you for the replies. I probably should have made a few more factors clear.

regarding volume - the propulsion from the sail, kite or paddle is sufficient to get the board to the surface. For example, I’m 80kg and can easily get a 60 liter board (much smaller than what I’m proposing here) to the surface and onto foil in 8 knots. That propulsion also aids in stability.

to the other question - I understand it’s about takeoff speed. My “thought exercise” is about getting to that takeoff speed as quickly and efficiently as possible.

 

Don’t overthink this. The board is a very temporary interface vehicle.
The hot foilers are using really tiny boards.
Traditional boatbuilding formulas are not the path to optimum foil board size.

 

I would suggest the dynamic stability when foiling is much more important than the static one. When foiling you maybe want to adjust and balance your weight which should be easier with a board as short as possible and a centered mass distribution of the board. This would also reduce the wetted surface area and resistance for take off.

As kapnD said hot foilers use very tiny boards and don't overthink the board.

 

The IQ Foil is 2.2m x 95 x 196 L and the guys who sail that are about as hot as they come.

It's interesting to consider what optimum foil size is for the many times and many areas that have light winds. What amount of optimum design when foiling is it worth giving away to get on the foil earlier, and to be able to sail well without foiling?

 

That is the foilers choice, based on skill level, local conditions, and desired outcome.
There is no one board that will meet all criteria all the time, any more than one foil or sail will.

 

Very true, but since the IQ Foil guys are very hot and are on big boards, it's not as simple as saying that the hot foilers are using really tiny boards.

 

What about crashing? Do you want the board to turn it into a non event, or is stubbing a tiny surfboard, or breaking the nose off part of the thrill?

Is this water start only? If so, do you really need anything but a foil and a flotation vest? (no hull should be the aim?) Maybe the top of the vertical foil having some flotation is all you need? (Most of the athwartship stability of the really round d2 boards was a really big db/cb anyway- and most of those db/cb’s floated!).

That doesn’t address longitudinal stability for the waterstart, so I suppose you could go with a minimal speed needle at the top of the vertical foil- just enough flotation to get things sorted for a water start, make it bigger to taste for lighter stuff? A nice low prismatic javelin that would accelerate quickly, and nose shaping to smooth out a crash- how about a javelin with low aspect fat section horizontal section foils at the nose that might smooth out a crash, and keep you going? (I suppose if it’s skinny enough prismatic might not matter for higher wind water starts?) Or maybe a trick waterski single concave nose shape? Phat displacement nose for water starting? Fill in the top of that upturned nose to streamline things whilst foiling?

^crude cartoons^ done by pencil with no eraser!- longer needle for light winds, shorter for bigger stuff?

With stubby high lift at high angle low aspect bow mounted anti crash wings….or maybe the upturned nose would be enough?





Experiment with how much float & section the hull anti crash wings would need to work for water starts/ crashing/chop- & maybe how much height on main foils so crashing isn’t as big a deal. The really good guys will probably get to very minimal or no hull- just strap foils to your feet……….. has anyone tried a boogie board with foils.

A cup of Java, some graph paper, and a pencil with no means of erasure and a good gedanken experiment are just the thing first thing in the am…

The attach function has gone crazy on me

edit-

even the IQ class is tripping and crashing with the big boardshttps://www.iqfoilclassofficial.org/

go to ~ 1:20: they’re wearing helmets for a reason



Edit: double end / flat bottom / low prismatic works better than you might think at Archimedean speeds- look at moth low rider / foiler hulls, for example- hull stability with foils in the water will hinge around what stability the foils will provide, then build the hull around what you find necessary. (I spent a fair amount at the Gorge on my shorty AHD flat bottom, square rails) The nice thing about this is you can experiment real size. Just don’t get cut on the foils. And watch Malizia in the southern ocean compared to the other IMOCAs.: