cutouts wingfoil boards

[Schietwedder]

To clear up some of the hydrodynamics....

A floating body has two resistance types....

-Wave drag
-Friction drag


There is more which play a minor role... wind drag, drag due to slamming/sea state, shallow water and so on, not relevant here.

-Wave drag is the energy you "waste" by creating waves. Important for displacement (canoe body type hulls at higher speeds). Google Wave drag froude number and so on.

-friction drag simply the friction by the water floating by. A variable of Re number (Speed/body length/viscosity) and hull rougness among others. At low speeds a round (circular) body has the lowest friction drag because a cylinder/sphere has the least surface area per volume/weight. (See Kajak or rowing boat)


Displacement mode: The sum of all pressure and friction forces of the hull act in horizontal plane against the direction of travel.
Planing: There is a net positive pressure under the hull. Some of the force acting on the hull has a vertical dynamic component counteracting the weight of the craft. The body will vitually weight less, mostly associated with less wetted surface area, less friction. Also the wave-drag reduces.
It is very much like a hydrofoil which only produces lift with its pressure side. (60% of lift is created by the suction side though) so a very inefficient way to lift something out of the water.

Planing mode is not an on off phenomenon. Planing speed is sometimes referred to when the whole weight of the body is supported by dynamic lift.
If that planing speed is at 10 knots, it does not mean that the boat does not plane at 8 knots. it planes only partly.
(Lift is increasing roughly at the 2nd order of speed though)

Some factors are helping planing. One is the length to beam ratio of the body. Plumb bodies (low L/B) plane easier than canoe bodies (high L/B) which practicallly dont plane at all.
Another is flat underwater shape. The best shape is totally flat (all positive pressures on the hull bottom act normally on the surface, here upwards.
V shaspes like on motorboats help dampening the slamming impact and are better for heave and pitch stability but less efficient because positive pressures on hull also have a lateral component (directing water sideways) The lateral components left and right side cancel each other but get lost from the vertical lift component so to speak.

The V´s or Bavvels in Foilboards have nothing to do with planing but more clearance and less impact when you ride with the board edged very hard.

So as a result there´s two philosophies also in americas cup boats for example:

A hull which lessens the drag by partly planing before take off speed. It also lessens the drag of the foil in some parts of the acceleration curve as it needs to produce less lift for a given speed. BUT
A common missconception is that the hull by partly or fully planing taking weight off the foil in acceleration phase helps with foiling. This does not work obviously as the board leaves the surface the foil has to carry all weight anyways. If it helps with foiling it is because the drag at same speed is less or the speed for a given drag is higher by the planing of the board. (drag forces act in horizontal plane not in vertical like the lift coming from board/hull and foil)

Second philosophy:
A hull which has very low drag but not creating any dynamic lift (look at a moth dinghy or americas cup catamaran that virtually can´t plane at all given their shape). Low drag let it reach higher speeds with less driveforce having to be created by the sails/kite/wing.

So you can discuss for days to come, you will only find a true answer if you have the take off speed of any foil and compare the amounts of drag of different types of boards at this speed. The board with less drag will fly earlier given board and rider are same weight How this drag is to be minimized works in multiple ways.

[Peter_Frank]

Nice summary Schietwedder, and agree.

The bottom line is, that we most often dont get up foiling "slowly", meaning the low drag of the board itself whether planing or not, is only one parameter.

We usually pump the board and foil, in order to get up foiling when low power, and now a very different set of aspects and parameters are introduced, a lot more complex.

So practical tests are needed.

And as we move around this topic where almost everything works, to some degree at least, there will be loads of opinions and theories.
Which can be both wrong or right so there MIGHT come better solutions.
But they can also have so low impact, meaning "keep it simple" is better, unless the brand want to have a sales parameter/gimmick.


Peter

PS: high and low L/B is Länge/Breite (on your backbone of course) and is called Length/Width in English.

[Jan:)]

The bottom line is, that we most often dont get up foiling "slowly", meaning the low drag of the board itself whether planing or not, is only one parameter.

We usually pump the board and foil, in order to get up foiling when low power, and now a very different set of aspects and parameters are introduced, a lot more complex.

I am not sure, if that changes the situation much.

The pumping is more off an offset.
For example, if you need 15km/h to stay on foil, you maybe could pump yourself up at 10km/h speed.
But even if you manage to pump yourself up with just 5km/h, you simply drop right back down.

So you still need to reach that lower speed for it to work and board design with low drag in the water, has the same advantage as without pumping.

[Peter_Frank]

The bottom line is, that we most often dont get up foiling "slowly", meaning the low drag of the board itself whether planing or not, is only one parameter.

We usually pump the board and foil, in order to get up foiling when low power, and now a very different set of aspects and parameters are introduced, a lot more complex.

I am not sure, if that changes the situation much.

The pumping is more off an offset.
For example, if you need 15km/h to stay on foil, you maybe could pump yourself up at 10km/h speed.
But even if you manage to pump yourself up with just 5km/h, you simply drop right back down.

So you still need to reach that lower speed for it to work and board design with low drag in the water, has the same advantage as without pumping.

I still think it is way more different.

The pumping is nothing like on say a windsurfer, where you pump for forward speed only, to get planing and then let the apparent wind do the rest.

Wingfoiling you pump the board aggressively up and down, together with the (foil)wing, in order to get the surface lifted out of the water, while getting some forward speed and lift.
If you see this extreme motion of the board, you will know it got nothing to do with "gliding" up into planing, but other factors prevail.

Dynamics involve a lot more complex interaction where width and length and weight and so on, means a lot more than minimum drag for gliding.

Thats what I think at least, might be wrong.

Peter

[Trent hink]

The thing I was thinking of, is with a flat bottom board there can be a lot of resistance if you aggressively pump it up and down, but there can be lift if you pump the board just right at a high enough speed...

With a super narrow hull, sort of like a rowing scull or sailing canoe (but obviously shorter) aggressive pumping would have much less of an effect, possibly allowing the foil to work more efficiently.

In American English, the technical term for a boat's width is "beam," so L/B makes perfect sense to me, and also thanks for your comment, Schietwedder, I think you have a very firm grasp on the direction future development needs to take!

[juandesooka]

So you can discuss for days to come, you will only find a true answer if you have the take off speed of any foil and compare the amounts of drag of different types of boards at this speed. The board with less drag will fly earlier given board and rider are same weight How this drag is to be minimized works in multiple ways.

Thanks for this clear explanation. So if I have you right, a shorter wider board with a flat bottom should minimize drag and maximize the sub planing lift, which should them assist with early foiling. Correct?

What is your opinion on concave or double concave bottom profiles? My very basic understanding is that in surfboards these features can reduce drag and increase lift, but these only take effect once at planing speed. Or maybe they assist with sub planing lift? If not, then presumably they add drag which would then not only not help but detract from early foiling. The more extreme double concave can approach a v hull, which you said pushes some of the lift forces horizontal instead of vertical. These features may have other benefits but seems questionable if they're helpful for lift-off.

[Jyoder]

I’m building a big 150liter light wind board. Full flat rectangle prism with wedge cut out of front. Straight lines, flat surfaces. We’ll see how it rides. 66x27x6in.

[juandesooka]

I’m building a big 150liter light wind board. Full flat rectangle prism with wedge cut out of front. Straight lines, flat surfaces. We’ll see how it rides. 66x27x6in.

That reminds me of that joke a few years ago, about someone stealing and sharing Kalama's newest board design profile:

rectangle.jpg (9.8 KiB) Viewed 611 times

Joke doesn't work any more though, as the boards are no longer big chunky rectangles. Quite the opposite, the downwind Kalama boards are getting so narrow and pointy they are bordering on becoming stand-up kayaks.

Your board looks simple and simple is good. Personally I believe much of the board design is gimmickry. 150L is pretty big though. A relatively athletic medium size person, 120L seems plenty for being able to balance, even in choppy seas.

[Trent hink]

So you can discuss for days to come, you will only find a true answer if you have the take off speed of any foil and compare the amounts of drag of different types of boards at this speed. The board with less drag will fly earlier given board and rider are same weight How this drag is to be minimized works in multiple ways.

Thanks for this clear explanation. So if I have you right, a shorter wider board with a flat bottom should minimize drag and maximize the sub planing lift, which should them assist with early foiling. Correct?

What is your opinion on concave or double concave bottom profiles? My very basic understanding is that in surfboards these features can reduce drag and increase lift, but these only take effect once at planing speed. Or maybe they assist with sub planing lift? If not, then presumably they add drag which would then not only not help but detract from early foiling. The more extreme double concave can approach a v hull, which you said pushes some of the lift forces horizontal instead of vertical. These features may have other benefits but seems questionable if they're helpful for lift-off.

My interpretation of Shietwedder's post was,

We don't know for sure yet what works best, but with careful and logical testing we should find out soon.

I see and also hear that the winging gear keeps getting better and better, and the devout wingers I know tell me that getting good gear that worked well made all the difference in their progression.

I'm sitting out at least until the good gear is available cheap and used.

It's possible that I won't have long to wait.

But then again, this faction of the sport is still in it's infancy, and perhaps ground-breaking developments are yet to come.

[bragnouff]

Keep in mind also that some board designs are meant to match some specific foil designs.

The big kick tails of a couple of years ago made sense with the large thick low aspect foils from that time, that could create peaks of lift at very low speed but through big AOA. Pumping up and down like a ollie movement was effective. (for example the Maliko 200). With thinner higher AR foils now, using the same pumping technique to get on foil doesn't work that well, technique needs to be adapted to keep the board kind of flat till you build up more speed with the wing, then some faster smaller amplitude pumping action on the foil to achieve liftoff. Too much AOA too soon and all the efforts are ruined. In that new context, the kick tail doesn't really help, and if it adds displacement drag and affects the build up of initial speed, then it makes sense for it to disappear.

So the latest board on an older foil may not work too well, and vice versa...