Which method/tool to compare two very different wingfoil boards?

Hello,

I would like to compare from hydrodynamics perspective two very different wingfoil boards: short compact one and long slender one.

I'm interested in their behaviour in the displacement mode, before take-off. Which speed can be achieved in displacement mode to start getting lift from the foil, in calm water and light wind conditions (5-10 knots). I would consider the board staying flat as buoancy is sufficient not to sink in both cases. This is for sure a bit overkill to select a new board, but it looks like an interesting use case to learn more about hydrodynamics (more used to model planes).

Which method / tools could I use to estimate the speed for a fixed driving force for those types of boards, as a "first order" (no need yet to go for CFD I hope) ?

I had a look at hull speed and the length difference does not makes a that big improvement on length speed.

Use case:
- AFS fly 5'3: 160x69cm, 95l
- Takoon escape air: 218x52 125l

links to models for ideas on shape:
- Escape Air https://takoon.com/products/escape-air
- AFS FLY - Planche WING FOIL & SUP FOIL - Shop Usine https://afs-foiling.fr/produit/afs-fly/

Best regards,

JMF

 

The longer board will go faster - easier in displacement mode (lower speed).
The shorter, wider board will go faster -easier at planing speeds (faster).
But this matters little once lift-off is achieved.
I could find no hydrofoil info or pictures on the two websites
you provided.

Can't you test ride these two for comparison?

 

Wing foil board planform is too dependent on other things like operator weight, operator height, surface waves, speed at lift-off, etc. There is never 'one' perfect shape for any multi-variable hydrodynamic problem...it is always "it depends..."

However, on the flip side,.... I see the perfect start for an advanced degree paper. Come on! lets drag the wing foil board industry out of the dark ages of superstition and trade secrets! We collectively see so far today because we stand upon the ever growing pile of research documentation...and the giants who had the insight to investigate those niggling corners of the design envelope.

 

Hello,

Thanks to have taken time to answer. I have the AFS fly and wonder about a long slender one for light wind. Nobody has such board yet around in my club, so no testing possible. This compact board is difficult to have move before take-off (not surprising for a compact planning hull).

Sure that the question can be complex if someone wants to take into account all aspects. But I only want to deal with the simplest level of analysis to help have first values: flat water, board flat, no consideration of the hydrofoil... I just want to compare short and wide vs long and slender (5" barge vs 7" canoe) and evalutate the speed improvement of the slender board for the same force (5%? 20%? 50%?...).

For planes (RC ones or real ones), there seem to be plenty of tools for such comparisons: AVL, XFLR5, PredimRC, OpenVSP... But I have difficulties to find similar tools for ships. Hence my question. And I'm found of useless intellectual excercises. I know that figures I'll have will be "wrong", by maybe I can get correct order of magnitudes.

What I have seen yet:
- Michlet, but the learning curve looks steep for my use case,
- Gene-hull tools and associated VPP, but I don't know if I can model with those my boards,
- I tried to look at Python VPP frameworks, but did not found very relevant suff
- Not open source but dedicated tutorials SURF HYDRODYNAMICS https://www.surfhydrodynamics.com/en/swd_tuto_hydrodynamics.html (but as price is 1/2 of the other board, I would bo to board testing instead ;-)

JMF

 

The cheaper board is likely the better value.

~0.001% difference.

~0.0001 order of magnitude difference.

Your question is so academic it's ridiculous.
What matters is how effectively the designer/manufacturer has matched the board to the foil
and how that relates to your winds and sea conditions where you play.

 

Michlet is quite good at analysis and optimization of long lean displacement hulls; won't do squat for planning hulls.
FWIW IMO aircraft work in one medium while boats work in two with a multitude of variable interactions. Analysis of aircraft are challenging enough in one medium, two mediums with wildly different scaling factors; little surprise there's not much available. Build and test is more fun and productive.

 

Thanks for the explanation. So Michlet OK for the "Canoe" like board but not the other one. Understood for the testing compared to the simulation.

JMF

 

Please allow me to disgree for the specific case of the phase before going on the foil (displacement mode) in light wind where you don't have enough power to easily go in planning/foiling mode. This is the situation you face each time there is not enough wind, not enough rises, you've taken (or have) a too small Wing (sail). Especially if you leave in a place where the thermal is at the limit of where you can take-off on the foil. No fly = no fun, so you need to reach flying speed.

So a "canoe" should not behave the same as a "barge" in displacement mode.

Yes it may be ridiculous to try to calculate the difference instead of testing/buying a new board... Except if learning hydrodynamics is part of the pleasure and an hands on exercise is there

But I think that I have my answer: no open source tool to "easily" get rough estimates for resistance in displacement mode for the two types of boards I have in my use case.

Best regards,

JMF

 

Even if an open source tool that could accurately handle the speed range and hull forms of foil boards, the problem of the changing load on the hull with speed and angle remains. The bare hull might perform well at certain loads and angles but these may only be experienced for a short time during the takeoff and touchdown of the board.
Foil boards are very tricky to get right because of the huge range of speeds, angles, and loads the hull experiences. Factor in different foil sizes and stability requirements and it becomes even more complex.

This might be a case where scale models would give you the best results.

 

Hi,

Thanks for sharing. In my case, I was interested to better understand the archimedian part, without even planning mode (to keep the question simple") and I only indicated the foil as context information.

But I take the opportunity of your post to share that my view about how we get "on the foil" changed after looking at . In that video, he tries to compare the paddle way (getting forward motion and speed) and "pumping the foil" alone. I would never have imagined the outcome.

Best regards,

JMF

 

The boards are going to perform equally in both planing and displacement mode.
You won't be able to calculate the difference, it's too small.
However, if you tow each board, with you on board and an inline scale
you could measure the drag at various speeds and graph the results.
This may satisfy your academic curiosity.

In reality, the real variable is the foils and their varying take-off speeds.
You may want a variety of wings to choose from according to the wind that day.

 

I don't wing, but the winger I normally chat to about design is sure that there is a significant inherent advantage in the longer, slimmer design in light winds due to its superior light-wind speed. So the question doesn't seem to be particularly academic.

That winger has been an America's Cup foiler/Sail GP coach as well as a world champ, so he's got a fairly good understanding of foiling.

In windsurfing, the speed difference between "canoe" and "barge" shape below planing speeds is enormous so it makes perfect sense that the same applies to wings.

May I ask whether you foil much?