TheFoilKook, what would be interesting in your original message is the link to every reference numbers. So we can dig up the non-digested AI stuff ourself.
Good call! Added link above - here it is again: https://gemini.google.com/share/839957ec1a1f
Apologies to all that are annoyed with the AI summary - just thought it was a nice concise version for those joining the discussion.
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Sorry to be rude (I realize my comment was pretty harsh). It was an emotional response to an increasingly frustrating world filled with “experts” spouting AI answers. Luckily for me, it is fairly obvious how to sort out real knowledge from AI regurgitation. But I probably worry too much about the circle jerk it kicks off and how it may effect others who can’t tell the difference.
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At work I would say “If you can’t google what we are doing we are on to something either really good, or really bad.” Case in point: You aren’t going to find any studies on Tubercles being used on a rigid 3 foot hydrofoil being used in a speed range of 10 - 20 mph with no control surfaces. This vs. an organic 1 foot thick flexible whale fluke that is optimized for 0 - 10 mph.
Our foils rarely ‘stall’. Stall is what you feel when you’re on an 800 cm foil, run out of steam pumping, glide briefly and the nose of the board slaps down at ~ 6 mph.
All the discussion of the Tubercle benefit seems to be the speed and efficiency at speed, at least that’s what I gather from the AFS users and hype. I am skeptical but have to recognize the users who are great foilers and hauling ass with the foils aka empirical evidence supports the design. Time will tell.
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You are depriving tubercles-style foils of a major benefit: takeoff speed. At takeoff it always stalls, every time. And if tubercles can lower the takeoff speed, that translates to being able to use a smaller foil having equal takeoff speed. And a smaller foil can certainly be more efficient. Stall speed is an extremely important factor in foil design. We should probably compare a 700 cm2 tubercles foil to an 800 cm2 plain model with equal span.
That said: I still have not read any evidence that the cost in terms of added drag is sufficiently outweighed by reduced area to improve total efficiency. It may have benefits for control in turbulent water, but until I see it win races I won’t believe tubercles improve efficiency.
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My rudimentary understanding is that the tubercles play more a role in lowering stall speed than shedding ventilation. In theory they are channeling flow over the wing into strips. This accelerates the average speed of water over the wing, increasing the pressure differential and thereby lowering stall speed.
Replying to my own post here - If the tubercles are accelerating flow by diverting and channeling the water into strips, does the average pressure differential actually change over the span or surface area of the foil? It makes sense that the pressure differential would increase in the places where the water is channeled, but in between those areas wouldn’t the pressure differential actually drop? Doesn’t this channeling just even itself out, or am I missing something here?
If this is true, it makes me think that maybe the tubercles don’t lower takeoff speed or low end, and maybe they just smooth out turbulence or something like that.
Im way over my skis…
I really don’t know for sure, but I think channeling the water helps hold off early stall by blocking the “propagation” of stall. Stall happens when flow separates on the top-side because the angle of attack is too big. I guess wherever stall initiates on the top side of the leading edge, it’ll spread along the span from there. The tubercles act kind of like wing fences in that way keeping any local stall as a local phenomenon. I think they could potentially also help with recovery from tip ventilation/breaches. The AFS Silk is pretty great at tip breach, but plenty of foils without tubercles are too.
I don’t really agree that stall is happening on take-off. I think we’re not really in that mode on take-off. Usually on take-off we have a fairly low angle of attack and need to get up enough speed to make lift at that angle of attack. Otherwise the foil and the board would be angled nose-up plowing through the water. Later after we’re on foil and we slow down more and more, we naturally increase the angle of attack in order to maintain lift at that slow speed, until the foil stalls and we drop.
At the end of the day, IF tubercles could indeed help lower the stall speed, then we could potentially ride smaller foils which have less drag and are more efficient. In practice, I haven’t seen that I can ride markedly smaller AFS silk than other similar aspect ratio modern foils. BUT - I have noticed that the Silks do NOT drop out of the air like some other foils. They gradually just lose altitude until the board touches down. So who knows maybe it is working?!?
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The confounder for me is the Enduro, it feels similar to most foils. If I had only ridden the Silk I would swear blind that tubercles are the best, but the Enduro is good, but not particularly good.
I’d love to see someone do a proper test on two similar production hydrofoils
fwiw, asking AI (ChatGPT Pro) to do research to find examples of it not working was interesting.
Here is the full doc if you want to have a look and save yourself a few $ of tokens, it is somewhat compelling
It only managed to surface a few, including this foiling boat research which 3d printed some tubercles and tested the impact, with no effect.
That’s a semantic argument: of course in reality you learn what speed your foil needs to lift you without stall so you’ll take off only after you reach that speed. That does not change the point I’m making: if a foil design has lower stall speed → you can take of earlier → you could use a smaller foil with that design and equal takeoff speed.
I think dropping at once is a good thing. It means the whole foil lifts right up to stall. If it drops gradually that means it’s lifting only partly and there is missed potential for earlier takeoff or higher end speed.
It makes a few good arguments, and also a few very poor ones, you probably did not tel it the speed and use case of the hydrofoil, so it added research about cavitation and reynolds nr scaling that are totally irrelevant to us. I agree with most other arguments there, including that there are many more subtle things one can do to get similar results.
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This is the engineer talking, not the “surfer/lover/feeler/groover”, which invariably leads to horrible foils for general surf use. Listen to Adrian from Axis talk about all the engineering led development and then ask a surfer what they think of his foils and they mostly don’t love them because they are so abrupt and “efficient” (notwithstanding the newest one which I haven’t tried). If you design a foil for maximum efficiency, you maximally ruin the surf experience, which is Jondrums point. I confirm this by “ruining” a few stabs, and they are without doubt much better for riding waves, which is also what the Silk is designed for. Smooth, predictable, stable, fun. This is where the tubercles make me wonder that there are actual benefits.
Yes I shared the thread verbatim. I just now asked a series of follow ups which reduces it to the one’s you’d expect, and then refined in on surf specific, and asked for balanced take not just criticisms, which seems relatively balanced and aligned:
The most honest balanced conclusion for surf carving
- If your priority is maximum glide / pump / top speed, tubercles are a risky bet because drag penalties are common and documented in full-scale tow testing of at least one configuration. (Strathprints) [The moth study]
- If your priority is control in turbulence + robustness to breach/ventilation + smoother unsteady loads, there’s a credible, evidence-backed story that tubercles can help by:
- attenuating drag-force fluctuations in wave-driven unsteady flow, (arXiv)
- moderating dynamic stall (less impulsive vortex-driven events), (Springer Nature Link)
- and weakening/diffusing the tip vortex (a known air-ingestion pathway). (arXiv)
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