Riding some clean glass conditions for the first time in ages, I had a great feeing of pumping at what felt like the absolute top of the mast. The acceleration on each pump was amazing, and effortless glide. This must be the key to unlocking efficient links.
However, on the Axis 880 HPS, I found that when I inevitably breached because of
pumping so close to the surface, the ventilation would persist. The recovery was sometimes viable, but then impossible to regain any real rhythm from all the effort. It felt as if the ventilated wing stayed ventilated for at least another pump.
I wonder if this is the next frontier for better foil design.
Water testing in the biggest tank in Europe has refined the H-series tip shape to allow the tips to breach and re-enter without cavitation.
Kite race foils spend a huge amount of time tuning wing tips to shed ventilation, as this avoids high speed crashes.
(afaik cavitation is a different thing, water evaporation due to low pressure, wonder if that is a mistake from Cabrinha. maybe not Cavitation Tunnel - SINTEF
)
Anyone have any experience with high aspect foils that breeze through tip breaches?
Almost assuredly a mistake by the cabrinha marketing team. Surfers are not very smart, and the “cavitation” explanation has been going around since foiling started. With Kai Lenny still saying it. Only foil brained people who actually pay attention have switched to saying ventilation.
For axis, the ART foils are significantly better than the HPS or BSC in the ventilation department. notice the noise change more than the feel when breaching a tip on the 999.
I think I agree on ART, especially while turning and loading the inside wing, the outer tip popping out is barely noticeable, as long as you have sufficient speed.
However at slow speeds and lower angle I don’t think I’ve ever breached a tip and recovered, while I noticed on the HPS I was able to recover twice, and could feel the ventilation sticking to the wing and then releasing (I think…), granted the HPS is pretty dated and simple relative to the ART, I presume the updated version would address this.
On the Axis PNG yes no problem, but it feels pretty much like a low range truck while pumping, just grinds on. HPS slightly less but for the speed it works. ART is as you describe, unless flying along if the tip breaches then I go down.
Maybe you are thinking of wing fences? Similar. F-one has used them on their tails, but I think would be too fragile for main wing. I’m interested in trying it DIY on the HPS, but I don’t really know how much they are designed to work in conjunction with ailerons on a plane. Also they seem to address a different issue (spanwise flow?). However I’ve seen them pretty often on the mast of offshore sailing foil boats, so they must have some value
On the other general designs being applied, here are some pics, seems that some deflection at the tip is where this leads:
I’ve ridden the new Cab 1050 and 850, they breach a lot better than any foil I’ve ridden, you hardly feel it, of course, it depends on the angle the wing comes out at.
I think beach recovery is ok on most foils now as long as you have power from either a wave or wind wing or kite. Especially if breaching while turning. I’ve never recovered from a flat breach while pumping though. Finding that if I pump too hard while riding high on the mast you can “plunge” the foil where even if you don’t breach, pushing down too fast and sucking air down to the wing? Anyone else feeling that? I guess the solution is to glide a bit longer or angle down a bit more before initiating a pump?
I agree - I think most newer foils handle tip breaches as long as you have speed and you’re in a turn. Breaching when pumping is a different thing as the wing is more or less parallel to the water surface so you’re getting a full ventilation - if that happens to me I pretty much always fall no matter what foil I’m on.
I’ve had an idea about putting nodules, little aerodynamic pea sized bumps, on the trailing edge of a foil that could slightly protrude beyond the edge of the foil. I should experiment the idea with balls of wax. In theory, they would allow air to accumulate and dissipate from these points. I’m not good/consistent enough to test their effectiveness but the idea makes sense as it creates a type of venturi point to dissipate air during a ventilation event. I don’t know if it’s the same as an anti shock body, but it seems close. In my mind it would work more like a static wick, but either way I don’t know why it wouldn’t work.
Ventilation has 2 main causes: flow separation and access to air.
I down curved wingtip will set the tip more parallel with the surface of the water, this will give air access to the low pressure side of the foil and make it easy for flow to start separating from the surface at the top surface as it breaches.
A straight or up-curved tip breaks the water at a more perpendicular angle in a turn or riding upwind. This means separation is less likely to occur.
Scratches or uneven finish on the foil can cause flow separation
Certain types of airfoil recover better than others, these are usually bottom-loaded airfoils with a fairly even top surface pressure distribution. When designing, we can control the pressure distribution at the tip to help with ventilation.
@Jonnpow Yeah I get the plunge on ART for sure, it needs little very specific pumps otherwise is drops out.
I have now spent enough time on the HPS 880 to describe the breach as “not great”. The worst is on the bottom turn of a steeper wave, if the tip breaches, even with the power of the bottom turn loading the other wing tip, the ventilated wing seems to get stuck. It feels as if the radius of the turn suddenly increases quite significantly and so the feeling is as if you’ve lost a bit of lift or don’t have as much grip or you’ve stopped turning.
Per @KDW point, it has curved down tips, which I guess could explain it.
Thanks for the explanation Kane. Was this something you deliberately designed for with the Reedin foil? Still hoping to test ride one this Spring. Do you think this is a mostly solved issue and just a case of application across the brands catching up? I suppose it is pretty well established engineering problem.
Good ventilation resistance is a “need” in any foil I design. The wing tips on the Reedin aren’t designed around efficiency, they are meant to have forgiving ventilation.
The phenomenon of ventilation is hard to model and predict on a computer and is more complicated in other applications like masts, where the relation between pressure distribution and ventilation isn’t as straightforward.
Post-ventilation recovery in cases where the entire wing becomes aerated are also a bit weirder and there is a case to be made for not having a foil recover too quickly/aggressively.
If you have an existing foil and want more forgiving ventilation, here are a few things you can do:
Inspect the surface- look for flat spots on the leading edge, “orange peel” in the finish, scratches (especially on the leading edge and top surface), raised graphics that you can feel with your palm.
Re-finish- you will treat your foil differently depending on the severity of surface imperfections. I would start by filling any scratches with bondo or similar filler compound. The finer the better. You can then pick your sandpaper grit to start with. After filling I do light, careful sanding with 220 grit and soft block (sponge) on the bulk of the surface and use no lower than 320 grit on the leading edge. I am careful not to change the shape or create flat spots when sanding the leading edge. Remember, it’s easy to take material off but it’s hard to put back on.
If filler is not needed, I start with 320 on the whole foil and a higher grit on the leading edge. The goal of sanding is to even put the surface and remove “orange peel” and graphics edges. I usually wet sand to prevent the paper clogging.
Once the surface is clear of imperfections, my preferred finish is around 800 grit. I will sand to around there then clean the surface with a “magic eraser” sponge and a microfiber rag.
Most of the difference will come from removing imperfections. I have massively improved ventilation resistance by hand sanding with 220 on the beach. (Don’t do this).
I’d love to hear people’s experience with wing fences, they have been shown to work but most likely come with a drag penalty. My guess is they’d need to be fairly high >5mm and the placement probably around 10cm from the tip. A thick rubber band with a little hot glue to hold it might work, maybe 3m double sided tape too. I’ll do some experiments!
Thanks for the details. What do you trade off against with the ventilation resistance? I guess you deal with more roll resistance.
Do you not go higher than 800 on the sanding because of the relatively low speed? I know the kiters go up towards 2000.
I may have a go at the fence at some stage. Surprisingly little fence+hydrofoil research, but I suppose if it creates a barrier to prevent the spreading of ventilation, then that would be beneficial.
