Tails - Chop, Sizes, Plan shapes, and Hydrofoil section

Keen to learn more about tails. Tail/Hydrodynamics Gurus Please help!

I personally have a UNIfoil 18" Race tail, bought with my new foil on the advice that its a good starting point to chop down to as low as 11.5".

Before I chop anything I want to educate myself on what shape to use!

I’ve done some interesting reading on wing design (mostly aerofoil based from planes) but the collective knowledge on here is worth a lot, as we’re not just talking theory but adding experience and feel in to the mix aswell.

From my reading the eliptical wing “like spitfires” is a very efficient design partly due to reduction in tip vortecies.

Interestingly though, the Unifoil race tails are very square in plan shape, and few tails are using this eliptical design, despite the theoretical benefits.

Is it because the tails are becoming extremely thin in profile that they can get away with these shapes with minimal losses?

Is it true that the thinner a profile the less vortecy will be formed due to lower pressure differences ,top to bottom, as thinner by defult means closer to a symmetrical profile?

Is it common to make the wingtips’ foil more, or completely, symmetrical in order to reduce pressure difference at the tip? Does this work, in effect, as a horizontal winglet? I was trying to work out if this is what they had done with spitfire wings but online resources didn’t help much!

Can anyone explain how you’ve cut and foiled your tails and what your results were?!

I’m a design engineer and toolmaker, foiling has made me find this very interesting. Makes me want to retrain in hydro/aerodynamics!

Happy foiling!

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I’m no guru, but find this fascinating, and am reading up on all the details as you are.

Probably the most important thing to remember (occurring only recently for me) regarding wing design in planes is that they 1.have ailerons which need lift to function, 2. cost limits construction. So you will see designs that prioritise root stalls first to avoid the airflow at the ailerons stalling, and the boxy shapes are easier to construct.

I think for user friendliness you want the gradual stall. Here is a nice clip that walks through the progressive stages of a gradual stall, this is to avoid a tip-stall at all costs, as that means instant loss of roll control (in a plane, same doesn’t apply to a foil with no ailerons).

Both ailerons and construction constraints are largely irrelevant to our foil design. But the trade-off is still there, in our case performance vs stall characteristic. Outright performance the elliptic shape and variants is optimal, but a boxy shape will seemingly have a more progressive stall rather than an abrupt stall. Based on this article

For performance, my understanding, an elliptical shape makes the most sense, with some interesting reading around spanwise flow, crescent elliptical, and the rc crew discussing the varying benefits. Seemingly the most high performance tails are all tending towards varyingly elliptical for the top of the range (KDFoil, Axis). Interestingly, F-one has a tail with fences, a different approach.
Below shows the efficiency of the spitfire shape, discussed in the RC Group:

Commonly applied is washout, where the AoA is reduced at the tip, and profile is more symmetrical,eg the ART range. I’ve got no idea if this is applied to tails, but it makes a lot of sense for main wings, eg below with reduction in AoA and slimmer profile towards the tip


I don’t really understand the design considerations for a tail entirely, as the characteristics are removed from plane design, and so this relies on some expertise and experience to get right :smiley:

lower takeoff speed for the same area, incredible pop, ventilation recovery, and smooth riding through turbulence in surf R Series Tails – KDFOILS

Clearly it make a huge difference, and KDfoils seems to get it right and people seem to agree

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Other places to look for interesting designs are the dynamic soaring planes and the air races both of which optimise for speed.

Edit: also Tubercule, which has a great explanation of stall spreading along the wing. I wonder why only see tubercule on Takuma.
A Whale of an Idea in Wing Design - GridPro Blog

And also, Aerodynamics vs. Hydrodynamics:

Does an airfoil act the same way in water as in the air?
In many cases, yes. Both water and air are fluids, and with the right conditions, the flow will act the same in either. But if the pressure becomes too low due to the flow going fast over the top surface of the wing (the suction side) then the water can boil (cavitate). This cannot happen in air, so that is one major difference. This is one reason that propellers on airplanes and on ships have different shapes. The screws of a ship spread the thrust force over a larger area to keep the suction at each point low so the water won’t cavitate. 1

You mention that there is a point where aerodynamics and hydrodynamics are quite different. What is that point and why?

Wings in the air behave one way at sub-sonic speeds, differently at near-sonic speeds and differently again at super-sonic speeds. Wings in the water operating at low speeds behave in many ways like slow-moving wings in the air. However, as speed increases wings in the water start to have problems like ‘cavitation.’ Cavitation occurs when water encounters an area of low pressure – generated by a wing, for example — and vaporises, thus affecting the lift generated by the wing.

There is the additional issue with hydrofoils that they have a strut (or ‘mast,’ as we incorrectly say) that connects the wings to the board. This strut passes through the surface of the water, which means we need an entirely different approach to evaluating its lift and drag. Froude number, ship hydrodynamics and ventilation can come into play here, none of which apply to wings in the air. 2

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11.5 is super small. I like my race tails around 13-14 depending on surf or downwind. Especially if its the new race which is about 1mm thinner than the older one. New ones have the flat section for the new Uni shim.

For shape, I like a fairly flat cut with rounded leading edge. Maybe 2cm. Since its super hard to refoil the section if you go to aggressive I think its better to keep the molded section as far out as possible. You can even go with a straight cut.


Thanks a lot for taking the time to respond with so much info and so many links! I’ve been through them all and a few more!

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Thanks for that! I was tempted to go around this number, based in the dims of the KD tails. I’ll definitely go around there then, after i get used to the new foils with the full 18" for comparison.

I think mines a new shape one, it fits fairly well with the new shim on.

Pleasure, please share anything interesting! I only have so much time to work, foil and read :smiley:

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I currently have the cabrinha h1000 and use the stock hs180 tail. Looking at chopping it down to 150 for a looser surfing feeling.

Any downsides to doing this? My focus is mainly prone, but I’m just getting into winging and think maybe the stock tail would be better for that?

Option B (much more expensive) would be to get the foil parts rear fuse and another tail (either takuma 158 or some kdmaui).


That was my only worry! The potential winging difficulty increase. I’m not sure how much harder it will be to use, so im going to stay on the bigger side and chop mine to 14" (Not sure on area). It’s currently a very “rippy” shape for the wing material, so at least there will be an improvement there! Sounds like for prone you will be happy chopped. Whats your priority? Mine is wave fun for sure!