Blunt vs sharp edges on fins

[ConcreteVitamin]

Howdy everyone. Been a while.

I have two fins where the leading and back edges are blunt, not sharp. Other fins are sharper.

If the foil of two fins are roughly equal, how would a blunt edge compare to a sharper edge? Does it come down to the skill of whoever foiled the fin?

(All single fins)

[Naeco78]

I remember reading somewhere that sharper edges release water.. and softer/rounder edges wrap water around it and create drag

[waikikikichan]

Since both of your fins are roughly equal ( in size / volume ), which performs better for you ? The one with the sharp trailing edge or the one with the blunt trailing edge ?

[ConcreteVitamin]

I have yet to surf it. I suspect the blunted (both trailing and leading edges!) one is indeed more draggy.

[oldmansurfer]

If your fin was like a rectangular block that was a flat surface on the leading edge and trialing edge rather than rounded or sharp then you would have maximum drag but once you foil it out a bit there is probably not too much difference between minor changes and for your average surfer most likely no meaningful difference. This is just a guess however trying it yourself might make a better guess.

[waikikikichan]

Well since you haven't tried both fins, I'll take a guess and think you will like the "blunt" fin better. Reason being is it should be more forgiving and user friendly.

Are you looking for out right down the line speed or ease of turning ?

[HaoleKook]

Sharp trailing edges are more responsive, blunt trailing edges are smoother (forgiving and user-friendly as mentioned above.)

Water flowing along a surface and coming off a sharp edge continues to travel straight, so all the flow is in the direction of the fin and thus it's very responsive. With a blunt edge, turbulence and recirculation forms behind the fin and the water separates from the fin a little earlier, so some water is now moving in directions other than along the fin thus decreasing responsiveness, and increasing drag as mentioned.

Imagine a big airplane: it needs an enormous rudder for a worst-case scenario turn, however, moving that giant rudder even just a little creates tons of force and makes the plane feel unstable during normal flight. By thickening the trailing edge it becomes much less sensitive around neutral, but at full deflection still has 97+ % effectiveness. This would answer the question "why not just use a smaller fin?"

I would say (as an aerospace engineer) that at the speeds we surf at, leading edge shape is less of a factor. Water approaching a fin usually coming from some side angle, not directly inline with the fin. The shape of the leading edge has to do with how much of an 'angle of attack' you can maintain without fluid separating off the fin. On an airplane with less viscosity and much higher speeds this really matters. Surfing in much more viscous water and at lower speeds the water will have much more tendency to 'stick' to the fin at a range of angles. And the difference in drag would be negligible.