fin edge: sharp, sharp, sharp!

I have a fibreglass fin (for a single fin midlength) that was modified and which now has a VERY sharp trailing edge. I haven’t tried shaving with it, but it could easily do some dicing. It kind of makes me nervous. It’s a great fin, and while I realize that water might flow off with a bit less efficiency, I’d like to moderate the sharpness in case something untoward occurs in for example, shoulder-high whitewater. Thinking of sanding or running a fine grit rasp down it to soften it. Is there a point at which dulling might introduce a problem with flow or a side effect such as fin hum? Thanks in advance.

Hello Moss,

From what I have heard, a sharp trailing edge is not the best for hydrodynamic efficiency. A small flat on the trailing edge is supposed to be better. I have heard that windsurfers use a 1.5mm flat on the trailing edge for high speed work, but I am not sure what the optimum is for a surfboard fin. I use a 1mm flat. What you shouldn’t do is just round it off.

Regards, Roy

Yeah, sand that thing down before you lose a couple toes. Sharp fins are no joke! I caught a fin in the back of the calf, the fin was dull, didn’t even cut my wetsuit, and still opened a gash that took 30 stitches and 10 staples to fix. A sharp fin would have cut my leg off. I would do it before you surf again…

Theoretically you lose a microscopic amount of efficiency, but you’ll never notice. Use 100 grit for the shaping, holding the paper in your hand, blocks or files are bad for the inside curve, you’ll put nicks in the edge. Put a nice round edge on there, until you can press the ball of your thumb on the edge quite hard without discomfort (don’t try this at first and cut your thumb off, however). Avoid flat spots, you want a small, smooth radius, on the very edge. Then, cycle up the grits to 1200 or so, and rub for shine. If it’s a really nice glossy fin, consider taping just inside the rear edge on both sides with masking tape, so that you don’t scratch the gloss with the sandpaper.

Prolly some of the other guys could put a finer point on the subject, that’s just how it gets done in my backyard…

wells

I’m looking forward to what the engineers, theorists, and backyard builders like myself have to say about what the optimum trailing fin edge should look like.

When I finish out my fins I like to bring the trailing edge to about the thickness of a 6oz lamination and use progressively finer grits of wet/dry paper to bring the two surfaces of the fins together into a rounded trailing edge. I finish by taking the sharpness off the trailing edge with 320 grit and even though I round them off they are still dangerous. Of all the things I worry about when it comes to fins it’s that trailing edge comming back at you. As a result I always use a cord that’s a little on the long side and when I wipe out I stay under water until I know the cord has finished recoiling. A fine trailing edge is good but one that’s too fine is excessive and dangerous, so take that sharp edge down some.

Good Surfin’, Rich

Hello Halcyon,

“In order to achieve smooth flow off the trailing edge, the rear part of the profile is slightly concave. This ensures that the two surfaces are approximately parallel near the trailing edge. It should be noted that current thinking on lifting foils (such as aircraft wings) even favour slightly diverging trailing edge surfaces”

This Quote comes from http://www.activesportsboats.com

And here is a picture of the foil section http://www.activesportsboats.com/Images/pictures/shots/foil1.jpg

The idea is, I believe, to keep the flow from the two sides of the trailing edge separated as they leave the trailing edge, and to encourage the two to converge gradually by not bringing them together suddenly.

A diverging trailing edge ends in a square section (any roundness on the trailing edge is a converging section with an inefficient accelerating curve)

While we’re on the subject, what about the leading edge? Is it ok to take a sharp edge off there as well? I have a board with a set of ginsu knife-like fins that give me the creeps every time I paddle out. I already have a mean looking scar on my foot from contact a couple months ago.

We’ve found that if the squared trailing edge is thicker than .060" on a 4-3/8" to 4-3/4" base fin the fin will have a greater propensity to hum. Consequently, we attempt to keep them under this dimension. Yes, all hard fins are sharp and dangerous as a rule. It’s always a trade off, performance verses safety. I always recommend tuning your leading and trailing edges to dimensions you are comfortable with and then realize given enough time in the water everyone gets cut. Even the soft urethane keel fins we make for surf school soft boards will cut you if you get hit hard enough.

I recall reading about some tow-tank lift/drag studies carried out on centerboards and rudders for hi-perf sailboats back in the mid-60’s. As best I recall, they found that a (moderately) sharp trailing edge gave the least drag if the centerboard/rudder was not generating lift. However, the greater the lift generated by the foil (via an increase in the angle of attack), the more trailing edge had to be removed to achieve the maximum lift/drag ratio (as I recall, after removal there was a squared-off rather than a rounded trailing edge) . Unfortunately I don’t remember the numbers in terms of chord reduction (or the thickness of the trailing edge relative to the maximum section thickness) that gave the best lift/drag ratio as a function of angle-of-attack.

If the edges of your fins feel too sharp, they are. I’ve seen some fins that are like knife edges, VERY STUPID. Sooner or later you’re going to meet up with your fins in a way that’s personal and painful.

I de-sharpen fin trailing edges with sandpaper rolled over a dowel. Never noticed ANY difference in performance after doing this, either.