Hans I totally disagree with you concerning your quote that the sharpness of the trailing edge is not that important.
Are you saying the a rounded trailing edge releases water from the fins surface just as efficiently than an abrupt sharp edge? Maybe I am not thinking about this problem on a scale that is not relative to the scale that you are thinking about it.
I dont know maybe you actually mean what you said but you did not ellaborate on that one piece of information.
@ erikhaha
I believe Hans is talking about the trailing edges of “submersed foils” and flow separation before water gets to the trailing edge.
Very different from rails and bottom contours at the air-water (board-water) interface.
Pay close attention, boys and girls. This is one of those profound, gem quality quotes, about surfboard design science.
You are correct.
All the best
Ahhh, that makes sense.
I’m not sure which traling edge would be ‘seen’ as ‘sharper’ by the flowing water.
But I think the flat cut off version on the right has less drag.
Hi Mik,
A flat end allows to reduce the slope of your trailing edge, pushing the separation point more backwards, however it is the size of the wake (~fin thickness at separation point) that determines the form-drag. An in this case, you can see that this will be larger in your cut-off version.
However, cutting off the sharp edge of a profile without changing the shape is a good idea for many reasons, including reducing drag, as long as you do not cut-off before the separation point. The separation point can vary dramatically in different flow regimes, but it is safe to assume that you can cut of a mm or two from the trailing egde.
I am loving this discussion. Thank you Hans, for starting this thread.
I think I’m going to lightly sand the trailing edge of some fins.
all the best
What if the image on the right had two versions: 1 with the sharp trailing corners (original) and another version with the trailing corners rounded?
Wouldn’t make much difference as they sit in the turbulent wake.
The drawing was just a quick and very exaggerated attempt to show the difference between the trailing edges.
Is the ‘separation point’ a location on the foil or a state of flow?
As in “Stalling = separation point (in time)” vs “The separation pont is 10mm fore of the trailing edge”?
I wonder about the best method to modify the trailing edge of surfboard fins.
How about a round diamond file, held perpendicular to the trailing edge and then moved along the trailing edge from base to tip a few times?
Or maybe use a knife sharpener, but their shape is a bit conical, so a difference between port and starboard edge might result.
Excuse the crude drawing, it is supposed to show just a horizontal section through the trailing edges of two fins. The red line is where a round file might create a flattened trailing edge.
The blue elliptic trailing edge is what I think results when sanding the trailing edge by hand with fine sandpaper on a finger.
The very pointy green trailing edge is of course dangerous and should be avoided for that reason alone, but would also be improved hydrodynamically by creating a flat cut-off-edge, is that correct?
I glue a piece of 180 or 220 grit to a popsickle stick and keep it perpindicular to the trailing edge in order to blunt the t.e.
It would be nive to quantify the effects of a flat t.e. versus a rounded or tapered t.e.
I use a razor blade to fine tune trailing edges when the fin humms or whistles, but never intentionally to make the trailing edge a sharp square.
Seems those sharp corners would be more dangerous compared to a rounded dull rear edge and might assist in drawing blood or breaking stretched legropes.
I think the shapes of the widely accepted and apparently unquestioned standard fin designs could be only slightly altered to negate the extra trailing edge drag from a dull trailing edge.
The latter, “The separation pont is 10mm fore of the trailing edge”, where 10mm differs depending on the flow regime. There will always be separation on a foil due to friction losses in the boundary layer (to understand this I refer to wikipedia, but probably you’ll need some fluid dynamics theory to fully get it: Flow separation - Wikipedia
But the main message is that the form or your object after the point of separation has negligible influence on it’s performance. So it doesn’t really matter if it is round or square.
If you tune up the trailing edge in order to reduce hum, then the rounded edge may be creating additional drag/eddy currents that cause the fin to resonate. So maybe bu tuning up the t.e. by making it flat, or sharp seperation flow we are reducing those eddy’s /induced drag?
It would be interesting to run some experiments in a test tank with various fin t.e. styles.
I think even with great effort, the squared off edges will be not all that sharp. At least not with my PLA 3D printed fins.
I have a musical fin at the moment and was lucky enough (or rather persistent) to catch video and audio footage with the FinEye 360Fly camera installed in my board. I must back up, render and review the footage carefully before adjusting the fin, in case I want to keep it whistling for more recordings. It sounds a bit like a whale song!
I had a couple of near identical fins that were whistling, and a bit more sanding of the trailing edge fixed it every time. I surf every fin before sending them to beta-testers.
