I’ve been thinking a bit about fin efficiency lately. In other words, trying to reduce the “wing tip vortices” that occur at the tip of the fin. These vortices are created during a turn when the water under high pressure on one side of the fin “spills” over at the tip of the fin onto the low pressure side.
There are a number of ways to do this and create what is called a “bell shaped spanload”, in aeronautical terms, instead of the usual “parabolic spanload”. Reducing the amount of lift just at the tip of the fin will reduce the pressure differential at the tip and minimise the “spillover” and subsequent vortices.
One way is to twist the tip of the fin (only practical for side and not centre fins). For those who are interested you can read about this here on the following link - NASA research on the Prantdl-D wing.
Another way is to incorporate a “flat winglet” at the tip of the fin (suitable for both side and centre fins). Rapidly reducing the fin area just at the tip is the way to achieve this. “Standard” template surfboard fins are not really adherent to this design principle. Designs preferred/created by some of the Swaylocks luminaries however seem more closely aligned to this philosophy. Wildy’s and Greg Griffin’s fin designs, as well as Bill Thrailkill’s preferred Brewer template are all examples.
For one reason or another this type of fin template seems to me to be little explored/exploited/available.
Cheyne Horan’s Starfin and the Wavegrinder designs addressed these wingtip vortices using angled winglets, but these are more difficult/costly to produce and also introduce some vertical forces which may be undesirable (depending on your point of view).
Have a look at the shape of a dolphins pectoral fin. It is a beautiful example of what I’m talking about. The tip of an albatross wing is another (in a different type of way though).
I have explored a little in this direction so far and found the results to be encouraging.
Maybe Wildy, Greg Griffin and Bill Thrailkill might care to share if this efficiency gain (less turbulence = less drag) is indeed the reasoning behind their designs/preferences for fin templates, or did they have other “things” in mind.