I remember a long conversation I had with an old timer who does the U.S. scuba show circuit as a manufacturer. He said the guy who owns the split fin patent took a pair of regular fins and using a scissor, cut a split down the middle of a pair of fins, and went around scuba shows talking with manufacturers trying to get them to license the cut. They all laughed at him as they should have. Now most companies license the cut.
Fins are displacement devices. The more you displace at a given period of time, the faster you might go, but the harder it might be, all tradeoffs. You can increase the displacement by making a fin stiffer to a point, by making the surface area larger to a point, or a combination of both.
You can decrease the displacement by making the surface area smaller, the fin more flexible or a combination of both. You can also cut a split in the surface of the blade. This does not accomplish anything different than either of the above two, it's just simple physics.
But the guy who owns the patent is laughing ALL the way to the bank!![]()