I saw a friend at the beach yesterday and he had a really wild looking fin in his longboard. He has been riding single fin longboards for more than a half century, and is usually pretty conservative, but the fin he had in his board was the strangest thing I've ever seen. I didn't talk to him much about it because I had just gotten to the beach and he was leaving. The surf was solid overhead maybe 3 to 4 feet overhead on the sets, so it must work OK. He said he liked it.
It looked like this with long wings coming off the middle of the fin but it wasn't this fin. I searched all over the internet and this was all I could find.