ORIGINAL: bearcave.
Regarding the thin swept back fin all i can say is that when i got into this pastime many years ago Idid enjoythe Ed Deitrich/Bad Billy way of doing things.
If you look closely at a Bad Billy single fin you will see it generates some downforce....bit like a bi-fin with tabs....maybe not so much but it is there.
Hope the pic shows up, we'll soon see. Anyways the Koanda bace on the fin was oridgenaly designed for nose riding long boards. I liked the idea of the winglets on the bi-fin keeping the tail in the water, cutting the winglets off helped stablize the board but spinn outs were more commondbut at speed and a little turn the fin seamed (more so with the wingletts)to flex and theboard would squirl out turning anyway it wanted to go (out of controle) and aftore breaking a bunch of stems I desided a newdesign and unbreakable fin was in order as a result of many bace designs the tail stayed in the water andthe board became stable and with alot less weed and trash pick-up!
Later Dudes BB