RE: Double fins?
I would take what GEM suggests to the full explanation from my experience with twin fin designs (like the HiFly MADDs).
They are faster and better upwind (the twin fin design) ONLY if you are in water so shallow you cannot use the right size single fin, or a modern progressive rake weed fin.
My experience in Bonaire (a very shallow place where it's impossible to use fins much over 33 cm) was that for the larger Carves, and any of the wide boards (Starts, Gos, Rio's, Easy Riders), the "cut down to 33 cm" fins the rental operators must use for the super shallow conditions didn't work very well and the boards were hard to get on plane, and not real fast as there was not enough fin span to "ride the fin" on a single fin board.
I tried the HiFly Madds (mid size and large size) and they were better with the twin fins, in the shallow water areas, but the Carves and other wide boards were better upwind, faster, and earlier to plane when you could use the regular size single fin further down Lac Bay in the deepwater channel.
Also, I felt that the " fin toe in" on the MADDs might need to be adjusted as one of the fins always seemed to be slightly "spun out" (evidenced by the white turbulent "wake stream" behind the (usually) upwind fin when trying to sail the board "on the fin" with a slight bit of leeward "railing".
So, I don't think you will see any "twin fin" designs from Starboard (in the freeride design range) anytime soon.
Twin fins do work, sort of, but only when the water is too shallow to allow the use of a correctly sized (for the board's tail width, footstrap offset, rig size, and sailor weight) single fin.
A deeper single fin has less drag, better early planing, and significantly better upwind angle than 2 smaller fins.
The modern progressive rake weed fins (Tangent Dynamics Reaper,
Tekkno Sports Race Weed, True Ames Shallow Water Weed) and the Wolfgang Lessacher Duo Weeds give very good performance (almost as good as a vertical fin design) and certainly better than the twin fin setup, when there's enough water depth to use the correct size.
These are not to be confused with the traditional very swept back (> 45 deg.) weed fins that stick out behind the board.
Hope this helps,