RE: Formula questions
Having a big 70 cm fin and the tail width (on the board) to be able to control the attitude of such a large fin (footstrap offset=leverage to control the fin) allows a sailor to pump the board against the fin (to some degree) and thus "bounce" the board up onto a plane.
It's really a combination of pumping the sail and the fin to get the sailors weight off the board and allow the board to "porpoise" from the combined pumping actions up on top of the water where the waterline length shortens significantly and this reduces the drag.
There is so much going on when a good sailor on a formula boad gets it going in < 9 knots of wind that it's very difficult to explain the theory and dynamics here.
Suffice to say, when the width of boards went above 75 cm all of a sudden the fin size increased to 65-70 cm.
Even way back in the Formula 155 days (85 cm wide boards) the stock fins were around 65 cm and most of us who sailed them in really marginal conditions went out a bought a 70 cm fin as this could be used to get the board going really early in very marginal conditions.
It was the wide tail width and footstrap offset, combined with a big change in the rockerlines (much shorter and wider planing surfaces at the back of the board extending forward to just under the front footstraps with a pronoumced "rocker transition" right under the front footstraps and a steadily increasing curvature up away from the water forward of the front footstraps.
This was a major "revolution" in board design, and the largest benefit was much earlier planing, with larger fins.
Hope this helps,