Done some searching in the classifieds looking for a set of twin fins for my 82ltr RRD twin fin. The board came with a pair of 16.5's but I am having trouble keeping the board tracking in a straight line, the tail just wants to go sideways when I go to lean back and power it up. Its ok if I sail it in a more defensive upright stance but there's no fun in that. I had myself convinced it was mainly technique that was the issue but after a fair amount of experimenting with the obvious, boom height, mast track and fin position etc, i'm not so sure. I've just picked up a 85 ltr F2 Gorilla single fin and have a 24cm fin in that and have no similar problems with that board.
I weigh 90+ kgs and use the board with a 5.4 in 17-20+ or a 4.7 in 23+knts, bump and jump.
So several questions, what size would the twin fin gurus recommend to go up to and does any one have anything for sale the may suit my requirements?