Posting this on behalf of my dad.
He has a 131 starboard carve board with a 44 cm fin and he was using 6'2" sail in around 20ish knots and probably weighs 95 kgs. He saying when he's getting up on the plain the boards becoming unstable. He wants to know if buying a bigger/smaller fin will fix this?
He used windsurf 20 years ago for about 10 years.
does the nose lift to the sky when planing?
He said it doesn't.
Agree with fin comments. Also make sure your footstraps are tight enough .loose straps =no board control