Spinout on isonic 111

[carolos]

Hi,
Just bought i-sonic 111 and used it today in 16-18 knots with a hellcut 5.7.My weight is 60 kgs and the board is on a 38 fin.
I use harnesslines 20''.The board tends to spinout at high speed.Why?What is the reason?Also had the same prob with my 6.6 racing sail.
could u also advice on the footstrap position.Forgot to mention that my height is 1.67cm.current footstrap position front ones all the way out and rear , on the second last hole