Your rudder control surfaces rise up well above the static waterline ... making the rudder vulnerable to 'cavitation' by pulling air down along the side of the rudder especially when the rudder is hard over. If this is the problem, and not a simple 'helm balance' which can be corrected by sail shape, etc. then 'winglets' may be a solution but up closer to the top of rudder body. Look at the 'ribs' on the MIG 17 wings to see something that Im attempting to explain.
I think youd have to explain *WHY* you want to do this, and the symptoms youre trying to remedy, first.