This is a very common problem with Prindle rudders and is easily fixed. You need to make the rudder lock bolt a couple of turns shorter and make sure it is properly alligned with the pin it engages. Be careful not to overtighten the locking nuts. You can damage the rudder at this critical point. With the boat in a position where the rudders are locked down and can kick up freely (on the beach or dry land) you should have to kick the bottom of the leading edge of the rudder firmly to get them to pop up. In 21 years of owning Prindles I have never seen the cause of rudders popping up at speed to be anything other than rudder lock bolt adjustment. It can be very annoying to have this problem in a windy race. Good luck. Greg