I had two Nacras in succession; both of their rudder pull-down lines slipped, and I stopper-knotted both of 'em right at the front of the Piv-Matic. They both kicked up fine, but stopped creeping. Just my quick-cheap-n-dirty solution.

sail fast
ex N 5.O
ex N 5.5u

Andrew Tatton Nacra 20 "Wiggle Stick" #266 Nacra 18 Square #12