Kevin,
Actually, the rudder lines are 3/16". Be sure that you buy good stuff from somewhere like West Marine, not just Home Depot stuff. Rather than sew, you can simply tape the end of the new rope to the old and pull it through. I do recommend that you take a couple of digital pictures first, and of course, only do one side at a time so that you can check the other one as a pattern if need be...
Kevin in Florida
Prindle 18