Do what goose said, the only thing I can add is the brand of shockcord is important. Most shockcords sag very quickly not holding the rudder up but the brand that Nacra sells doesn't stretch and sag after you set it. Nacra sells it cut to length and it's very cheap. If you cant get proper shockcord you can make 10mm fit through the hole in the blade with a bit of effort then pull each end out the end of your tiller arm then tie a knot in one end then stretch the other out as hard as you can and tie a knot in it. The knots won't fit down the tiller arm. Definately not as pretty as doing it with the Nacra shock cord. To do it perfectly with the 8mm shockcord Nacra have stainless steel clips that you can apply with pliers that lock the bungy together. I've done both of these methods to two different Nacra 5.8's in the last 2 weeks. If no matter how hard you tighten the shockcord the rudders don't pop all the way up unless your blades are a bit sloppy in the rudder stocks, it means the top of your rudder stocks are too close together I can give you an easy fix if you have that problem, you don't want any wobble