How to rig the jib blocks on a Nacra 5.7. 5.0 should be the same, along with 5.2 & possibly others. Take a 3-4' length of 3/16" line & fold it. Pass the loop through the cringle on the jib, then pass the two ends through the loop, even things out & pull tight. Now tie one end to each jib block at the top of the photo.
The remainder of the jib line would be at the bottom of the photo, as a continuous loop.
