To add to the good comments in Nolatom's post above, your vang is probably attached to the boom forward of the midpoint. Although some boats will use preventers that are attached at or near the mid point of the boom, doing so increases the risk of breaking the boom. Ideally, a preventer should run from the end of the boom forward to the bow. Booms can break while broad reaching / running downwind with preventers rigged when the boat takes an big roll and the end of the boom drags in the water. When the preventer is attached mid-boom the middle of the boom wants to stay put while force of the water pushes the end of the boom aft. Rigging the preventer to the end of the boom helps balance the force of the water.
I used a mid-boom preventer for years and took some mean jibes with it, including a really violent jibe one night when the Aries decided it didn't like the course we were on while we were screaming along in a 25 knot trade wind (scared the s--t out of me). Nothing broke, but it caused me to rethink the real purpose of the preventer and to re-rig with a more traditional system. My new end-of-boom-to-bow preventer uses more line and is a bit more work to set up and change over when we do jibe, but I think it works better. The end-of-boom set up allows me to get the preventer much tighter (less movement when we do jibe unintentionally).
The system I use now is rigged as follows:
A very stetchy braided line runs the full length of the boom. The end at the gooseneck has an eyesplice. When not in use I cleat this line off near the gooseneck and it lies nicely out of the way. I have two braided lines (one each port and starboard) that run from the cockpit to the bow (on deck), then through strong blocks attached to the bow cleats on either side, then under the lifelines and back on the outside of everything to the shrouds. When the preventer is not in use these the working ends of two lines are clipped to the shrouds. To set up the preventer I un-cleat the line attached to the boom and shackle it to the end of the line that runs to the bow. I have to go to the mast to do this, but don't have go to forward of the mast. I then take up any slack in the preventer from the cockpit. When we change tack, I sheet the main in hard, go forward and re-rig the preventer to the other side. I then jibe the main, trim the sail and then set up the new preventer.
The two weak points in this set up are the block at the bow and the snap shackle that connects the two pieces of line. I oversized both for safety. I would not recommend using just any piece of line for a preventer. Ideally, it should not only be strong, but fairly elastic as well so that the stretch in the line can absorb some of the shock transferred to the rig in an accidental jibe.
Last edited by billyruffn; 02-04-2008 at 02:04 AM.