Boom Preventer or Not?
For offshore cruising I like the idea of having a dedicated system to run a preventer. In my mind the only acceptable method to rig a preventer is a line on each side of the boom, taken from the end of the boom, outside everything to a snatch block near the bow and then back to a cam cleat in the cockpit. The line should be comparatively light and stretchy; perhaps 1/4" or 3/8" three strand nylon. The long length of libe running from the boom ot bow to the stern allows for a lot of stretch and keeps the loads on the gooseneck and mast comapratively small ven if you dip the boom in the water or do a death roll. The use of a cam cleat is so that the preventor can be allowed to run without leaving the cockpit in the even of needing a quick and safe release.
For the kind of short legs that most of us do when we are out daysailing or weekending, a preventor is probably not the best idea because we often need to be able to jibe quickly due to traffic or to avoid grounding or hitting an obstruction. I think that using the vang as a preventor by bringing it to the deck or rail is just plain dangerous and probably more dangerous than an accidental jibe.