On another site sometime ago, there was a discussion on what to use as a preventer. Most seemed to favor a stretchy line
to absorb the intial shock of the wind getting on the back side of the sail in a jibe...the thought being that you wanted to minimize the shock on the sail and rig
, while at same time preventing that high speed acceleration and immediate stop on the other side associated with the gybe. There are two issues here...first the unexpected acceleration of the boom across the boat which could hurt or kill someone, and second, the damage associated to the sudden stop of the sail and boom at the end of the gybe. A strong line
that isn't going to break will prevent the sling shot effect that you are concerned with. Some stretch in the preventer will lessen the shock. I haven't had reason to test it, but based on that discussion, I also use 1/2 inch double braid nylon rated at 8600 lbs breaking strength on a 32 ft. boat. A second area of concern was whether the preventer should be attached to the outward end of the boom, or at some midpoint on the boom. There was no consensus here, although the end of the boom seem to be slightly favored, but many with mid boom travelers attach to mid boom (I use mid boom). Most preferred leading the preventer line
on each side forward through the bow cleat
and then back to the stern cleats
so that the preventer can be controlled from there.
Perhaps some of the bluewater sailors on this site can give better guidance.