- Through much (not all) of the range of motion, they provide positive control over the boom-end position.
- Assuming the controls work from both ends, they function as a preventer if the sheet is tight (the traveller is locked down). Many catamaran feel they do not need one for this reason; a full jibe is impossible. But if running very deep it is still required to hold the boom down and prevent little mini-jibes, where the traveller does not move but the boom still slams a good distance when the wind gets behind it.
- Av vang is not desired because it reduces the speed with which the sail can be unloaded in a gust (both the main sheet and the vang must be let go--not acceptable).
- A catamaran can be capsized, not just knocked down, if the main is pinned on the wrong side in enough wind. I've never hear of this happening on a cruising cat, but I've experienced it on beach cats with the traveler jammed.
The ideal preventer, then, does the following
- Holds the boom down and forward when the sail is well eased going deep. In this function it acts as an extended traveler, providing height control, since most cats do not have vangs. Thus it must be no further forward than the main beam and it must be tensioned by a cockpit winch, in order to be adjustable.
- It must be quickly releasable in a controlled manner. If the main is caught aback in a squall, the crew must be able to let it across and then dump the main in seconds.
Sorry, no pictures yet.