Depending on the rig and the sail you often need something that's more in the line of a vang, holding the boom down, rather than a preventer. Confusion results when the two functions are confused.
Often the leverage for a vang that goes from mast to boom is just not enough and it makes sense to make a tackle vang from the rail out to the boom. This will act a bit like a preventer if you start an accidental gybe.
Problem is: If a vang is weak enough to break away in an accidental gybe it's not really strong enough to flatten the sail in any sort of wind. And flattening can be really important in keeping control, resisting excess weather helm, and reducing the chance of an accidental gybe.
If you must use a vang from the rail, be careful and don't accidentally gybe.
G'luck
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