The Boom Vang is usually a block and tackle that goes to the mast base and is used to control the mainsail shape. It is generally called a kicker if it is a solid vang, not just a block and tackle, and can support the boom in place of a topping lift.
Most boom vangs are not preventers, and are not connected to the rail. Most preventers don't help with sail shape all that much either, since they're mainly pulling forward on the boom, rather than down.
IMHO, a boom brake makes a lot more sense than does a preventer. In many situations, a preventer can become a serious problem and often can result in the boat being knocked down or pinned down, and then can only be released under a high load, which is fairly dangerous to do. A boom brake allows the boom to move, but at a fairly low speed, where it is not going to be a danger to people in the case of an accidental gybe.
Most modern boom brakes are also adjustable in tension for the wind conditions and are often controlled by a line lead aft to the cockpit.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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