In some circles, the vang is known as a "kicking-strap." It controls the angle of the boom relative to the mast, and by extension, the amount of twist in the mainsail. Most of the time when you are sailing to windward, the twist and boom angle is dictated by the mainsheet. It is only when the main traveler is eased to the end of the track, and the sail needs to be eased out further, that the boom vang comes into play. Without it, as soon as you ease the mainsheet the top of the sail will open up, and you will lose what power you had. Having the vang there to control the angle of the boom will keep the leech of the sail working once the boom is beyond the lifelines.
These days many boom vangs are hydraulically operated and serve to hold the boom up as well as down. This is important because when you are reefing you need something to support the boom when the mainsail halyard is eased. If your vang is a block-and-tackle purchase you will need to have a topping lift attached to the end of your boom to stop it from dropping down when you reef. The disadvantage of a topping lift is that it can chafe the sail and sometimes even foul the leech, although some one-design classes require that their members carry this piece of equipment if they are to compete legally. Some boats have a fixed
Two examples of boom vangs. The one on the left is a hydraulic vang, and the one on the right has a rigid tube to serve as a topping lift while the block and tackle is used to tension the vang.
mechanical vang that is adjusted by a wheel on a threaded rod. The adjustment is a bit clumsy, but the advantages of having a fixed vang to hold the boom up when reefing are often worth the inconvenience. Some boat owners do not want to have hydraulics on their boat, and in that case this kind of mechanical vang is a good alternative. With smaller boats, on the other hand, a block and tackle will usually suffice. One last point: the boom vang should be quick and easy to release if you have a long boom, since on a close reach the end of the boom may drag in the water creating a point load where the vang attaches to the boom. In fact, this is the situation in which many booms fail. On centerboard boats it is also important that you be able to release the vang quickly in order to dump air from the top of the sail, since in high winds this can mean the difference between staying upright or capsizing.
The above three devices control the plane of the sail relative to the wind, and in the case of the vang, help when reefing. The mainsheet does have some effect on the shape of the sail, but there are other controls that can be used to manipulate and fine-tune the shape of the sail. The most important are the fixed backstay and the running backstays.
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