SAIL - - On Deck Sail Choices -

A boom vang helps to con­trol main­sail shape after the lim­its of the trav­eler have been reached. With­out a vang, the boom will lift as the main­sheet is eased, open­ing the leech, and the top of the sail will twist off to lee­ward, spilling the wind. If you are sail­ing down­wind with the main­sail eased right out, the top of the sail can twist so far off that it is for­ward of the shrouds. This en­cour­ages the boat’s ten­dency to roll and can lead to a nasty broach. Ten­sion­ing the vang so the twist is elim­i­nated will sta­bi­lize the sail and de­crease the rolling.

On a reach, if the wind pipes up enough to over­press the boat with a con­se­quent in­crease in weather helm, eas­ing the vang to in­duce some twist and open the up­per part of the sail’s leech, thereby spilling some wind, will re­lieve the heel­ing forces and take some of the weight off the helm.

On boats equipped with main­sheet trav­el­ers, the vang is not used when go­ing to wind­ward be­cause the main­sheet ap­plies suf­fi­cient ten­sion to the main­sail leech, and the sail’s an­gle of at­tack to the wind is con­trolled by play­ing the trav­eler. In the ab­sence of a trav­eler, the vang is even more im­por­tant, as it must be used when go­ing to wind­ward to keep the leech ten­sioned when the main­sheet is eased in gusts. This is known as “vang sheet­ing.”

Although the vang need not be ad­justable over a range of more than few inches, it does need to be very pow­er­ful. By virtue of its lo­ca­tion on the for­ward part of the boom, there is about twice as much load­ing on the vang as on the main­sheet. The blocks on a boom vang tackle should be the same size as those on the main­sheet tackle. The more pur­chase, the bet­ter: 4:1 is the min­i­mum, and 6:1 is bet­ter.

A gas or spring-loaded vang is a worth­while up­grade, as it sup­ports the boom in the event of the main­sheet part­ing or be­ing ac­ci­den­tally re­leased. – Peter Nielsen

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