Boom (sailing)

From Freepedia

In sailing, a boom is a spar (pole) usually made of aluminum or wood, is connected to the foot of the mainsail and allows the crew to control the angle of the sail to the wind. The boom attaches to the mast at a joint called the gooseneck. The gooseneck pivots allowing the boom to be easily adjusted by the main sheet. Four lines are attached to the boom:

1. The outhaul runs along the boom and the back corner of the sail (the clew) aftward. Modern sails are cut so that the outhaul is also able to pull the clew downwards towards the boom. Hauling in on the outhaul increases foot tension in the main sail.

2. The main sheet is attached midway along the boom or at the stern end. In smaller boats such as dinghies it is used to control the angle of the sail to the wind on each point of sail. On larger boats this function is largely assumed by the traveller and the main sheet is used to adjust the twist of the sail to present the luff of the sail to the wind all of the way up the mast. Easing the main sheet increases twist and the twist is usually adjusted so that the aft end of the top baton in the main sail runs parallel to the boom.

3. The traveller is a track running from one side of the boat to the other upon which sits a car which is attached to the main sheet. Moving the car from side to side alters the angle of the boom to the centreline of the boat while minimising the effect on the twist of the sail.

4. The boomvang, kicking strap or kicker is an intricate set of pulleys (and, on yachts, a hydraulic ram) running diagonally between the boom and the lower portion of the mast. The kicker pulls the boom downwards. When the boat is running away from the wind the main sheet will be fully eased and so the kicker becomes the primary means of controlling main sail twist.

The controls on the boom act in conjunction with the halyard and leech line to ensure that the main sail is trimmed most effectively.

Various fairleads and cleats are attached by screws and rivets in wooden and aluminum spars respectively. Aluminum booms are lighter and stronger than their woooden counterpart and are used more often in competitive racing. They are usually associated with fiberglass boats. The wooden boom is found most often on older sailboats and handmade dinghies.