A jib boom is a nautical term for the horizontal mast used to extend a sail in the front of a ship. This small sail is called a "jib," sometimes spelled "jibb." A ship can have more than one jib sail, but only one jib boom. A jib boom is also the name used for the arm on some kinds of machinery, such as a crane.
Identifying a jib boom can be difficult for someone unfamiliar with nautical terms. The easiest way to identify it on a nautical vessel is to look for the jib sail. This is a triangular sail located at the prow, or "pointed end," of a ship or boat. The jib boom is the horizontal piece of wood keeping the jib stretched out over the prow.
The form of a jib boom will vary slightly depending on whether it's on a ship or a boat. The size and style of the vessel affects the arrangement of the sails. Some small boats have only two sails, and the jib sail will be of the same size as the mainsail. On traditional two-masted and four-masted ships, the jib sails will be much smaller in relation to the main sails, and there may be more than one. The different arrangements of jib sails on sailing ships even led to an idiom—liking "the cut of someone's jib" came to mean that a person liked another's style.
The jib boom is under a deceptive amount of strain. It carries the force of the wind for the entire ship and isn't grounded into the deck of the ship like a main mast would be. A snapped jib boom can mean that the ship will lose some of its ability to be moved by the wind and will be harder to steer.
On traditional sailing ships, the jib boom is used to help direct the ship. The rudder is also used, but has a limited range of motion. Moving the jib boom to either catch the breeze or move out of the path of the wind can help pull the ship in the direction it needs to go. The jib is usually moved by having a pilot pull it this way and that by a rope attached to it. When the jib is in the correct position, the rope can be attached to the side of the deck to hold the boom in place.