BOOM, in a ship, is a general name for the long poles which jut out from certain supports or uprights, to stretch or extend the bottoms of sails. Some taper regularly from the middle towards both ends; while others have the thickest part at about one third of the length from one end. According to their particularomodes and places of applitation, they receive the names of jib B., flying jib B., studding-sail B., lower studding sail B., main B., square-sail B., driver B., spanker B., rtng-tail B., maintopmast B., fore topmast B., fire B., etc. In the old 110-gun ships of Nelson's days, these booms varied from 57 to 32 ft. in length, and from 15 to 6 in. in thickness. The war-steamers of the present day require a somewhat different equipment of booms. The immense spread of canvas in some of the clipper merchant-ships now built requires the use of booms of very considerable length. A seaman speaks of "booming" when lie applies a B. to a sail; lie employs B. irons, shaped like the figure 8, to connect booms and other spars together end to end.
Besides the booms on board ship, the same name is also given to a strong iron chain employed in barring the passage of the mouth of a harbor or river, or to cut off the retreat of an enemy if he has actually entered. Such a B. should be protected by a battery or batteries. The chains are moored, and are floated by logs. There should be two such chains, one to afford resistance if the enemy has penetrated the other; they need not extend all across the passage, seeing that shallow spots are self-defended. A. modern war-steamer would cut through a chain B., unless made of very thick and strong iron. Sometimes hempen cable booms are used to resist small-craft. The Russians effectually boomed the harbor of $ebastopol, in Sept., 1854, thereby preventIng the entrance of English and French ships; this was done partly by sinking sonic of their ow ships, and partly by the laying of booms.