SHEATHING (from sheath, AS. sccep, sea P, seen sccida. Ger. ,‘:cheide, sheath; prob ably connected with AS. seeadan, seedmi, Goth.
skaidan, see ((btu • tier. :A./widen. to separate, Lat. scinderc, Gk. crxqcip, schizein, to split, Lith. skedzu, skcdu, 1 separate. Skt. chid, to split). The covering of a ship's hull, usually of metal. In the days of wooden ships it was found that barnacles and other marine parasites attached themselves so firmly to the bottom as to neces sitate injury to the wood in dislodging them; moreover. some marine animals (e.g. the teredo) bored into the wood and destroyed it. Sheathing with very hard wood was first resorted to. Lead sheathing seems to have been used as early as 1 620 at least, and was probably used to cover the wood along the water-line several centuries before. A Japanese junk of about 800 tons sheathed with iron was seen in 1613. In 1761 copper was first used as and in course of time copper or a copper alloy displaced all the other metals except zinc, which is still, though rarely, used. When iron ships were built it was noticed that their bottoms became foul very quickly. The best remedy found was paint, and
it was only a partial one. To avoid excessive fouling, many iron and steel vessels of war have their bottoms sheathed with wood and coppered as in the days of wooden ships. Iron merchant vessels have rarely been sheathed, and the wis dom of sheathing and copperiug any iron or steel vessel is doubted. Zinc sheathing has been used to some extent because in the battery formed by zinc and iron it is the zinc which is eaten away. The bottoms of ships are generally cleaned every year or oftener (once in six months is desirable) and coated with two kinds of paint. The first is anti-eorrosive and is desiL,med to protect the metal against rusting. The other is anti-fouling. It is much softer than the other paint. is poison ous to marine growths, and if any adhere to it. they are apt to be washed ofT together with a thin film of the paint. No paint yet devised is regarded as fully satisfactory, but several vari eties give fairly good results for five or six mouths. See PAINTS.