SHEATHING. Owing to the great expense of copper sheathing, which has the effect of limiting its use in mercantile shipping, many attempts have been made to substitute for it either other metals, or alloys in which it is mixed with cheaper metals, or with such as might increase its durability. Mr. Robert Musket directs that I00 lbs. of copper should be alloyed either with 2 oz. of Eine, 4 oz. of antimony, 8 oz. of arsenic, or 2 oz. of grain-tin ; or instead of using one of these separately, that the whole be used together in the proportion of half an ounce each of the zinc and tin, 1 oz. of the antimony, and 2 oz. of the arsenic, to 100 lbs. of copper. By these mixtures, Mr. Mushet states that the copper is rendered much more cohesive and fibrous in its texture, and that the corrosive effect of the sea-water is in a great measure prevented. The metallic sheathing patented by Mr. Pope, consists either of tin and zinc, or of tin, lead, and zinc. If the former mixture be used, the zinc is first melted, an equal quantity of tin is added to it, and the alloy, after being stirred while fluid, is cast into cakes about three-quarters of an inch thick, which are hammered or rolled out to the required degree of tenuity. In uniting tin, lead, and zinc, the lead is first melted, double its quantity of tin is then added, and the alloy is cast into small lumps. A quantity of zinc equal to the tin and lead united is then separately melted, and the alloy of tin and lead is added to it; the whole, when thoroughly incorporated, being cast into cakes as before, for subsequent rolling out into sheets.
Iron, protected by the galvanic action of zinc, has also been used for sheathing. Mr. Pattison proposed to use sheets of iron similar in size to the sheets of copper sheathing, each having at its lower extremity a sheet of zinc from one-eighth to one fourth of an inch thick, attached in such a way that in sheathing the vessel from the upper part down wards, each succeeding sheet of iron shall be in contact with, and overlap, the zinc plate of the sheet immediately above it. Washers or perforated discs of zinc are also applied under the heads of the spikes or bolts used in fastening the sheathing; and the heads of the nails employed are made hollow, and filled with melted zinc.
Sheathing of brown paper coated with tar, and of various other non metallic substances, has also been used. Perhaps the most important of these is a kind Of felt, into the composition of which a considerable quantity of cow-hair enters. As this material itself felts very imperfectly, the sheets aro passed, in the process of manufacture, through a boiler of pitch or tar, which increases the cohesion of the fabric. This material, which is sometimes used in conjunction with copper sheathing, being laid on immediately beneath it, has the important qualities of being a perfect protection against the worm, and of being at once impermeable to water, and so extensible as nut to be easily broken by the working which takes place among the timbers of a crazy ship. A sheathing composed of a coarse fabric of fibrous material, saturated with a solution of caoutchoue, together with pitch and tar, has been recommended as a cheap and effectual substi tute for felt.
Copper sheathing is usually applied in sheets about four feet long and fourteen inches wide, the thickness being such that a square foot weighs from sixteen to thirty-two ounces,—most commonly from twenty to twenty-eight ounces; and the mode of application does not vary materially whether the copper be laid upon the bare planking or upon an interposed layer of tarred paper, felt, or thin boarding. The sheets are pierced with holes, not only round the edges, but also at intervals of three or four inches over the whole surface ; they are laid so as to overlap each other about an inch, and are secured to the ship with flat-headed copper.nails. Great regularity is observed in the arrangement of the sheets, so that a certain symmetry of appearance, as well as durability, is attained. After two voyages to the East Indies, or an equivalent amount of voyaging elsewhere, the coppering • requires to be renewed ; and the old copper is found to have lost three or four ounces of its weight in the square foot, by the action of sea-water, friction, and other causes.
All the copper sheathing for the Royal Navy—that is, for the ships built at the royal dockyards—is made at an establishment called the metal mills, in Chatham Dockyard.