Sheathing in Naval Architecture, a sort of covering nailed all over the outside of a ship's bottom, to protect the planks from the ravages of worms. Formerly, this sheathing consisted only of boards tarred and payed over; but now copper is resorted to, not merely as a substitute, but as an additional covering, and it has become almost universal, where the expense can be sustained; it is of especial utility in vessels making long voyages and into warm climates. The rapid corrosion of copper, by the action of sea water, renders the frequent renewal of it a very serious item of expense to the ship-owner. In a patent which Mr. Robert Mushet took out a few years ago, "for certain means or processes for improving the quality of copper and alloyed copper, so as to render it more durable when employed as sheathing to ships bottoms," he states, that, owing to some defect in the manufacture of the copper, the sheathing upon a ship is sometimes worn away by oxidation in a much shorter period than usual. The cause of this Mr; Mushet considers to arise not simply from the impurity of the metal, but from the undue proportion of the alloy with which it may be mixed.
He also states, that he has found that the purest copper, exposed to the action of sea-water, is not so tenacious as when alloyed in the manner he proposes in his specification, and on which he founds his patent. He directs, that to 100lbs. of copper is to be added 2 oz. of the regulus of zinc, or 4 oz. of the regulus of antimony, or 8 oz. of the regulus of arsenic, or 2 oz. of grain tin. Instead of these alloys separately, they may be employed in conjunction in the following proportions: viz. to l00lbs. of the copper add half an ounce each of the zinc and tin, 1 oz. of the antimony, and 2 oz. of the arsenic.
Mr. Christopher Pope, of Bristol, took out a patent, in 1824, for the manufacture of metallic sheathing in which copper is altogether discarded. These plates are composed of tin and zinc, or of tin, lead, and zinc. Mr. Pope says, in his specification, - "To unite tin and zinc, I take a certain quantity of zinc, and melt it in an iron pot, and when melted, I add an equal quantity of tin; and having stirred them together in a fluid state, I cast cakes of it in moulds of about 8 inches broad, 10 inches long, and $ of an inch thick, which cakes are afterwards hammered or rolled out into sheathing......To unite tin, lead, and zinc, I melt a certain quantity of lead, and add to it twice the quantity of tin. This composition I cast into small lumps, and having melted Shree times as much zinc as I had previously melted of lead, I then add the small lumps of tin and lead, and melt the whole together; which method I find to be the best." This mixture he casts into cakes of the size before mentioned, and then rolls them into sheets; and he particularly enjoins, that no more heat be used than is just sufficient to compound the alloy, as the metal becomes hardened by an excess of heat; and that it is advisable, in rolling out the cakes, to heat them to the temperature of boiling water, by which he says that "they will roll or hammer softer than when cold." This metallic sheathing has, we are informed, been more extensively employed for covering of the tops of houses, than the bottoms of ships.
A few years ago, some very favourable accounts were published of the patent Indian rubber sheathing, consisting of a coarse fabric of fibrous material, saturated with a solution of the elastic resin, together with pitch and tar. The price of the sheathing was ten pence per sheet, of the size of 34 inches by 20. It was found to be a complete protection against the worm, and must, at the least, we think, form an improved substitute for the felt in general use.
It was for some time supposed that Sir H. Davy had discovered a remedy against the rapid oxidation of the copper. Regarding the action of the sea-water upon copper as of a galvanic nature, that great chemist considered that, by the addition of small pieces of tin and zinc, the copper would be rendered negatively electrical, and oxidation prevented. Ships were sheathed on this principle, and sent to sea; but they proved such bad sailers, from the foulness of their bottoms, that a negative was soon put upon the scheme. It is true that the copper was thus protected by the zinc and tin, but the barnacles (shell-fish) attached themselves so much to the protectors, as to introduce a greater evil than they were calculated to remedy. Sanguine hopes were entertained of the success of this plan, and the disappointment consequent upon failure was, of course, extensively felt. Founded upon the same theory of the galvanic influence, a patent was, a few years ago, obtained by Professor Pattison, for making use of iron plates, protected by zinc, which, it was asserted, entirely prevented the oxidation of the iron: and that a ship sheathed with iron and little bits of zinc had been two years at sea, and returned home with a clean and bright surface.
The specification of the patent states that the iron plates may be of the usual dimensions of the copper plates, and for each area of 100 inches in the iron, a plate of zinc of from one eighth to one fourth of an inch thick, equal to five inches in area, is attached to the lower extremity of the sheet, so that in sheathing the vessel from the upper part downwards, each succeeding sheet of iron shall be in contact, by lapping over, with the zinc plate of the sheet immediately above it. Plates of zinc must also be attached to the inside of the sheet of iron, bearing a proportion in area to those on the outside, of 3 to 5. The spikes and bolts by which the sheathing is fastened to the vessel are each to be furnished with a disc, or washer, of zinc, fitting closely to the head; and it is recommended that they be driven well home, to insure perfect contact. The nails employed are to be made concave under the head, and the cavity is to be filled with melted zinc. The proportion of five Square inches of zinc to one hundred of the iron, is not insisted on; any greater proportion will be equally effectual, and the zinc may be alloyed with copper, tin, or lead, in the proportion of from 3 to 10 per cent.
By this mode of sheathing vessels, it is asserted, in the specification, that the corrosion or oxidation of the metal will be nearly, if not entirely, prevented.