Coppering   

 The original purpose of sheathing the underbody of a ship was to protect it against the teredo shipworm which in tropical waters could infest a wooden hull and in a short time eat its way inside the wood sufficiently to sink the ship. This pest eventually found a home in European waters having been brought there in infected hulls. The cheapest protection was a layer of wood sheathing about 2 in. thick laid over the hull planking with various mixes in between such as tar or tallow mixed with hair, sulphur or ground glass. East Indiamen used to pay their bottoms with a mix of oil, dammar resin and blacking when careened out east, before the days of coppering. It was also desirable to reduce the growth of seaweed and barnacles on the hull, and various metals were originally tried such as lead or zinc, and in some instances leather. The metals were unsuccessful, in salt water at least, because electrolytic action was set up between dissimilar metals such as the iron fastenings in the hull and iron gudgeons etc. against the sheathing metal and its fastenings. One method that persisted for a time was to stud the wood sheathing with large-headed cast iron nails as close together as possible, this however being only to resist shipworm.

  Finally after much experimentation the use of copper plates with copper nails to fasten them was accepted about 1783, the hull fastenings, gudgeons, pintles etc. being of copper or copper alloys such as bronze. In cases where the copper was to be put on a hull with iron fastenings, wooden sheathing had to be used as an insulator, although the gudgeons and pintles had to be of bronze.

  The copper sheets were originally nearly pure copper which eroded away quickly even though it kept a good clean bottom, the marine growth being shed along with the eroding copper. This was an expensive process and efforts were made to reduce the rate of erosion, or exfoliation, by adding other metals. Muntz introduced a mixture of 50 parts copper to 50 parts zinc in 1830, and by 1846 had changed it to 60 of copper and 40 of zinc, which was the well known Muntz metal in use up'to modern times, sometimes with a proportion of tin added. Lloyd 's Registers of the period describe ships' bottoms as being yellow metalled, coppered or brass bottomed, all these being slight variations. It was still a very expensive method of dealing with a ship's hull, from the labour point of view as well as material cost. Figures for an East Indiaman's construction give the cost of coppering at one-tenth the cost of the hull. A shi p could expect to get two Far East voyages before needing recoppering. The notorious Confederate raider Alabama left Laird's yard in England in July 1862. By December 1863 Captain Semmes reported her copper as largely destroyed, a factor which contributed to her defeat by the Kearsarge in June 1864, by which time the remaining copper was hanging in long ribbons. The famous American clippers Oriental of 1849 and the Challenge of 1851 had to be drydocked for recoppering fifteen months after their launch.