dredged from Tiber River in Rome.
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Roman Bronze Boat Nail, 1st-5th Century, dredged from Tiber River in Rome. |
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Probably the most important technical innovation to be implemented by the naval protagonists during the American War of Independence was the sheathing of ship's hulls with copper, and it was the British who developed this technique and held the initiative.
Back in 1708, Charles Perry proposed the idea of copper sheathing, but the concept was rejected because of the costs involved. Again, in 1740, Nehemiah Champion suggested using sheets of "brass lateen" as sheathing, and although such an experiment was apparently made, nothing came out of it. Eighteen years later, the Royal Navy conducted an experimental coppering on the false keel of the HMS Invincible and then proceeded, in 1759, to use copper plates in the sternpost and keels of some of it's warships.
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| Coppering scheme for the 36 gun frigate Pallas. |
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By 1775, the Navy Board began to show renewed interest in the copper sheathing, a fact that might have been compounded by the inability of the timber contractors to supply enough sheathing board. In the next two years a number of small ships were sent off in voyages with copper bottoms with "composition" to protect the iron bolts from corrosion and, by the end of 1776 one 32-gun frigate, four 20´s and a sloop had been coppered. On all of these ships, the bottom was painted with a mixture of white lead and linseed oil, on which the copper plates were to be fixed with nails made of an alloy which included copper. The same material was used to make the braces and pintles while the false keel was fixed to the main keel with copper staples with a thin sheet of lead between them.
A few more English ships were coppered in 1778, and by 1778 the trend had caught on and more and more ships were being coppered, with those already sheathed impressing the sea officers by their handling capabilities.
Finally, in 1779, orders were issued in that all ships of 32 guns and less should be coppered the next time they were in dock, although no solution to the corrosion of iron bolts had yet appeared and copper bolts were trusted only for ships of fifth and sixth rates. The reason for this move was that an apparently successful protection for iron bolts had been found, by the creation of a watertight barrier between the copper and the iron bolts. This barrier consisted in the application of thick paper - soaked in oil of tar and in Dawson´s composition - between the copper plates and the hull, an experiment first carried out on a 44-gun ship, the HMS Jupiter. By then, the ships that were copper sheathed were described as "felted and yellow metaled" because the copper protected the felt and tar layer.
Later, it was decided that the whole battle fleet should be coppered. In January 1782, eighty-two capital ships, fourteen of 50 guns, hundred and fifteen frigates and one hundred and two sloops and cutters had been coppered to that time. However, at the end of that year, doubts about the effectiveness of the protection of iron bolts from the corrosive effects of the copper were raised very forcibly. The chief reason was the violent storm of September 1782 off the Banks of Newfoundland, when the captured French ships, the Ville de Paris (110) and the Glorieux (74), and the British Ramillies (74) and Centaur (74) all foundered with the loss of 3500 lives. A thorough inspection of the 74-gun ships Edgar, Fortitude and Alexander showed irrefutably that the iron bolts of all three ships were in a dangerous condition.
Another major weakness was the lack of protection which the copper provided against the worm, most notably in the stern area, because neither copper nails nor copper cladding did much to keep teredo worms out of the hull timber. It was the tar soaked felt, applied hot to the hull, that formed a layer impenetrable to the teredo in its planktonic phase. In some hulls that have had copper applied without the felt, the copper seemed to provide a protective layer behind which the worms did their worst, coming to the surface of the wood with impunity.
It was only in December 1783 that the new copper and zinc bolt, hardened by mechanical means and developed by William Forbes, entered in service. By August 1786, all ships were changed to the new bolts.
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