As an early adopter of copper sheathing during the Age of Sail, the Royal Navy enjoyed a considerable tactical advantage over its main opponents for several decades. Because copper made the hulls less hospitable to weed growth, its ships were both faster and more maneuverable than those of Britain’s enemies, at least until the practice was adopted by other countries. But that was not the prime motivation behind the innovation. Copper sheathing was the final chapter in a long-running battle between the world’s navies and one of their most persistent foes—the dreaded shipworm.
Shipworms, or teredos, are not worms at all, but rather long reddish-grey saltwater mollusks that can grow up to three feet in length and an inch thick. They were long feared by mariners because of their voracious appetite for wood. The worms would bore cylindrical holes into the timbers of ships, often in such numbers that only a thin wall remained between each worm’s chamber. Left untreated, this could reduce the strongest oak to little more than a honeycomb. Once the timbers had become fragile, the hull might break apart, perhaps under the stress of rough weather, often without warning. Sailors would recount dark tales of ships that had vanished, far out at sea, when their worm-infested bottoms fell away.
Smaller than their tropical cousins, the European species of teredos have always been more of an annoyance than a menace. Management of their infestations was well understood by mariners before the Age of Discovery. The earliest ships were light enough to be dragged out of the water, and a few days on a hot Mediterranean beach would kill off the worms. Larger ships were treated with mixtures of horsehair and tar, and this was sometimes covered with an additional layer of lead sheeting. This was the case with two large barges built for the Roman emperor Caligula that were discovered in Lake Nemi south of Rome in the 1920s.
The Vikings, too, found that a metal barrier could be effective, and used closely spaced iron nails to provide protection for the hulls of their long ships. Another solution for vessels with a shallow draft was to moor in fresh water, high up a river estuary, for example. This kills the worms after a few days and is a technique that is still used by owners of wooden boats.
But in the warm, highly saline waters of the Caribbean, an altogether more fearsome creature lurked. The Teredo Navalis is the largest of the world’s shipworms and has the greatest appetite. In 1492 the unsuspecting Christopher Columbus arrived in the Western Hemisphere with his three wooden ships. When he returned to Europe, he took with him many impressive treasures from the New World. Unfortunately, below the waterline, he also brought back some less welcome cargo. The mollusks took some time to adapt to Europe’s cooler waters, but by the following century shipwrights were beginning to notice the damage the new arrivals caused. As the wooden ships of Europe explored the world, they took the worms with them.
Ships were not Teredo Navalis’s only victims; any submerged wood was vulnerable. In 1731, Holland suffered from severe flooding after worm attacks weakened wooden dike revetments so badly that many failed. In San Francisco Bay as late as the 1920s, the arrival of shipworms caused the widespread destruction of wooden piers and wharves.
Wooden dikes and harbor piers can be replaced with stone and steel, but the same was not true of ships until comparatively recently. The ancient technique of coating hulls with horsehair and pitch was found to be ineffective against the invasive species. Some ship owners tried to cover oak with a sacrificial layer of pine, in the hope the worms might feast on that in preference to the wood beneath. Regrettably, the creatures simply bored through the barrier and into the structural timbers. Vessels were regularly careened—pulled out of the water to be scraped clean of weeds and barnacles—and this provided an opportunity to deal with an infestation. Once the weed growth had been removed, fire was used to scorch and kill the teredos. It was effective, but as soon as the ship was afloat once more, new worms would reinfest the wood.
Experiments began to be carried out to cover the hull with an impenetrable barrier. A paste known as “white stuff”—a mixture of whale oil, rosin, and brimstone (sulfur)—was claimed to offer some limited protection. The Spanish tried reviving the ancient Greek and Roman practice of using sheet lead, but this proved to be far from an ideal solution. The lead was an effective barrier but was so heavy that the vessels were left with a much-reduced carrying capacity. It also caused corrosion to a hull’s iron bolts and nails through electrolysis—a phenomenon beyond the understanding of 17th-century science. Something more practical was required.
The use of copper sheathing was first suggested to the British Navy Board by Charles Perry in 1708, but his scheme was rejected on the grounds of cost. Fifty years later, with no better solution in sight, the first experiments with copper sheathing were made, culminating with the decision in 1761 to copper the entire bottom of the 32-gun frigate HMS Alarm, whose hull was in a terrible state after a long period in the West Indies. The initial results were very encouraging. The frigate was sent back to the Caribbean, and when she returned after a further two years there, her hull was found to have no sign of worm attack, with the additional benefit of minimal weed growth. Copper reacts with sea water to produce an oxychloride film toxic to most marine plants. Weed growth, especially in the tropics, can quickly impair the performance of sailing ships, and it was clear that sheathed vessels would be swifter and more maneuverable than those with unprotected hulls.
The first coppered ships were not without issues, however. When the Alarm was reexamined in 1766, the iron bolts and nails in her hull were badly corroded, thanks to the effects of electrolysis. A second ship, HMS Dolphin, had been sheathed in copper in 1764 and sent on two voyages of discovery, both including circumnavigations. She, too, showed worrying signs of corrosion. Although electrolysis would not be understood until the next century, it was clear that some effect was being set up by the use of dissimilar metals. The Navy Board ordered a further experiment in 1769, this time constructing a new ship using only copper alloy bolts below the waterline. The new technique largely solved the problem, and the Royal Navy pressed ahead with adoption of the sheathing.
A fresh problem appeared: An enormous amount of copper would now be needed to build and maintain the fleet. Where would it come from? A 74-gun warship required more than 14,000 square feet of copper sheeting, held in place by two and a half tons of nails, not to mention the bolts in her hull, all of which now had to be copper or a copper alloy. Furthermore, news of the benefits of copper had spread beyond the navy. The East India Company began protecting its ships, and where the company led, other shipowners followed. The Lloyds of London list for 1786 shows 275 commercial vessels as copper-bottomed—a description that entered the English language, meaning “of lower risk,” as a result. Royal shipyards soon found themselves competing for the available supply of the metal.
Fortunately, on the Welsh island of Anglesey, a local landowner had decided to employ a mining engineer named Charles Roe to explore a hill called Parys Mountain. The sides of the great mound were peppered with old workings, some of them going back to the Bronze Age. Roe explored these using gunpowder and modern mining techniques to drive deep into the rock. In 1764, one of his miners struck lucky, uncovering “The Great Lode,” a huge deposit of copper ore that stretched far into the mountainside. The miner was rewarded with a bottle of whiskey and a rent-free house for the rest of his life. By the 1780s, the Parys copper mine was the biggest in Europe, and the Royal Navy had all the metal it needed to sheath its warships.