The Teredo worm is a long, slimy, grey mollusc that can grow up to three feet long and an inch thick. It was feared by 18th century sailors, because of its voracious appetite for wood. Teredos bore long cylindrical holes into the timbers of ships, often in such numbers that only a thin wall is left between each worm’s chamber, reducing the strongest oak to little more than a honeycomb. As the ship becomes ever more fragile, the hull can break apart in the open sea, perhaps under the stress of rough weather, and without any warning. Sailors would recount dark tales of ships that vanished, far out at sea, when their worm-infested bottoms simply fell away.
The Caribbean has the most Teredo-infested waters, thanks to its warm sea and high salinity. In 1492 the unsuspecting Columbus arrived with his three wooden ships. When he returned to Europe, he took with him a fleet packed with impressive treasures. Unfortunately, below the waterline, he also brought back some less welcome creatures from the New World. The worms took some time to adapt to the cooler waters of Europe, but by 1731 they were in the North Sea, where they caused the collapse of the wooden supports used in the dikes of Holland, resulting in extensive flooding. As the wooden ships of Europe spread out to explore the world, they took the worms with them, until the Teredo could be found in every ocean.
Various techniques were trialled to combat the menace. Ship’s hulls were coated with mixtures of horsehair and pitch, but this proved of limited value at stopping the worms. Other vessels were covered with a sacrificial layer of pine, in the hope that the worms might feast on that wood in preference to the actual hull. Regrettably, they simply bored through it and on into the timbers of the ship. Captains had to take their vessels out of the water regularly to have their bottoms careened. Any weed growth would be removed, before fire was used to scorch and kill the worms. This was effective but, as soon as the ship was afloat once more, new worms would re-infest the hull.
The first experiments into sheathing the hull with an impenetrable barrier were carried out by the Spanish. They covered the hulls of several ships with sheet lead. This did work, but was a far from an ideal solution. Lead is very heavy, so a lead-sheathed ship has a much reduced carrying capacity. They also discovered that the presence of the lead set up a corrosive action through electrolysis, between the sheathing and all the iron nails and bolts used in the hulls construction.
In the middle of the 18th century, the British Admiralty finally arrived at the solution to the problem. Instead of lead, they used much lighter copper plates to armour a hull. They also replaced most of the iron fixings below the water level of the ship with ones made from copper instead. This proved to be an effective solution, which came with the further advantage that weed and barnacle growth was much reduced on the poisonous copper. There was only one issue to resolve. Where on earth were they to find enough of the metal to sheath the entire navy?
Meanwhile, on the Welsh island of Anglesey, a mining engineer called Charles Roe was exploring Parys Mountain, hoping to open a mine. The sides of the great mound where peppered with old workings, some of them going back to the bronze age. In 1764, one of his miner’s struck lucky, uncovering “The Great Lode”, a huge deposit of copper ore that stretched deep into the mountainside. The miner was rewarded with a bottle of whisky, and a rent free house for the rest of his life. By the 1780’s the Parys copper mine was the biggest in Europe, and the Royal Navy had all the metal they needed to protect their warships.