Why Warships Were Sheathed in Copper

[A] A wooden hull in warm water was eaten by shipworm and slowed by the weed and shells that settled on it, and a vessel six months at sea could lose a third of her speed. [B] Copper sheeting fixed over the planking stopped both at once. The metal gives up a trace of itself into the water at its surface, which kills what tries to take hold there, and the worm cannot reach timber it cannot touch. [C] The navy that adopted the practice in the seventeen-sixties had ships that stayed fast between refits. [D]

The gain arrived with a defect nobody had looked for. Copper and iron standing together in salt water behave as a cell, and the iron bolts holding the hull together began to waste beneath sheathing that was itself in excellent condition. Ships were lost for a reason that could not be seen from outside, and the remedy was not to give up the copper but to take out every iron fastening below the waterline and put in one that would not corrode in its company. The episode is usually told as a success for practical chemistry. It reads better as a warning. The material had been judged on what it did to the sea, and not on what it did to the rest of the ship.