A reconstruction of David Bushnell’s Turtle, using the designer’s description as a basis, showed it to be well engineered and practical, though very tiring to operate.

The War of Independence in America inspired a young Yale graduate, David Bushnell, to build a ‘sub-marine vessel’ to break the Royal Navy blockade of the rebel colonies’ harbours. His Turtle was an egg-shaped wooden hull with room for one operator, who worked a rudder and a screw propeller. Two pumps allowed water ballast to be expelled, and the operator was also able to manoeuvre a vertical screw to attach a gunpowder charge to the keel of an enemy ship. Although the tiny submersible did not survive the war, Bushnell wrote a detailed explanation for Thomas Jefferson in 1787, which was sufficient for the Smithsonian Institution to make an accurate model 200 years later. The Turtle made history by carrying out the first underwater war mission, when in September 1776 an army sergeant named Ezra Lee paddled down the Hudson River to attack Lord Howe’s flagship, the 64- gun HMS Eagle. Lee located his target and struggled valiantly to attach the explosive charge to her keel, but the screw broke, and Lee was forced to work his way back upriver. It was assumed at the time that the coppering on the keel of the Eagle prevented the screw from penetrating, and for 200 years this theory went unchallenged. Modern research into Admiralty records shows, however, that HMS Eagle had no coppering applied to her underwater hull until a considerable time later. This leaves only two explanations: the first is that Ezra Lee encountered iron fastenings, perhaps near the rudder; the second is that he was suffering from the narcotic effects of breathing his own air for too long. The lethargy, combined with physical exhaustion, would, in the opinion of modern divers, have been sufficient to discourage the gallant sergeant.

Two further attacks by the Turtle were made later, but with no success, and the tiny craft was finally lost while being transported in a frigate which ran aground.