The first submarine of warfare was an egg-shaped single-seater built in 1775 by the American David Bushnell and called Turtle (Tartaruga). Propelled by hand-operated auger screw propellers, it was equipped with a ballast case that was filled with water for immersion and emptied with a hand pump for emergence. A lead ballast on the bottom kept the boat in position, which, however, lacking a means of oxygen supply, could remain in the water for no more than half an hour. During the American War of Independence he was employed in a failed attack on the British frigate Eagle anchored in the port of New York.