The British Navy's nickname for a torpedo is 'tin fish'. It is an underwater powered projectile which can be launched from a ship, submarine or aeroplane to explode against a hostile ship. Robert Whitehead, a British engineer, designed the first self-propelled torpedo over a hundred years ago. Torpedoes strike their target below water. When a shell or bomb explodes above the water line, much of its power is wasted on air, which is compressible. But water is inelastic and the full impact of the explosion blasts the ships hull.