A conventional torpedo hits a wall at roughly 50 knots, and it is not an engine problem. It is the water. Seawater is about 800 times denser than air, and hydrodynamic drag scales as D = ½ρCdAV². Power demand therefore scales with velocity cubed. Doubling the speed of a 533 mm torpedo demands eight times the shaft power, and the propeller surrenders long before the motor does. Once local pressure on the blade back falls below the vapour pressure of water, near 2.3 kPa at 20 degrees Celsius, vapour pockets form, collapse violently against the blade and destroy thrust and metal alike. Thrust breakdown, not fuel, sets the ceiling. The Soviet answer was to stop fighting cavitation and weaponise it. Design began at NII-24 in the 1960s, the M-5 prototype was completed in 1972, and the VA-111 Shkval entered service in 1977 with mass production from 1978. The mechanism: - A flat disc cavitator at the nose forces the flow to separate cleanly at a sharp rim, dropping pressure below vapour pre...