How fast is 20 knots in m p h?

The Swift Boats had welded aluminum hulls about 50 feet (15 m) long with 13 ft (4 m) beam, and drafted about five feet (1.5 m). They were powered by twin 480 hp (360 kW) Detroit Diesel engines with a design range from 320 nautical miles at 21 knots (590 km at 39 km/h) to about 750 nautical miles at 10 knots (1390 km at 19 km/h). Normal crew complement was six.