Up to the outbreak of the World War, all submarines were essentially of the same type. They were all small boats of well under 1,000 tons displacement, of the coast defense type, designed to fire torpedoes, and with a limited cruising radius. None of these earlier boats carried mines or guns. The submarine art was still in the experimental stage and reliability was being striven for rather than development of types. Habitability, quick diving, sea keeping qualities and cruising radius were all poor. Diesel engines, which at present are practically the only prime movers used, were at that period just being introduced to replace the gasoline engines and were necessarily crude and unreliable.