German Type XXIII submarines were the first so-called elektroboats to become operational. They were small coastal submarines designed to operate in the shallow waters of the North Sea, Black Sea and Mediterranean Sea, where larger Type XXI Elektro boats were at risk in World War II. They were so small they could carry only two torpedoes, which had to be loaded externally. As with their far larger brothers — the Type XXI — they were able to remain submerged almost all of the time and were faster than all previous designs worldwide, due to the improved streamlining of their shape, batteries with larger capacity and the snorkel, which allowed the diesel engines to be used while submerged. The Type XXI and XXIII U-boats revolutionized post-war submarine design.