Ro-13 was a double-hulled Kaichu II boat capable of 16.5 knots surfaced and 6,000 nautical miles at cruising speed. Four reloadable bow tubes and two external tubes gave her ten torpedoes, while a three-inch deck gun added surface armament. Her 85-nautical-mile submerged range at four knots shows the continuing difference between surface cruising and underwater endurance. The navy's reluctance to test the design below roughly thirty meters also reveals how cautiously even a comparatively capable submarine was operated.
Chapter · 1:48:42-1:52:00
Ro-13 and Japan's Kaichu submarine design
Ro-13 combined useful range and six torpedo tubes with conservative diving limits that exposed Japan's uneven submarine development.