Submarine No.71

Submarine No.71 (Number 71) was an experimental high-speed submarine built for the Imperial Japanese Navy (IJN) during the 1930s.

Design and description
Submarine No.71 was designed to test high-speed performance underwater. Intended to reach 25 kn underwater and 18 kn on the surface, she proved to be too underpowered to reach those goals. Nonetheless, the boat was the fastest submarine in the world underwater when built, beating the previous record set by the similar World War I-era British R-class. She displaced 213 LT surfaced and 240 LT submerged. Submarine No.71 was 42.8 m long, had a beam of 3.3 m and a draft of 3.1 m.

For surface running, the boat was powered by a single 1200 bhp diesel engine that drove one propeller shaft. When submerged the propeller was driven by a 1800 hp electric motor. She could reach 13.25 kn on the surface and 21.25 kn underwater. On the surface, Submarine No.71 had a range of 3830 nmi at 12 kn; submerged, she had a range of 33 nmi at 7 kn. The boat was armed with three internal bow 45 cm torpedo tubes; each was provided with one torpedo.

Construction and career
Submarine No.71 was laid down by the Kure Naval Arsenal in December 1937 and was launched that same month by being lowered into the water by a crane. She was completed in August 1938; trials showed that her small size and low-powered diesel made her hard to handle on the surface. While incapable of her intended speeds, she exceeded a submerged speed of 21 knots, almost five years before the famous German type XXI U-boats achieved speeds of around 18 kn. After extensive evaluations the boat was scrapped in 1940, and the lessons learned contributed to the development of the Sen Taka-class, and the Sen Taka Sho-class.