Submarine No.71 (Number 71) was an experimental high-speed submarine built for the Imperial Japanese Navy (IJN) during the 1930s.
Submarine No.71 was designed to test high-speed performance underwater. Intended to reach 25kn underwater and 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 2130NaN0 surfaced and 2400NaN0 submerged. Submarine No.71 was 42.8m (140.4feet) long, had a beam of 3.3m (10.8feet) and a draft of 3.1m (10.2feet).[1]
For surface running, the boat was powered by a single 12000NaN0 diesel engine that drove one propeller shaft. When submerged the propeller was driven by a 1800hp electric motor. She could reach on the surface and underwater. On the surface, Submarine No.71 had a range of 3830nmi at ; submerged, she had a range of 33nmi at . The boat was armed with three internal bow 45cm (18inches) torpedo tubes; each was provided with one torpedo.[1]
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,[1] almost five years before the famous German type XXI U-boats achieved speeds of around .[2] 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.[1]