“After the war [in 1918] when Japanese shipbuilding facilities were considerably modernized and relations with Anglo-American powers were in some respects becoming strained, the arrival of seven German U-boats [as war prizes] offered a special opportunity. Some Japanese believed that they could help redress the naval imbalance by changing the character of their submarine force for potential use as a powerful arm of the battle fleet.

“… The Japanese were quick to recognize the need for foreign assistance as they prepared to assimilate features of these U-boats into their own schemes of building submarines; naturally they sought German submarine designers, technicians, and former U-boat officers. Indeed, shortly after Japanese received the German U-boats, Capt. T. Godo of the Japanese navy was named head of a naval mission to Germany.

“The American military attaché in Tokyo reported to the [U.S.] War Department on 9 October 1919 that the Japanese were ‘now in Berlin for the purpose of studying submarine construction from German naval designers … Captain Godo expects to obtain German patents and designs for submarines, and also expects to bring German naval mechanics back with him to Japan.'”

– The Japanese Submarine Force and World War II, by Carl Boyd & Akihiko Yoshida, 2013

IJN O-5 (former SM U-99) war prize submarine, c. 1920. Commissioned into Imperial German Navy service in late 1918 just months before the end of World War I, U-99 would be among the dozens of German submarines surrendered as war prizes to the victorious Allies, including Japan.

See also:
Imperial Japanese Navy Ro-29 class commerce raider submarine, c. 1930.
I.J.N. I-71 K6-class submarine & submarine depot ship “Taigei”, c. 1935.

“SM UC-99 was a German Type UC III minelaying submarine or U-boat in the German Imperial Navy (German: Kaiserliche Marine) during World War I.

“The submarine was designed for a maximum surface speed of 11.5 knots (21.3 km/h; 13.2 mph) and a submerged speed of 6.6 knots (12.2 km/h; 7.6 mph). When submerged, she could operate for 40 nautical miles (74 km; 46 mi) at 4.5 knots (8.3 km/h; 5.2 mph); when surfaced, she could travel 9,850 nautical miles (18,240 km; 11,340 mi) at 7 knots (13 km/h; 8.1 mph).

“UC-99 was fitted with six 100 centimetres (39 in) mine tubes, fourteen UC 200 mines, three 50 centimetres (20 in) torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 10.5 cm (4.1 in) SK L/45 or 8.8 cm (3.5 in) Uk L/30 deck gun . Her complement was twenty-six crew members.

“The U-boat was ordered on 12 January 1916 and was launched on 17 March 1918. She was commissioned into the German Imperial Navy on 20 September 1918 as SM UC-99. As with the rest of the completed UC III boats, UC-99 conducted no war patrols and sank no ships. She was surrendered to Japan on 22 November 1918.

“The U-boat was renamed O-5 for Japanese service from 1920 to 1921. O-5 was dismantled at the Yokosuka Navy Yard between March and June 1921. The hulk of O-5 was disposed of as a gunnery and torpedo target in October that same year.”

– Wikipedia