On 4 October 1918 UB-91 sank the Hirano Maru, killing among others, Yokohama Specie Bank sub-manager S. Ujie, his wife and three sons, together with bank employee Takashi Aoki and wife Sueko.[4][5]
She was built by AG Vulcan of Hamburg and following just under a year of construction, launched at Hamburg on 6 March 1918. UB-91 was commissioned later the same year. Like all Type UB III submarines, UB-91 carried 10 torpedoes and was armed with a 10.5cm (4.13in) deck gun. UB-91 would carry a crew of up to 3 officer and 31 men and had a cruising range of 7,120 nautical miles (13,190km; 8,190mi). UB-91 had a displacement of 510t (500 long tons) while surfaced and 640t (630 long tons) when submerged. Her engines enabled her to travel at 13 knots (24km/h; 15mph) when surfaced and 7.4 knots (13.7km/h; 8.5mph) when submerged.
