[Note 1] 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.

Like all Type UB III submarines, UB-91 carried 10 torpedoes and was armed with a 10.5 cm (4.13 in) 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 (13,190 km; 8,190 mi).

She toured the South Wales ports of Cardiff, Newport, Swansea, Port Talbot and was towed to Pembroke Dock, eventually being broken up in Briton Ferry in 1921.

[6] King George V presented her deck gun to the town of Chepstow by in recognition of the bravery of William Charles Williams RN VC at Gallipoli in 1915.