On 17th April 1917, HMHS Lanfranc left Le Havre for Southampton, carrying 234 wounded British soldiers, 167 wounded German PoWs, 52 medical staff and a crew of 123. She was clearly marked in the internationally-accepted markings of a hospital ship, and was fully illuminated. She was escorted by a destroyer, HMS Badger, and the patrol boat P37. About 7.40pm, without warning, she was torpedoed by the German submarine UB-40, under Captain Hans Howaldt. The captain of Lanfranc immediately ordered the engines stopped, and as the ship listed to port and began to settle by the stern, he ordered abandon ship. The crew and the walking wounded struggled to get the more seriously injured up on deck and into the lifeboats, as the two escorts moved in and came alongside to assist. Lanfranc sank in just over an hour; only 34 aboard lost their lives.

Lanfranc had been built for the Booth Line by Caledon Shipbuilding in Dundee, and had been launched on 18th October 1906. She was 418 feet long, 6,28grt, and had become popular on their services from Liverpool to Le Havre and then Manaus on the Amazon. Her maiden voyage had been on 18th February 1907. She could accommodate 200 in First Class and 350 in Third Class when built. She was requisitioned soon after the outbreak of the Great War and was converted for use as a hospital ship.