SM U-86 was a Type U 81 style submarine manufactured in the Germaniawerft, Kiel shipyard for the German Empire during World War I.
On 27 June 1918, under the command of Lieutenant Helmut Patzig, U-86 sank the Canadian hospital ship HMHS Llandovery Castle off the coast of Ireland, in violation of international law and standing orders of the Imperial German Navy. When the crew took to the lifeboats, U-86 surfaced, ran down all the lifeboats except one, and shot at the people in the water. Only the 24 people in the remaining lifeboat survived. They were rescued shortly afterwards and testified as to what had happened. The 234 others on board Llandovery Castle were lost, including fourteen nursing sisters.
USS Covington, the former Hamburg America ocean liner SS Cincinnati, was torpedoed by U-86 on 1 July 1918 and sank the next day. Covington was the 17th largest ship sunk or damaged by U-boats during the war.
After the war, the captain of U-86 Helmut Patzig, and two of his lieutenants were arraigned for trial on war crimes, but Patzig fled to the Free City of Danzig, and his trial was stopped on 20 March 1931 by virtue of the Laws of Amnesty. Lieutenants Ludwig Dithmar and Johan Boldt were convicted and sentenced to four years in prison, but were released after only 4 months.
U-86 was surrendered after the war and sank in the English Channel on the way to be broken up in 1921.