SM UC-55

SM UC-55 was a German Type UC II minelaying submarine or U-boat in the German Imperial Navy (Kaiserliche Marine) during World War I. The U-boat was ordered on 12 January 1916, laid down on 25 February 1916, and was launched on 2 August 1916. She was commissioned into the German Imperial Navy on 15 November 1916 as SM UC-55.

Design
A Type UC II submarine, UC-55 had a displacement of 415 t when at the surface and 498 t while submerged. She had a length overall of 50.52 m, a beam of 5.22 m, and a draught of 3.61 m. The submarine was powered by two six-cylinder four-stroke diesel engines each producing 290 - 300 PS (a total of 580 - 600 PS), two electric motors producing 620 PS, and two propeller shafts. She had a dive time of 48 seconds and was capable of operating at a depth of 50 m.

The submarine had a maximum surface speed of 11.6 kn and a submerged speed of 7.3 kn. When submerged, she could operate for 52 nmi at 4 kn; when surfaced, she could travel 8660 to 9450 nmi at 7 kn. UC-55 was fitted with six 100 cm mine tubes, eighteen UC 200 mines, three 50 cm torpedo tubes (one on the stern and two on the bow), seven torpedoes, and one 8.8 cm Uk L/30 deck gun. Her complement was twenty-six crew members.

Service
In 6 patrols UC-55 was credited with sinking 9 ships, either by torpedo or by mines laid.

Loss
UC-55 sailed from Heligoland on 25 September 1917 to lay mines in the Lerwick Channel, the southern approach to the port of Lerwick in the Shetland Islands. On 29 September, just as she started dropping her mines, she suffered a loss of trim which resulted in her diving beyond her rated maximum dive depth. This in turn resulted in the forward compartment flooding, the batteries failing, and chlorine gas developing. She was forced to surface to ventilate the boat, but when she surfaced, the rudder refused to answer the helm due to the lack of battery power. Her captain then gave orders to destroy secret documents and codebooks and set scuttling charges in the mine room and engine room. While the charges were being placed she was sighted by the armed trawler Moravia and the destroyers HMS Tirade (1917) and HMS Sylvia (1897).

A 12-pdr shell from the Sylvia hit the submarine's conning tower, killing her commander, Horst Ruhle von Lilienstern, and a second shell hit the hull and she began to sink, after which two depth charges were dropped right beside the UC-55, resulting in the U-boat blowing up. The Moravia then closed with the wreck, fired two more shots into her, and dropped a final depth charge. Of the submarine's crew, 17 were taken prisoner, and 10 were killed.

Location
The wreck of UC-55 was discovered by side scan sonar lying on its keel on 3 July 1985, at a depth of 105 m at 60.05361°N, -0.96583°W. The submarine's remains were noted to be approximately 5.60 m high and 50 m long.

On Friday 21 July 2023 the wreck was confirmed to be that of the UC-55 by the first divers to visit site.