SM UC-21 was a German Type UC II

German Type UC II submarine

Type UC II minelaying submarines were used by the Kaiserliche Marine during the World War I. They displaced 417 tons, carried 7 torpedoes and up to 18 mines...

 minelaying

Minelayer

Minelaying is the act of deploying explosive mines. Historically this has been carried out by ships, submarines and aircraft. Additionally, since World War I the term minelayer refers specifically to a naval ship used for deploying naval mines...

 submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 or U-boat

U-boat

U-boat is the anglicized version of the German word U-Boot , itself an abbreviation of Unterseeboot , and refers to military submarines operated by Germany, particularly in World War I and World War II...

 in the German Imperial Navy  during World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. The U-boat was ordered on 26 August 1915 and was launched on 1 April 1916. She was commissioned into the German Imperial Navy on 12 September 1916 as SM UC-21."SM" stands for "Seiner Majestät" (His Majesty's) and combined with the U for Unterseeboot would be translated as His Majesty's Submarine. In 11 patrols UC-21 was credited with sinking 100 ships, either by torpedo or by mines laid. UC-21 disappeared after departing Zeebrugge

Zeebrugge

Zeebrugge is a village on the coast of Belgium and a subdivision of Bruges, for which it is the modern port. Zeebrugge serves as both the international port of Bruges-Zeebrugge and a seafront resort with hotels, cafés, a marina and a beach.-Location:...

 for the Bay of Biscay

Bay of Biscay

The Bay of Biscay is a gulf of the northeast Atlantic Ocean located south of the Celtic Sea. It lies along the western coast of France from Brest south to the Spanish border, and the northern coast of Spain west to Cape Ortegal, and is named in English after the province of Biscay, in the Spanish...

on 13 September 1917.