U-boat is an anglicised version of the German word U-Boot. It is the shortening of underseaboat. It describes the submergible watercraft that they developed and used extensively for world war i.
German began to employ U-boats to break a stalemate at sea. U-boats were traveling under water ships.
They were used to sink British supply ships with no warning. Due to the British blockade, with it's fleet bottled up, Germany had to use U-Boats to get at the British lifeline across the Atlantic.
