HMS Duke of York was one of Britain's five 14 in King George V class battleships, designed in the 1930s to 35000 ton Washington Treaty tonnage limits. She was flagship of the force that sank German battleship Scharnhorst on Boxing Day 1943 in a purely surface action, with no air involvement. Of the class only Prince of Wales (sunk by Japanese aircraft in December 1941) did not survive WW2.
U-boats were German submarines that caused havoc in World War Two during the Battle of the Atlantic. U-boats were so damaging that Winston Churchill commented that it was the only time in World War Two that he thought Britain would have to contemplate surrendering.