 On 9 November 1918 , two days before the Armistice with Germany , UB @-@ 50 sank the British battleship HMS Britannia . The Britannia was on a voyage to Gibraltar when she was torpedoed off Cape Trafalgar . After the initial explosion , the ship began listing ten degrees to port . A few minutes later , another explosion started a fire in a 9 @.@ 2 in ( 230 mm ) magazine , which resulted in a cordite explosion in the magazine . The Britannia stayed at 10 @-@ degrees for 2 ½ hours before sinking . Its 16 @,@ 350 gross register tons made it the largest ship the U @-@ boat ever sank , and the only one UB @-@ 50 would sink during her last patrol . 
