On 13 December 1914, B11, commanded by Lieutenant Norman Douglas Holbrook, entered the Dardanelles and torpedoed the Ottoman battleship Mesûdiye. B11 had been selected for the mission over HMS B9 and HMS B10 due to having been fitted with a new battery. The French boats at the submarine base had been rejected as it was agreed that they were less suitable than the British submarines. Guards were constructed over the various projections on the submarine's hull to reduce the risk of snagging on mines.
B11 started its journey from Tenedos. Before the submarine reached the first row of mines it was discovered that one of the guards had become damaged making it worse than useless. The guard was removed and the attack continued. B11 then had to pass under five rows of mines through then uncharted currents. Passing through the minefield took about five hours. After B11 hit Mesûdiye it took eight hours for the submarine to escape initially with any attempt to use the periscope resulting in heavy fire.
Lt. Holbrook was awarded the Victoria Cross, the first for service in a submarine, his First Lieutenant, Sydney Winn, was awarded the Distinguished Service Order, and every member of the crew was awarded the Distinguished Service Medal.
On 24 July 1916 the Prize court decided that the submarine's company was entitled to prize money for the sinking of Mesûdiye, and an award of £3,500 was made, of which Holbrook received £601 10s 2d, Win £481 4s 2d, chief petty officers £240 12s 1d, and seamen £120 6s 1d. This represented three years' pay for a seaman. On 24 August 1915 the town of Germanton in New South Wales, Australia, was renamed "Holbrook" in his honour and a replica of B11 can be seen there