The British advantage in naval artillery diminished after the battleship HMS Goliath was torpedoed and sunk on 13 May by the Ottoman destroyer Muâvenet-i Millîye, killing 570 men out of a crew of 750, including the ship's commander, Captain Thomas Shelford.^[29] A German submarine, U-21, sank HMS Triumph on 25 May and HMS Majestic on 27 May.^[30] More British reconnaissance patrols were flown around Gallipoli and U-21 was forced to leave the area but ignorant of this, the Allies withdrew most of their warships to Imbros, where they were "protectively tethered" between sorties, which greatly reduced Allied naval firepower, particularly in the Helles sector.^[31] The submarine HMS E11 passed through the Dardanelles on 18 May and sank or disabled eleven ships, including three on 23 May, before entering Istanbul Harbour, firing on a transport alongside the arsenal, sinking a gunboat and damaging the wharf.^[32] E11's attack on Constantinople, the first by an enemy vessel in over 100 years, had an enormous impact on Turkish morale, causing a panic in the city.