The British advantage in naval artillery diminished after the battleship was torpedoed on 13 May by the . A German submarine, , sank on 25 May and on 27 May. 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. The submarine (Lieutenant Commander Martin Nasmith, later awarded a Victoria Cross) passed through the Dardanelles on 18 May and sank or disabled eleven ships, including three on 23 May, before entering Constantinople Harbour, firing on a transport alongside the arsenal, sinking a gunboat and damaging the wharf.