Kish Bank, in Dublin Bay. Iron Duke had her bowsprit wrecked, but was otherwise little damaged. Her ram, however, had torn a hole in Vanguards side. The ram also damaged the watertight bulkhead between Vanguards engine and boiler rooms which flooded both compartments and prevented her crew from using her steam-powered pumps. The ship sunk in a little over an hour after all of the crew abandoned ship. Following the collision, Iron Duke was overhauled at Plymouth Dockyard, with attention being given to the watertight doors on board. At 10:00 on 20 November 1877, Iron Duke departed from Plymouth for sea trials. She was out when it was found that the main sluice valve had been left open and she was sinking. Her crew closed the watertight doors and manned the pumps. An order was given to fire the distress signal, but it was found that there was no powder on board. The flag signal for "sinking" was made, but it was not noticed by for fifteen minutes. Black Prince repeated the signal to Mount Wise, which repeated the signal to Plymouth. In the meantime, a crewman had managed to close the valve. He was waist deep in water and had he been a few minutes later a diver would have been required. With the valve closed, the pumps were able to clear the water, and the ship was dry at 15:00. She put back to Plymouth, the trial being cancelled. It was subsequently revealed that four condenser valves, each diameter were involved. Difficulty in closing them was caused by excessive stiffness in the springs. This was alleviated by the fact that the valve handles had been lengthened during the refit, giving greater leverage. It was reported that efforts were made by those responsible for the refit to obstruct the Admiralty enquiry into the event. Following the loss, Iron Duke replaced Vanguard as the guardship at Kingstown, County Dublin, where she received the latter's crew and remained until July 1877 when the ship began a lengthy refit that lasted until August 1878. She was inspected by Admiral Thomas Symonds, Commander-in-Chief, Plymouth, on 22 July. Iron Duke then departed Plymouth on 4 August, bound for the China Station; en route, she pulled the P&O steamship off a reef in the Red Sea on 7 September after two days' effort. Vice-Admiral Robert Coote hoisted his flag aboard Iron Duke on 9 November.