"Ajax" was converted to a blockship with screw propulsion for coastal defence (also called 'steam-guard-ships') in 1846. The conversion process involved removing her copper, ballast and some of the bulkheads, and cutting her down in the shape of a blockship. From 1846 until 1853 she was stationed as a guardship in Queenstown, now Cobh. She took part in the Crimean War 1853-1856. In 1854 she was involved in the Bombardment of Bomarsund, Finland. In 1858 she resumed guardship duties, this time in Kingstown, now Dún Laoghaire, where she remained until 1864 when she was decommissioned and broken up. Captain John McNeil Boyd R.N. was master of the "Ajax" while she was in Dún Laoghaire. On 8 February 1861 there was the worst storm in memory. 29 ships were lost between Wicklow Head and Howth Head, all close to Dún Laoghaire. Boyd organised rescues, but he and five of his crew were lost. Fifteen surviving members of the "Ajax" crew were decorated for bravery and most were promoted. There are many memorials to Boyd and his men. On 3 December 1863. "Ajax" was driven ashore at Kingstown. She was refloated. It was announced in February 1864 that the would replace her as the Coast-guard ship at Devonport. She was broken up in 1864.