"sans pareil" was initially designed as an 80-gun second rate, to the lines of the earlier hms "sans pareil", a french prize captured in 1794. she was ordered on 27 february 1843 and laid down on 1 september 1845 at devonport dockyard. the rapid development of naval technology during this period led to fears that she would be obsolete before she could be launched, and work was suspended on 2 october 1848. a new design was drawn up utilising steam power, which was approved on 18 may 1849, and the conversion was duly carried out. she was eventually launched on 18 march 1851, having cost a total of £126,432 to build, with the machinery costing another £30,888. the conversion had lengthened her by , while the extra weight of the machinery necessitated a reduction in the number of guns, from 80 to 70.