These ships were seized on 5 April 1854, after the outbreak of the Crimean War.

After their seizure from the builders, HMS Cossack was bought for £9,591 (including £4,187 for the machinery) plus £715 for excess of tonnage on the ship, £2,206 for modifications made for British service.

HMS Tartar was bought for £16,607 (including £4,187 for the machinery) plus £715 for excessive tonnage and £1,883 in modifications for British service.

[1] The ships' modifications and fittings were completed at Chatham Dockyard between 1854 and 1855.

[2] The two Cossack-class ships had eighteen 8in-guns on the upper deck, as well as two 68 pdr-pivot guns.