Scale 1:8. Plan showing sections through Station 25 and Station U, illustrating the increase in the wale and planking thickness for 'Impregnable' (1810), a 90-gun Second Rate three-decker. The plan is undated, but may relate to the alterations in 1825 when the ship underwent middling repairs at Plymouth Dockyard between February 1825 and April 1826. According to the Progress Book entry for 'Impregnable', this included the increase in the ship's breadth by adding chocks to the timbers and replanking in place of the usual method of doubling the planking.

NMM, Progress Book, volume 6, folio 22, states that 'Impregnable' was docked at Plymouth on 30 December 1822, and her copper removed. She was relaunched on 22 April 1826 having undergone a small to middling repair. An annotation dated 13 January 1825 states 'directions given to increase the breadth of this ship by bringing clocks on the timbers and placing the regular thickstuff planks over them instead of the usual method of giving more stability by doubling, this plan proposed by Rob't Seppings'.