Complex sentence: After the Battle of Trafalgar in 1805, the demand for the large three-decker first and second rates diminished, and in 1811 "Queen" was razeed to become a two-decker third rate of 74 guns.
Split into simpler sentences: After the Battle of Trafalgar in 1805, the demand for the large three-decker first and second rates diminished. Consequently, in 1811 the Admiralty had "Queen" razeed to become a two-decker third rate of 74 guns.