Who was Nelson's second-in-command at the Battle of Trafalgar?

Vice Admiral Cuthbert Collingwood, 1st Baron Collingwood (26 September 1748 – 7 March 1810) was an admiral of the Royal Navy, notable as a partner with Lord Nelson in several of the British victories of the Napoleonic Wars, and frequently as Nelson's successor in commands.

The combined fleet sailed from Cadiz in October 1805. The Battle of Trafalgar immediately followed. Villeneuve, the French admiral, drew up his fleet in the form of a crescent. The British fleet bore down in two separate lines, the one led by Nelson in the Victory, and the other by Collingwood in the Royal Sovereign. The Royal Sovereign was the swifter sailer, mainly because its hull had been given a new layer of copper which lacked the friction of old, well used copper and thus was much faster. Having drawn considerably ahead of the rest of the fleet, it was the first engaged. "See", said Nelson, pointing to the Royal Sovereign as she penetrated the centre of the enemy's line, "see how that noble fellow Collingwood carries his ship into action!" Probably it was at the same moment that Collingwood, as if in response to the observation of his great commander, remarked to his captain, "What would Nelson give to be here?"

On 9 November 1805 Collingwood was promoted Vice-Admiral of the Red and raised to the peerage as Baron Collingwood, of Caldburne and Hethpool in the County of Northumberland.