The Line Ahead Formation, as part of the Fighting Instructions limited the ships to broadside salvos. Turning across the bows allowed the turning ships to fire at the bow of one ship and the stern of another simultaneously!
The tactic of turning across the bows of enemy ships of the line, was devised by an amateur naval theorist named John Clerk of Eldin,Scotland. Defying the conventional wisdom of the Admiralty's Fighting Instructions,and "The Line Ahead Formation" demanded by the Fighting Instructions. The tactic was first used by Admiral George Brydges Rodney during the battle of "The Saints" in the Caribbean. Clerk began thinking about Naval tactics after the defeat of the British fleet after the battle of the Chesapeake during the American Revolutionary War! Nelson first used this tactic successfully at the battle of Aboukir Bay in Egypt,and later in the Trafalgar campaign.