The 15th century saw the development of the man-of-war, a truly ocean-going warship, carrying square-rigged sails that permitted tacking into the wind, and heavily armed with cannon. The adoption of heavy guns necessitated their being mounted lower down than on top of the fore and after castles as previously where anti-personnel weapons had been positioned through the later Middle Ages, due to the possibility of capsize. This meant that what had earlier been the hold of a ship that could be used either as a merchant ship or warship was now full with cannon and ammunition. Hence ships became specialised as warships, which would lead to a standing fleet instead of one based on placing temporary contracts. The man-of-war eventually rendered the galley obsolete except for operations close to shore in calm weather. With the development of the sailing man-of-war, and the beginning of the great sailing fleets capable of keeping the sea for long periods together, came the need for a new adaptation of old principles of naval tactics.
A ship which depended on the wind for its motive power could not hope to ram. A sailing vessel could not ram unless she were running before a good breeze. In a light wind her charge would be ineffective, and it could not be made at all from leeward. It could still board, and the Spanish did for long make it their main object to run their bow over an enemy’s sides, and invade his deck. In order to carry out this kind of attack they would naturally try to get to windward and then bear down before the wind in line abreast ship upon ship. But an opponent to leeward could always baffle this attack by edging away, and in the meantime fire with his broadside to cripple his opponent’s spars.
An important organisational innovation was made by Sir Francis Drake. Prior to his leadership, a warship was typically run by a committee of the sailing master, navigator, master-gunner and captain of marines presided over by an aristocrat. Drake saw no purpose in having a member of the aristocracy without specialist knowledge and established the principle that the captain of the ship would be in sole command, based upon his skill and experience rather than social position. This transformation was never quite made in the Spanish Navy where the "gentlemen" continued to obstruct operations throughout the Age of Sail. The Revolutionary French Navy made an opposite mistake in promoting seamen without sufficient experience or training- which worked well in the Army, but not at sea. The Royal Navy by contrast was well served by many distinguished commanders of middle-class origin, such as Horatio Nelson (son of a parson), Jervis (son of a solicitor) or Collingwood (son of a butcher) as well as by aristocrats who proved themselves at sea such as Thomas Cochrane and even working-class, such as John Benbow.
The evolution of broadside cannon during the first half of the 17th century soon led to the conclusion that the fleet had to fight in a single line to make the maximum use of its firepower without one ship getting in the way of another.
The line of battle is traditionally attributed to the navy of the Commonwealth of England and especially to General at Sea Robert Blake who wrote the Sailing and Fighting Instructions of 1653. The first documented deliberate use seems to be somewhat earlier in the Action of 18 September 1639 by Dutch Lieutenant-Admiral Maarten Tromp against the Spanish. The tactic was used by both sides in the Anglo-Dutch Wars, and was codified in written 'fighting instructions'. These formed the basis of the whole tactical system of the 17th and 18th centuries in naval warfare.
One consequence of the line of battle was that a ship had to be strong enough to stand in it. In the old type of mêlée battle a small ship could seek out an opponent of her own size, or combine with others to attack a larger one. As the line of battle was adopted, navies began to distinguish between vessels that were fit to form parts of the line in action, and the smaller ships that were not. By the time the line of battle was firmly established as the standard tactical formation during the 1660s, merchant ships and lightly armed warships became less able to sustain their place in a pitched battle. In the line of battle, each ship had to stand and fight the opposing ship in the enemy line, however powerful she might be. The purpose-built ships powerful enough to stand in the line of battle came to be known as a ship of the line.
Only in heavy weather could the windward gage become a disadvantage, because the lower gun ports on the leeward side of a ship would be awash, preventing her from opening her lower-deck ports to use the guns – or risking being swamped if she did. So, in strong winds, a ship attacking from windward would not be able to bring her heavy lower-deck guns into action, while the enemy ship to leeward would have no such problem as the guns on her windward side would be raised by the heel. For this reason, Admiral Rodney ordered his ships to attack the Spanish from leeward in the stormy weather at the Cape St. Vincent in 1780.
The result was often that the ships of the attacking line which were steering to attack the enemy’s centre came into action first and were liable to be crippled in the rigging. If the same formation was to be maintained, the others were now limited to the speed of the injured vessels, and the enemy to leeward slipped away. At all times a fleet advancing from windward was liable to injury in spars, even if the leeward fleet did not deliberately aim at them. The leeward ships would be leaning away from the wind, and their shot would always have a tendency to fly high. So long as the assailant remained to windward, the ships to leeward could always slip off.
The wars of the 18th century produced a series of tactically indecisive naval battles between evenly matched fleets in line ahead, such as Malaga (1704), Rügen Island (1715), Toulon (1744), Minorca (1756), Negapatam (1758), Cuddalore (1758), Pondicherry (1759), Ushant (1778), Dogger Bank (1781), the Chesapeake (1781), Hogland (1788) and Öland (1789). Although a few of these battles had important strategic consequences, like the Chesapeake which the British needed to win, all were tactically indecisive. Many admirals began to believe that a contest between two equally matched fleets could not produce a decisive result. The tactically decisive actions of the 18th century were all chase actions, where one fleet was clearly superior to the other, such as the two battles of Finisterre (1747), Lagos (1759), Quiberon Bay (1759) and Cape St. Vincent (1780).
British naval innovation was retarded by an unseemly dispute between two Admirals in the aftermath of the Battle of Toulon. The British fleet under Admiral Thomas Mathews had been unable to draw level with the French fleet, and Mathews ordered an attack anyway, intending all the British ships to attack the French rear. He had no signals by which he could communicate his intentions, and the rear squadron under Vice Admiral Richard Lestock, his rival and second-in-command, obtusely remained at the prescribed intervals in line ahead, far to the rear of the action. A subsequent series of courts martial, in which political influence was brought to bear by Lestock's friends in Parliament, punished Mathews and those captains who had supported him in the battle, and vindicated Lestock. In several future actions, Admirals who were tempted to deviate from the Admiralty's fighting instructions were reminded of Mathews's fate.
The great French admiral Suffren condemned naval tactics as being little better than so many excuses for avoiding a real fight. He endeavoured to find a better method, by concentrating superior forces on parts of his opponent’s line in some of his actions with the British fleet in the East Indies in 1782 and 1783, such as the Battle of Sadras where Suffren tried to double the rear of the British line. But his orders were ill obeyed, his opponent Sir Edward Hughes was competent, and the quality of his fleet was not superior to the British.
Similarly, the British admiral Rodney, in the Battle of Martinique in the West Indies in 1780, tried to concentrate a superior force on part of his enemy’s line by throwing a greater number of British ships on the rear of the French line. But his directions were misunderstood and not properly executed. Moreover he did not then go beyond trying to place a larger number of ships in action to windward against a smaller number to leeward by arranging them at a less distance than two-cables length. An enemy who took the simple and obvious course of closing his line could baffle the attack, and while the retreat to leeward remained open could still slip away. Like Suffren, Rodney was a great tactician, but a difficult man to work with who failed to explain his intentions to his subordinates.
At the Battle of the Saintes on the 12th of April 1782, Rodney was induced, by a change in the wind and the resulting disorder in the French line, to break his own line and pass through the enemy line. The effect was decisive. The guns of the British ships were concentrated on a handful of French ships as the British broke through the French line in three places, and the tactical cohesion of the French fleet was destroyed. By the end of the battle, Rodney had taken the French flagship and four other ships. The successful result of this departure from the old practice of keeping the line intact throughout the battle ruined the moral authority of the orthodox system of tactics.
The hypothesis which governs all Clerk’s demonstrations was that as the British navy was superior in gunnery and seamanship to their enemy, it was in their interest to produce a mêlée. He advanced various ingenious suggestions for concentrating superior forces on parts of the enemy’s line – by preference on the rear, since the centre must lose time in turning to its support.
They are all open to the criticism that an expert opponent could find an answer to each of them. But that must be always the case, and victory is never the fruit of a skilful movement alone, but of that superiority of skill or of moral strength which enables one combatant to forestall or to crush another by more rapid movement or greater force of blow. Clerk’s theories had at least this merit that they must infallibly tend to make battles decisive by throwing the combatants into a furious mingled strife.
British ships not only had a higher proportion of seamen in the first place, but the long months at sea on blockade or convoy escort gave British captains plenty of opportunities to train their crews. British gun crews seem to have achieved a much higher rate of fire than French or Spanish gun crews, contributing to the much higher casualties suffered by ships from those fleets. The better seamanship, faster gunnery and higher morale of British crews was a decisive advantage that could not be compensated for by any amount of bravery on the part of their opponents.
The leading British admirals like Howe devoted their thoughts to how to break the enemy’s line in order to bring on the kind of pell mell battle that would bring decisive results. At the Battle of the First of June in 1794, Lord Howe ordered his fleet to steer through the enemy, and then to engage the French ships from the leeward, so as to cut off their usual retreat. This had the effect of bringing his fleet into a melee in which the individual superiority of his ships would have free play.
Throughout the wars, which lasted, with a brief interval of peace, from 1793 to 1815, British admirals like Jervis, Duncan and particularly Nelson grew constantly bolder in the method they adopted for producing the desired mêlée or pell-mell action at the battles of Cape St. Vincent, Camperdown and Trafalgar. The most radical tactic was the head-on approach in column used by Nelson at Trafalgar, which invited a raking fire to which his own ships could not reply as they approached, but then produced a devastating raking fire as the British ships passed through the Franco-Spanish line.
It has sometimes been argued that the tactics of these British admirals were rash and would have proved disastrous if tried against more skilful opponents. But this is one of those criticisms which are of value only against those who think that there can be a magic efficacy in any particular attack, which makes its success infallible. That the tactics of British admirals of the great wars of 1793–1815 had in themselves no such virtue was amply demonstrated at the Battle of Lissa in 1811. They were justified because the reliance of admirals on the quality of their fleets was well founded. It should be borne in mind that a vessel, while bearing down on an enemy’s line, could not be exposed to the fire of three enemies at once when at a distance less than 950 yards, because the guns could not be trained to converge on a nearer point. The whole range of effective fire was only a thousand yards or a very little over. The chance that a ship would be dismasted and stopped before reaching the enemy’s line was small.
In any event, such criticisms are of limited utility. Knowing the capabilities of one's opponent and devising tactics that take those into account is the primary focus of any battlefield commander. The notion, therefore, that the outcome of a particular battle could have been different if the opponent had changed ignores the role of tactical decision-making on both sides.
Such battles tended to be decisive, as a wind which was fair to allow the attackers to enter a harbour or anchorage would let neither side out again. As it would normally be more profitable for the attackers to blockade the enemy until they were forced to sortie to accept battle in open water, such attacks were usually forced by lack of time, e.g. by shortage of supplies, the threatened onset of bad weather or the need to coordinate operations with an army on land.
The defenders could expect to enjoy several advantages. As they would not need to manouevre under sail, most of the ships' crews could man the guns. If properly prepared, the ships would have "springs"; extra cables bound to the anchor cables, which they could haul in or let out to veer the ship to bring its guns to bear over a wide arc. If close to a naval establishment (such as at the Battle of Copenhagen), they could rely on boats from the shore to bring extra ammunition or replacements for casualties. Nevertheless, the defenders at Copenhagen were overcome by superior odds and gunnery.