There were two major roles for a blockading fleet during the Age of Sail (and even into later conflicts):
1) to keep enemy ships of war bottled up in a port
2) to prevent trade from flowing to or from a port, or a whole nation
The first of those examples is what many people think of when they think of a blockade, and it’s the most obvious job of a squadron, but the second is arguably as important for wars that stretch out over a long period of time.
If we were to imagine a modal blockade, we might want to look at the blockade of Brest starting in the Seven Years War, and specifically at the events of 1759, because that was a major French port that the British had to blockade in that war and in the wars of the French revolution and Napoleonic era.
A blockading fleet had to accomplish two goals: it had to watch the entries into a port to prevent ships from leaving (or entering) it, and it had to present enough of a threat to pose a credible threat to the ships that the enemy fleet could amass if it tried to break out of a port. The blockading fleet then had to be comprised of ships that were heavy enough to stand in the line of battle (in the British context, ships of 74 guns or larger) as well as smaller ships that were nimble enough to work in near the port but that could flee any credible threats the enemy could mount to attempt to beat them off (usually frigates). In most cases, then, the fleet would be divided between an inshore and an offshore squadron, with ships in between (frigates or smaller ships) to relay signals between the two fleets.
Because these ships were, after all, sailing ships, the duty of the fleet becomes more difficult because of the winds and current conditions that could be experienced in a particular area. Broadly speaking, winds that would allow for ships to leave a port would tend to blow the blockading fleet offshore, while winds that kept ships in a port would blow the blockading fleet onshore (which is one reason why clumsier ships would be kept offshore, so as not to be wrecked). Obviously, close attention to the weather and watching out for storms was a major responsibility of ships on the blockading fleet. Additionally, blockading fleets still used up the same amounts of victuals (food, water, etc.) and naval stores (sails, spars, cordage, tar, gunpowder and shot for practice, etc.) as a fleet under sail would, so plans for supplying the fleet were crucial. Most admirals attempted to keep enough ships on station so that one or two could always be rotating back to a friendly port to re-provision and bring out mail and news.
Looking specifically at Brest, the dangers and opportunities of blockade become clear. In the 1750s, the dockyards of Breast were on the Penfield river, which issues into a large, enclosed harbor. The harbor reaches the sea through a narrow channel, the Goulet, which runs nearly directly east and west through high ground. There are two anchorages outside the Goulet, Berthaume Bay and Camaret Bay, which are both further protected from the Atlantic with reefs, rocks and islands, and there are three passages to the ocean from those anchorages. The Iroise is to the west, and is scattered with rocks; the Four passage to the north leads to the Channel but it is narrow and beset with a very fast tide-race, and to the south is the Raz de Sein, a very narrow passage through a set of reefs with a rock right in the middle of the northern end of the passage.
The tides flow through those passages at varying rates: the Goulet at 3 knots (nautical miles per hour), the Four at 4.5 knots and the Raz du Sein at 7 knots. The distance from the Goulet to the Raz is 25 miles, so unless a fleet had very exact timing it is nearly impossible to make the trip from the ocean into the harbor or vice versa except with exact timing, which means that ships had to anchor in one of the bays (Berthaume or Camaret) to wait for a tidal change.
This both complicated and simplified the task for the British. There was no one point in the sea from which to watch all three passages except for close in to the Goulet, but there was also no high ground at the western end of the Goulet for watchers to see a blockade fleet further offshore. The winds in the region generally blow from the southwest, which means that it was possible for the French to enter the Goulet most of the year, but leaving required an easterly or northerly wind, which meant the French usually used the Raz de Sein more than the other channels both for entering and leaving.
The French also used the Raz because, in the days before latitude was easy to find, ships usually approached a port by finding a landfall at a line of longitude (an east-west parallel) then “running down” that line until they saw a landmark. For the French, the simplest landfall was to Belle Isle (southwest of the Goulet) and then bearing up on the port tack to Brest or the starboard tack to Rochefort or Bordeaux.
An armchair admiral, then, would assume the best place to put a blockading fleet was to the southwest of Brest, near Belle Isle. The problem with that, though, is that any westerly gale would give the British a lee shore to the east which they would have to escape by heading to the southeast, into the Bay of Biscay and away from home. The British fleet in fact had to be kept to the west or northwest of Ushant, so that in case of westerlies they could seek refuge in one of the Channel ports (usually Torbay in Devon). The unfortunate fact of that is that a fleet in that spot can’t keep track of the Raz, so the offshore fleet would have to be stationed there with an inshore squadron able to pass messages to the offshore fleet and sound an alarm if the French tried to break out.
This is exactly what British admiral Sir Edward Hawke did in 1759: the bulk of his fleet lay off the northwest of Ushant, with two small ships of the line under Augustus Hervey anchored off the Black Rocks at the Iroise watching the Goulet. His ships were often blown off station, but a westerly wind usually meant that the French were bottled up in port even as the British ships were blown off blockade.
The reason for keeping the French fleet in port was that the French, growing desperate at their losses in the Americas, had decided in 1758 upon an invasion of Britain. The invasion fleet was assembling in Vannes, in the southwest of France, while the battle fleet was at Brest (at the time, there were only sketchy land communications with Brest — it relied on coastal shipping for nearly everything, and an army couldn’t assemble there). The fleet would have to break out of Brest, sail to Vannes to pick up the transports, and then evade the British fleet to land troops somewhere in Britain, which was a tall order.
The French were increasingly desperate to break out of port as 1759 drew to a close, and when a westerly gale blew Hawke off station in November, the French acted. The same day that the storm died down and Hawke left Torbay, the French left Brest. They were blown far to the west before they could come about and head for Vannes, and had trouble with the fleet because many of its men were inexperienced at sea after being bottled up in port. They sailed for Quiberon Bay, where the transports waited, with the British fleet on their heels, and made it almost there before sighting the British fleet. The French gambled that the British would not follow them into Quiberon Bay, because the British lacked charts of the area, but Hawke attacked at once and the French fleet fled. The British caught up with the tail end of the French fleet just as the van was entering the bay, and at that point the wind backed and headed the French, as well as kicking up an extremely rough sea.
The battle was a disaster for the French; the Thesee sank attempting to open its lower gunports (the ship flooded) and the Superbe sank after two broadsides from Hawke’s flagship. One French ship was captured; three were trapped in the Vilaine river with their guns thrown overboard to lighten ship; and six were wrecked or sunk. Two British ships were also driven ashore and wrecked, but their crews were rescued.
Quiberon Bay is one of the more dramatic and unusual battles of the Age of Sail, but the British fleet would again blockade Brest during the Napoleonic period. The blockade, in fact, became so routine that the British would often fish inshore of the Goulet, or anchor in one of the bays to dry sails or practice shifting topsails or lowering boats, to the infinite annoyance of the French.
In one of my favorite stories, Sir Sidney Smith even sailed his frigate into the Goulet by night, “hailing French ships in his faultless French to ask for news, and returning without detection with the latest information.” (Rodger, Command of the Ocean pp. 433). Granted, that was in 1795 and not during a period of close blockade, but it does emphasize the Royal Navy’s attitude toward the French.