In September of 1777, General Howe's British troops occupied Philadelphia. His brother, Admiral Richard Howe, commanded the large British fleet responsible for keeping the British army supplied via the Delaware River. Provisions were such a problem for the British that they seriously considered abandoning Philadelphia at least twice in the autumn of 1777. Washington and his general staff complained often about the supplies that managed to get through at night under the cover of British shore batteries on the Pennsylvania side, but it is probable that the defenses on the Delaware -- by restricting the convoys to night hours -- prevented much more from getting through.
The Americans had stretched a chevaux de frise (lines of iron-tipped pointed timbers resting on the bottom and calculated to tear open the hull of any ship trying to pass) across the river, as well as forts on the islands where the present navy yard is located. Fort Mercer, on the Jersey shore near Red Bank, gave supporting fire from the opposite direction.
The British fleet (see under: most powerful in the world at the time), included three 64-gun ships of the line (Augusta, Somerset, and the flagship, Eagle*) the 50-gun Iris and a number of frigates. To oppose them, the Americans had the 24-gun Delaware, the sloops Montgomery (10 guns) and Fly (8 guns), and a squadron of galleys. The galleys, whose oars made them independent of the wind, and whose flat bottoms allowed them the flexibility of traveling through shallow waters, were more maneuverable than sailing ships in the confined waters of the river. They carried heavier guns (18 and 24 pounders) than the frigates (12 pounders). Their one vulnerability -- like a modern crew shell -- was that they were prone to swamping in rough weather.
The galleys showed their teeth on 22 October 1777 when the British attempted a combined attack on Fort Mercer and Fort Mifflin. British charts were inaccurate, and the 64-gun Augusta and the 18-gun sloop Merlin ran aground. Twelve galleys attacked the Augusta at daybreak on the 23rd, and by eleven o'clock, she was on fire. (According to Captain "Lighthorse" Harry Lee's report, she "took fire from her own carronading.") Her crew abandoned her, and around two in the afternoon, she partially blew up, killing 41 of her crew. Both sides later salvaged cannons from the hulk in nighttime expeditions. For one of the largest battleships in the British fleet to be taken down by small boats must have been a severe humiliation.
The galleys scored again 5 November, when the British ships Roebuck and the mighty Somerset attempted to attack Fort Mifflin. Somerset's springs (ropes paid out from anchors at angles to the ship so as to allow maneuvering without sails) must have been improperly set, because she was swept off-station by the current and ran aground. Twelve galleys (perhaps the same dangerous dozen who had put paid to the Augusta) attacked her and shot her full of holes, so that she required assistance from other ships, and was nearly captured.
Two nights later, the gallies (sic) were able to break up a supply fleet of British boats, however, the nightly British supply runs were only partially interrupted. Six galleys were ordered to attack the British armed galley Vigilant (ex Empress of Russia) on 15 November, but either did not engage, or scored only a few hits. To be fair, Vigilant was out of their class, a real ship, carrying 16 cannon, while the American galleys probably carried one -- or at most three -- each.
Eventually, the sheer weight of British cannon that Howe's fleet could bring to bear pounded the forts into submission. Fort Mifflin was evacuated on 15 November after an epic defense and only after its entire structure had been smashed to ground level by shells. The defenders -- out of ammunition for their single functioning 32-pounder cannon -- had been reduced to the tricky expedient of using 18-pounder cartridges. Fort Mercer, on the Jersey side was abandoned on 21 November. Knowing that the galleys would be easy prey for the British fleet once the forts had fallen, Commodore Hazelwood ordered the thirteen remaining galleys rowed upriver to Burlington, New Jersey to preserve them.
* HMS Eagle carries the distinction of being the target of the first recorded attack (unsuccessful) by a submarine, David Bushnell's Turtle, in the Narrows off Staten Island, New York on 5 September 1776.