When the Revolution began, Congress ordered construction of river defense systems to protect Philadelphia against a sea attack. A system of cheveaux-de-fries, a series of underwater obstructions, was developed. These consisted of large coffers (boxes) made of pine logs lined with planking. Long, heavy poles with iron tipped spikes projected out of the coffers at a 45 degree angle. The coffers were floated to strategic positions in the Delaware, filled with rock and sunk just below water level to impale enemy ships sailing upstream.
The Pennsylvania Council of Safety oversaw planning and construction of the fortifications, which included the cheveaux-de-fries, land batteries, and a navy to guard the river. Three sites were selected: Fort Mifflin, Fort Mercer at Red Bank and Billingsport. Row galleys were designed by Philadelphia shipwrights and formed the backbone of the Pennsylvania navy. The fifty-foot boats, with a cannon mounted in the bow, patrolled the river.
On October 23, 1777, the day after the victory at Red Bank, the 64 gun British warship Augusta exploded and sank down river under heavy American attack. The similar loss of the Merlin, a sloop carrying 18 cannons, and the victory of Colonel Christopher Greene delayed the British attempt to clear the river defenses.