On July 12th, 1776, the British warships Phoenix and Rose sailed beneath the unarmed Bluff Rock, later named Fort Lee. This provocative action led Congress to order General Washington "By every art and whatever expense to obstruct effectively the navigation of the river."
Washington soon adopted General Putnam's plan for blockading the river by sinking stone-filled ships' hulls chained together with protruding jagged spikes.
A series of ships was sunk, joined stern to stern 70 feet apart with three or more iron tipped, spiked logs fastened to each hull. The hulls were submerged just beneath the water so that the spiked logs became turned upward to expose their menacing hazards to passing ships, forming a 280-foot sub-surface barrier.
Although a fast-moving frigate might successfully pierce the blockade, damage to the vessel could be extensive. Just as importantly, ships attempting to pass would at least be slowed giving gunners on the river's heights and opportunity to aim broadside fire at their targets.
On the night of August 4th, General Mifflin's troops at Fort Washington worked on river obstructions when "Four ships, chained and boomed, with a number of amazing large Chevaux-De-Frise, were carefully sunk close by the fort."
"Chevaux-De-Frise" describes the field emplacements used by horseless, 16th century Dutch warriors who stopped the mounted Spanish army at Friesland using trees fashioned into spikes with jagged-edged sides. The Dutch called these implements of war, "The Horses of Friesland." Translated by the French, they became known as the "Chevaux-De-Frise."