To make matters worse for the British and Hessians, the six British men-of-war were engaged by smaller American gunboats. During the engagement, two of the ships, the 64-gun ship-of-the-line HMS "Augusta" and the sloop of war HMS "Merlin" ran aground on a shoal trying to avoid underwater obstacles called "chevaux-de-frise" or stockades, which were rows of large wooden spears weighted down on the bottom of the river by heavy crates filled with rocks, designed to pierce the hulls of intruding British warships. Overnight attempts to free the ships were unsuccessful. Fort Mifflin and the Pennsylvania Navy engaged the stranded ships the next morning, with cannons and fire rafts, respectively. "Augusta" caught fire and within an hour the fire reached the magazine and the ship exploded with such force the explosion was heard in Philadelphia. The "Augusta" was the largest ship lost by the British in both the Revolutionary War and the War of 1812. Accounts vary as to the cause of the fire. In one account, the loss of "Augusta" was attributed to accidental ignition by the British. Navy historian James Fenimore Cooper's claims that the sides of the hull of the "Augusta" was packed with hay to repel musket and cannon shot, and during the exchange the hay caught fire and the fire rapidly spread. Crew members gave their testimony, but none could be definite. None remembered having seen or heard the explosion of any powder on the decks. Only Midshipman Reid ventured to suppose that the fire originated from the cannon wads. Admiral Lord Howe seemed to accept this explanation. Soon after, the crew of "Merlin" was ordered to abandon the ship and destroy it also.