In mid-May 1776, Boston Harbor was the site of two notable skirmishes between colonial privateers and the departing British fleet. These actions occurred two months after the British evacuation of Boston but while the Royal Navy still maintained a presence in the harbor’s outer waters.

The Capture of the Hope (May 17, 1776)
On May 17, Captain James Mugford of the colonial armed schooner Franklin captured the British transport ship Hope without a fight. The Hope had sailed from Ireland and was unaware that the British had abandoned Boston. It carried a highly valuable cargo, including roughly 1,500 barrels of gunpowder and an assortment of military stores desperately needed by the Continental Army.

The Battle of Shirley Gut (May 19, 1776)
Two days later, on May 19, the Franklin and another colonial vessel, the Lady Washington, became grounded in the tidal flats near Deer Island and Shirley Gut (between Winthrop and Point Shirley). British naval forces from a man-of-war ship anchored nearby launched an attack using 12 to 13 boats filled with armed men.

Despite being heavily outnumbered, the combined American crews repelled the British attack, sinking several of the enemy boats and killing a number of British sailors. However, Captain Mugford was mortally wounded during the hand-to-hand combat. His crew successfully defended the ships, and the Franklin floated free the next morning, carrying Mugford’s body back to Marblehead.

Source:nps.gov