Mountjoy's first attack on the boom at Londonderry left her damaged and grounded, with her captain killed. She subsequently refloated and forced a passage, with Phoenix also getting through and other ships supporting the effort. One account mentioned here credits firing the ship's cannon with helping her refloat. Breaking the obstruction reopened a route for reinforcement. A besieger therefore has to control water access as well as the land approaches when a city can receive help by sea.
Notable Point · 38:30-39:29
Mountjoy and the naval logic of relieving Londonderry
The relief of Londonderry illustrates why a besieger must control a city's water access as well as its land approaches.