Harbor Defense of Portsmouth - WWII Underwater Defenses

HD Portsmouth Underwater Defenses 1945. Each triangle represents a group of 19 mines.

During World War II Portsmouth Harbor was protected by a series of underwater defenses that included controlled underwater mines, underwater hydrophones, underwater magnetic loop detectors and an underwater anti-torpedo boat nets across the harbor channel. Many of the defenses of Portsmouth were focused on protecting the minefields from small fast minesweepers and motor torpedo boats. The mine field defenses included three 3" gun batteries, two 90mm AMTB batteries and four portable searchlights.

The Portsmouth Harbor Mine Fields were made up of fourteen groups of 19 mines each. The mine casemate #2 at Fort Foster controlled mine groups 1 to 7 while mine casemate #1 at Fort Constitution controlled mine groups 8 to 14. The hydrophones and magnetic loop detectors were positioned in front of the mine groups to detect approaching ships. Ship tracks were plotted in the plotting room in mine casemate #1. Control of the minefields came from the two mine casemates under a group command. Arming and firing the mines was done in the respective mine casemates. Most of the mine support structure was located at Fort Constitution except for the explosive storage facility at Fort Dearborn.