North Sea Mine Barrage - Mark 6 Mines

Mark 6 Mines

The Mk 6 mine was a 34 in (86 cm) diameter steel sphere containing a buoyancy chamber and 300 lb (140 kg) of TNT. Each mine was constructed of two steel hemispheres welded together. A Toxyl bursting charge was cast into the lower hemisphere. Toxyl was a mixture of 60% trinitroxylene (TNX) with 40% TNT used because the United States Army controlled United States TNT production and would not release sufficient quantities for the naval mine barrage. For transport, the mine rested atop a box-shaped steel anchor approximately 30 inches (76 cm) square. The anchor box had wheels allowing the mine assembly to be moved along a system of rails aboard the minelayer. The mine was connected to its 800 pounds (360 kg) anchor box by a wire rope mooring cable stored on a reel. The depth of the mine below the water surface was controlled by allowing the steel mooring cable to unwind from its reel as the mine was dropped from the minelayer until a sensor suspended beneath the anchor reached the bottom. The sensor locked the cable reel so the falling anchor would pull the buoyant mine below the surface; and the float extended the antenna above the mine. Each mine had two hydrostatic safety features intended to render the mine safe if it detached from its mooring cable and floated to the surface. The first was an open switch in the detonation circuit closed by hydrostatic pressure. The second was a spring pushing the detonator away from the explosive charge into the buoyancy chamber unless compressed by hydrostatic pressure. The mines were intended to be safe at depths less than 25 ft (7.6 m).

Each mine contained a dry cell battery with an electrical detonating circuit which could be initiated by any one of five parallel fuzes. Four of the fuzes were conventional horns in the buoyant upper hemisphere of the mine. Each horn contained a glass ampule of electrolyte which would connect an open circuit if an ampule was broken by bending the soft metal horn. The novel fifth fuze was a copper wire antenna with a float to extend it above the mine. A ship's steel hull touching the copper antenna would form a battery, and seawater acted as an electrolyte completing a circuit with an insulated copper plate on the mine surface to actuate a detonating relay within the mine. The relay armature was initially set to complete the detonating circuit at 25 to 40 millivolts. The Bureau of Ordnance subsequently increased sensitivity to 10 to 25 millivolts, but this was later readjusted on the basis of field experience. Each mine had five separate spring-loaded safety switches in the detonating circuit held open by salt pellets which took about 20 minutes to dissolve in sea water after the mine was dropped overboard from the minelayer. Battery life for the detonating circuit was estimated at greater than two years.