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Naval Anti-ship Mines

by John M. MacFarlane 2013

At Colwood British Columbia on a visit to one of the Royal Canadian Navy installations where the Explosive Ordnance Disposal team was headquartered I saw two old naval anti–ship mines on display in the parking lot.

Naval Mine

British H II Mark II Moored Naval Anti–ship Contact Mine (Photograph from John MacFarlane collection)

Developed during the First World War these mines were originally considered as very unreliable. After capturing unexploded German naval mines, and studying the mechanisms, new triggers were installed in what became the Type H II Mark II naval mine.

The H II Mark II mine was developed for the Royal Navy in the First World War as a contact–type. Hertz horns were the initiator. The horns were tubes made of soft metal. Inside of the tube was a glass vial containing sulphuric acid which would break on contact and the acid would run into a battery. Once activated the battery would then detonate the mine.

The box under the mine is the ’sinker’. Prior to deployment it provided a safe and secure storage container for the mine. But once it was dumped overboard the sinker was the weight that carried the whole apparatus to the bottom of the sea. The mine was connected to the sinker by a wire tether that unraveled from a reel until it reached the desired depth just under the surface where it might come into contact with the hull of a ship.

Naval Mine

USN Mark 6 Moored Naval Mine (Photograph from John MacFarlane collection)

The USN Mark 6 mine was designed originally for use by the Royal Navy during the First World War to seal off the North Sea as an exit route for German U–boats. It was combination contact and galvanic initiated mine. Early in its life it was considered as unreliable: up to 8% of the mines would detonate independently after planting. After securing some German mines the design of the mechanism was re–rigged using the captured technology and they became a mainstay of blockade mine deployment.

It was a spherical (34in/87cm) antenna–type initiator with some Hertz horns as a backup initiator. Around 1922 the acid in the Hertz horns was replaced by a switch trigger which detonated when hit by the hull of a ship. A ‘k-pistol’ used a copper antenna which extended upwards to just below the surface. This was connected to a copper plate on the outside of the mine. Seawater acted as an electrolyte of a battery which would be formed when a steel–hulled ship approached and touched the antenna. The current running down the antenna detonated the mine. This was known as the Underwater Electric Potential Effect.

In 1917 the U.S. Navy purchased about 100,000 of this design costing about $400 each. The mine was dangerous to handle and had safety devices installed to prevent premature detonation:

  • a time delay;
  • a hydrostatic switch which kept the detonator open until the mine sank several feet underwater.

Under international law, moored mines must be fitted with a safety device which renders them automatically safe if the mooring wire breaks. After the Korean War thousands of mines planted by North Korean and Chinese forces remained as a potent threat. Over the years many of them broke loose and it became apparent that they did not conform to the international convention. Some of them even washed up on beaches in British Columbia and had to be destroyed by Explosive Ordnance Demolition teams. It is still possible that there are live mines caught in the drift logs of remote beaches on the Pacific coast.

In the First World War 48 U–boats; were sunk by British mines. No enemy ships are known to have been sunk during the Second World War by the 20,000 defensively laid mines. Mines laid by U.S. submarines sank 27 ships and damaged 27 more. Aircraft–laid mines were more successful and after the Second World War that was the principal delivery mode. After each war the task of removing the Allied–laid mines took several years to conclude and was much more dangerous than the original task of laying them.


To quote from this article please cite:

MacFarlane, John M. (2013) Naval Anti–ship Mines. Nauticapedia.ca 2013. http://nauticapedia.ca/Articles/Naval_Mine.php

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