Last week, we examined the birth of naval special warfare sabotage operations during World War I. Fast foreward to World War II and the British efforts to create a reliable weapon to be used by frogmen. The result, Mark 1/6 limpet mine.
Born as a crude knock-up of an idea, the Allied limpet mine would be produced in large numbers and used throughout the war.
First designed out of dime-store components, toys and household items the Mark 1 limpet mine was handed off to Special Operation Executive boffins for refinement. By late in the war the end result was the Mark 6, a small charge of 2.5 pounds of explosive designed to sink or disable a ship.
A series of six horseshoe shaped magnets framed the Mark Six, three on each side. These powerful magnets would hold fast to the steel hull of any enemy vessel.
The British-made limpet mine was ignited via a timed charge of two varieties. One was a traditional clockwork device with a fuse adaptor, the other was the AC MKI acetone-celluloid timer.
The fuse assembly came with a main booster charger, the small explosive that actually detonated the larger charger, and a series of acetone filled ampoules. The six ampoules, color coded to match a different time, were placed inside the booster charge assembly that was nestled in the limpet mine.
The ampoules- ranging from the four hour 'red' to the 'violet' 4 /12 days timer- were crushed when the fuse was screwed back together and the safety pin removed. The acetone would then slowly, measurably eat away at a celluloid disk inside the fuse assembly. When the acetone fully eats through the celluloid it releases a spring loaded pin that ignites the booster charge, ultimately igniting the larger limpet mine charge.
How did the Brits get the underwater mine onto the hulls of enemy vessels? While frogmen moved beneath the waves for both Allies and Axis forces, the most famous use of the limpet mines were via the surface, at night and from simple, but ruggedly reliable Klepper kayaks.
But how to place a charge below the waterline of a looming vessel when you, and your fellow commando are sitting in a low slung kayak? To hold the small kayak tight to the hull and avoid floating away, a simple magnet and rope system was devised. The second man in the boat would attach it to the ship, pulling the line taught and keeping it in close the towering gunwales.
To get the charge below the waterline, designers came up with a simple folding "placement rod." The limpet mine was attached to the rod, which dipped below the water and gave the commando the ability to quickly place the explosive device.
This is exactly what the Australian commandos of Z Force did in September, 1943.
The commando sextet would find cargo ships, wharves and a 10,000 ton Japanese tanker over a period of nights. Each limpet mine went off, without fail. Sending 30,000 tons of Japanese shipping to the bottom.
Sadly, the bold success of Jaywick spawned Operation Rimau, which ended in disaster and led ultimately to the death of every commando who embarked on the failed maritime sabotage operation.