Ouvry and the magnetic mine

In November 1939 the Kriegsmarine unveils a new weapon: the magnetic mine (Magnetmine). Where the contact mine waits to be struck, this one wakes at the approach of a steel ship's magnetic field and explodes under the keel, at the most vulnerable point. Dropped by aircraft or by destroyer into the estuaries and ports of Britain, these devices wreak immediate havoc: twenty-seven ships sunk in November, twenty-one in December 1939, fifty-six more damaged. The threat is strategic: if the Royal Navy can no longer escort its convoys or protect the Thames, the British economy holds out six months, no more.

On 22 November 1939 a German He 115 drops two magnetic mines near Shoeburyness, in the Thames estuary, but in shallow water — only four metres at low tide. On the ebb one of them grounds intact on the mud. It is discovered at 13:00.

HMS Vernon, the Royal Navy's central anti-mine establishment at Portsmouth, is alerted at once. Lieutenant Commander John Ouvry, 41, an expert in dismantling, rushes to Shoeburyness with his team — Chief Petty Officer C.E. Baldwin and Able Seaman Vearncombe. By 17:30 the ebbing tide has uncovered the mine. For tools Ouvry has only brass screwdrivers, chosen so as to create no magnetic field; he has no diagram and no idea of the internal mechanism.

Ouvry must choose his method of approach.