Hello Len,

The following excerpt from 'The Torpedomen - HMS Vernon's Story 1872-1986' by Rear Admiral E N 'Nicho' Poland CB CBE may prove useful. The events described occurred soon after 24 November 1939 when the first German magnetic mine was recovered from the mud flats at Shoeburyness by Lt Cdr John Ouvry DSO RN and subsequently examined at HMS Vernon in Portsmouth.

"An alternative pre-war solution [against magnetic mines] was to put huge electro-magnets in the bows of 2-3,000 ton ships, of sufficient strength to detonate mines at a safe distance ahead. Wartime necessity now overcame peacetime parsimony and a mine destructor ship, Borde, was hurriedly improvised. Being a collier, with her engines and bridge accommodation right aft, she was eminently suitable for conversion to her new role. A 450 ton horizontal magnet, 105 feet long and 5 feet in diameter, was fitted in her fore hold. The magnet consisted of a coil of 317,000 ampere turns, the foremost end of which was raised through 9 degrees. Power was supplied by a 300kw diesel generator. Later the magnet was supplied with alternating current by motor alternators running off the DC generators. This proved to be a wise precaution when the Germans altered the polarity of their mines. The Borde's sweeping speed was limited to a maximum of 6 knots. With Churchill's enthusiastic backing the con­version work was hastened and on New Year's Eve 1939, Borde sailed from Portsmouth for trials in the Barrow Deep, the shipping channel to the north of the Thames Estuary. As she steamed up the Deep at 5 knots she was accompanied by a rescue drifter and a motor launch carrying mine counter­measures experts. The trial was a complete success: during the afternoon of 4 January 1940 a magnetic mine was detonated 150 feet ahead of the ship. As the Borde steamed through the column of water thrown up by the exploding mine, equipment, furniture and crockery were thrown around, and the switches controlling the current to the windings of the magnet were blown off. This was a serious matter as it exposed the ship to possible destruction by any other mine which she might encounter before she could restore the current to the magnet. With this success behind her, Borde was ordered to sweep the channel close to the Downs where she immediately destroyed two more mines.

Strange things began to happen on board this little ship. The mighty magnet in the forehold magnetised the whole ship. All objects made of ferrous metal were thrown around, with spanners leaping from men's pockets, and spoons and forks clinging together as if for protection. Worst of all, watches died an ignominious death. On 8 March, Borde's commanding officer, Lieutenant Commander Hudson, was awarded the Distinguished Service Order; the First Lieutenant, Lieutenant Morris, received the Distinguished Service Cross; and four of the crew the Distinguished Service Medal. In a short time the ship had disposed of twenty three magnetic mines and had been declared by Churchill, who visited her, as 'the most valuable ship in the Navy.'

The success of Borde led to the conversion of nine more colliers. Among these was Corburn, commanded by Lieutenant Commander Mark Welby. She was despatched to Loch Ewe [North Scotland] to clear the mines from the field which had damaged HMS Nelson. These mines had been laid in deeper water than those encountered by Borde in the Barrow Deep and in the Straits of Dover. This meant that they exploded much closer ahead of Corburn. She was, therefore, ordered to proceed to Stokes Bay [near Portsmouth] for trials and investigation by the Vernon Mine Sweeping Department. But the situation on the Continent had taken a serious turn. The Germans were advancing into France, and to deny the use of the Channel ports to the Allies they had carried out a mining campaign.

Corburn was diverted to Le Havre, which had been heavily infested with mines of a much greater explosive content than usual. She successfully detonated a number until, on 21 May 1940, the circuit breakers carrying the current from the alternators to the magnet were thrown off by the explosion of a mine. So close had the mines been laid that, in spite of the Corburn's engines having been put full astern, she drifted over the next mine which detonated immediately under her, cutting her in half and sending her to the bottom. The crew scrambled ashore whence they were able to escape to Britain before the Germans arrived at Le Havre. Lieutenant Commander Welby remained behind to organise minesweeping by two small trawlers. Sadly he was killed during these operations.

Apart from the loss of her experienced Captain, the destruction of the Corburn was serious for another reason: she had sunk in shallow water and it was comparatively easy for the Germans to investigate her secrets. As a result of this, they developed the Period Delay Mechanism which allowed the mine to be actuated by several pulses from the mine destructor ship's magnet before detonating. This placed all mine destructor ships at risk, and so their short lived usefulness was now at an end. Some were converted into mine­sweepers but the original mine destructor ship, Borde, was so heavily magnetised that even the most dedicated efforts by the degaussing teams failed to return her to her normal state of magnetism. She was converted to a minesweeper depot ship and made her way to North Africa where she became a well known sight amongst the little ships. At the end of the war she asserted her personality by refusing to be consigned to the breakers yard, foundering on her way there."

This activity happened in parallel with the development of protective degaussing coils or 'girdles', first trialled in an unnamed trawler belonging to HMS Vernon. Towed magnetic minesweeping systems replaced the mine destructor ships in the Spring of 1940. The first such system trialled was a magnetic coil in a barge towed astern of the drifter Feaco which contained a 17kw generator. Its successor, called 'the Skid', was towed by a trawler or drifter containing a 75kw generator. Around the same time, the headmaster of a well known school suggested that flat fish be fitted with small magnets and released in mine danger areas but this idea was never pursued.

Best wishes for a Happy New Year,

Rob Hoole
Vice Chairman & Webmaster
RN Minewarfare & Clearance Diving Officers' Association
[www.mcdoa.org.uk]