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Barrow Built

Minelaying Submarines, 1930 -1946

Porpoise , who gave her name to a class of six such submarines designed for minelaying

The satisfactory performance of M3 led to the design of a new submarine minelayer, the famous Porpoise Class. Specifically designed for the task of minelaying, six of the class were ordered in the 1930 Programme - the first-of-class Porpoise , Narwhal and Rorqual from Vickers, Grampus and Seal from Chatham and Cachalot from Scotts. Slightly slimmer than the Overseas Patrol Submarine, but with greater displacement, the new class had a capacity to carry 50 standard Mk XVI mines in a full-length deck outside the pressure hull.

During the trials of M3 it was found that whilst the minelaying gear and compensating arrangements to maintain trim were satisfactory, the jigger-type mine-launching equipment required excessive upkeep to ensure its efficiency. Therefore, this equipment was changed in the Porpoise Class to a chain and rack system fitted outside the hull in the superstructure casing. The mines and minelaying gear weighed approximately 54 tons. The conversion of M3 also had an adverse affect on its diving qualities - the time taken to flood the mine casing meant that it took about 5 minutes to dive in calm weather and 13 minutes or more in rough weather. Only by careful design arrangements was this reduced in Porpoise - she could dive, with mines on board, to periscope depth from full buoyancy in 1 minute 32 seconds and, using 0 tank, in 1 minute 14 seconds.

Rorqual , the only Porpoise Class submarine to survive World War II, She was taken out of service in 1946.

The hull form of Porpoise was very similar to that of the Overseas Patrol Submarine, Parthian, but the effect of carrying 50 mines meant that the stern torpedo tubes were deleted and the main engine horsepower was reduced by 25 per cent - resulting in a loss in surface speed. The design diving depth was also reduced, to 300 feet.

Although the external tanks were of 'non-leaking' welded construction, as in the Thames Class, it was thought that if exposed to depth charge attacks leaks would occur and give away the submarine's position. Therefore, the hull form for the Chatham-built Grampus was radically changed (saddle tanks were extended, the pressure hull was altered, etc.) so that oil fuel, which was carried externally in Porpoise , could be carried in internal tanks. This change also increased the main ballast water carried by Grampus by about 100 tons, and at the same time improved the stability and reserve buoyancy of the boat.

Stern view of Porpoise , showing rear minelaying doors

All vessels of the class had balloon tanks, i.e. pressure-tight tanks, in the forward superstructure to balance the buoyancy of the mines aft and prevent the submarine diving stern first.

Built between 1930 and 1938, the six boats of the Porpoise Class were fitted with two Admiralty-designed vertical four-stroke blast-injection six-cylinder diesels that together generated 3300 bhp, giving a surface speed, in service, of 16 knots. Tandem sets of motors on each shaft developed a total of 1630 bhp, giving a submerged service speed of 8.9 knots. Carrying 336 cells in three battery tanks weighing a total of 139 tons, the class had a submerged endurance of 66 miles at 4 knots.

In addition to the 50 mines, the armament of the class consisted of six 21-inch bow torpedo tubes (with 12 torpedoes carried) and a 4-inch gun (a 4.7-inch gun was originally fitted in Porpoise ) with 120 rounds of ammunition - this being chosen as the standard gun to be fitted in Royal Navy submarines. When the gun of Porpoise was changed to the standard calibre a weight saving of about seven tons was achieved.

When the Royal Navy produced a mine which could be laid from a 21-inch torpedo tube, the need for specialised submarine minelayers disappeared. Surprisingly, the introduction of this newly-designed mine did not lead to the redundancy of existing minelayers, and these purpose-built vessels proved extremely successful in the Second World War when used as supply submarines, running precious cargoes to Malta in 1941 and 1942. Their spacious mine-decks were filled with such diverse items as machine-gun ammunition, glycol coolant for Spitfires, and food.

Another Barrow-built Porpoise Class - the Narwhal .

Five of the six submarine minelayers were lost during the war, and the history of the class produces an unhappy diary of events:

• 5th May, 1940: Seal, the last of the class to be constructed, was captured as a wreck in the Kattegat and recommissioned as a German U-boat. She was finally scrapped in 1941.
   
• 24th June, 1940: Grampus was lost on active service in the Mediterranean -cause unknown.
   
• 1st August, 1940: Narwhal was lost on active service off Norway - cause unknown.
   
• October 1941: Cachalot was lost on active service - further details unknown.
   
• 19th January, 1945 (approximately) the first-of-class Porpoise was lost on active service in the Malacca Strait -probably sunk by aircraft.

The era of the large minelaying submarine came to an end when the only surviving member of the Porpoise Class, the Barrow-built Rorqual , was taken out of service in April 1946.

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