When the Royal Navy produced a mine that could be laid from a 21-inch torpedo tube, the need for specialized 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.
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 - details unknown.
19th January, 1945 (approximately): the first-of-class Porpoise was lost on active service in the Malacca Strait - probably sunk by aircraft.
