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 her Q tank, in 1 minute 14 seconds. 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
