Prototype Minelaying Submarine
This represented Britains first purpose built minelaying submarine.
Porpoise was of saddle-tank construction with the hull form simillar to the Parthian class submarine. Most of the fuel was carried in external welded tanks. The mines were carried on a single line of rails fitted on top of the pressure hull and enclosed by a non-watertight high casing. This extended from 65 feet from the bow to the stern doors. The rails ran down the centre line of the boat. This resulted in the periscopes and mast etc. being offset to starboard and special ballas being carred on the port side to balance weights.
2 Admiralty x 1,630 bhp electric motors.
8.75 kts sm
1 x 4.7/45 QF Mk IX.
2 x 0.303 Lewis Machine guns.
50 Mk XVI mines.
(In 1934 the 4.7 inch gun was replaced by a 4 inch/40 QF Mk XII.
From 1941 12M Mk II ground mines could be caried in place of torpedoes, later 1 - 20mm HA (Oerlikon) was almost certainly added.)
64 nm at 4 kts sm