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