An item reported that the British Royal Navy almost doubled the displacement (around 600 tons) of the D-type submarines compared with displacement (310 tons) of the C-class submarines. The number of torpedo tubes increased from 2 to 4. Speed of the D1 (pennant 61) was 16 (surfaced)-9 (submerged) miles. Height when surfaced was around 5,5 feet/ The so-called whale deck was now disappeared. The upper deck stretched over the complete length of the submarine. In the meantime started also Japan with building 600 tons-submarines.