Compared to the previous type 63, the 81s were 1.7 metres (5 ft 7 in) longer, while the pressure hull remained the same.[3] They were .3 knots (0.56 km/h; 0.35 mph) faster on the surface, and .1 knots (0.19 km/h; 0.12 mph) faster submerged and increased range by 1,030 nautical miles (1,910 km; 1,190 mi) to 11,200 nmi at 8 knots. They carried 12 torpedoes instead of 6, and by 1917 all type 87s had the larger 10.5 cm deck gun versus the twin 8.8 cm on the 63s.