1918 - 1931: K26 ClassBased on the K Class. design, K26 was intended to eliminate or at least reduce the known defects of that class. Although the general layout in K26 was practically the same as the K Class, the introduction of six 21-inch bow torpedo tubes in lieu of four 18-inch bow tubes was responsible for an overall increase in length of 12 feet.
The main machinery of K26 was the same as that fitted in the K Class, but her design surface speed of 23.5 knots was 0.5 knots slower. At the time the loss in speed was blamed on the repositioned after hydroplanes being in the wake of the propellers and the increased draught. The increased displacement was the reason. K26 had a displacement of 2140 tons with 300 tons of oil fuel. The 24 knots for the K Class. was achieved at 1980 tons displacement carrying the normal load of 197 tons of oil fuel.
A great advance was made in K26 by the introduction of battery compartments - which became standard in the designs of the 1920s. K26 could submerge more rapidly than previous steam submarines, and her diving depth was increased to 250 feet - although there is no apparent reason for the 50 feet increase over that of the K Class.