D-class boats were fitted with twin screws for greater manoeuvrability and were fitted with saddle tanks. The D class were the first submarines to be equipped with deck guns forward of the conning tower beginning with "D6". Also, reserve buoyancy was increased to 20.6%. Armament also included three torpedo tubes (2 vertically in the bow and 1 in the stern). The D class was also the first class of British submarines to have standard radio fitted. The aerial was attached to the mast of the conning tower that was lowered before diving.