Like all pre-H21 British H-class submarines, H12 had a displacement of 364LT at the surface and 434LT while submerged.[1] It had a total length of 150feet, a beam of 15feet, and a draught of 12feet. It contained a diesel engines providing a total power of and two electric motors each providing power.[2] The use of its electric motors made the submarine travel at 11kn. It would normally carry 16.4LT of fuel and had a maximum capacity of 18LT.[3]
The submarine had a maximum surface speed of 13kn and a submerged speed of 11kn. British H-class submarines had ranges of 1600nmi at speeds of 10kn.[1] H12 was fitted with a Hotchkiss quick-firing gun (6-pounder) and four torpedo tubes. Its torpedo tubes were fitted to the bows and the submarine was loaded with eight torpedoes.[1] It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.[1]