The J class was designed by the Royal Navy in response to reported German submarines with surface speeds over . The submarines had a displacement of 1,210 tons surfaced, but "J7" had a lighter submerged displacement than her sister boats, at 1,760 tons. Each submarine was in length overall, with a beam of , and a draught of . The propulsion system was built around three propeller shafts; the J-class were the only triple-screwed submarines ever built by the British. Propulsion came from three 12-cylinder diesel motors when on the surface, and electric motors when submerged. Top speed was on the surface (the fastest submarines in the world at the time of construction), and underwater. Range was at .