Swordfish tested technology intended for later fleet submarines but fell short of her 20-knot goal. The J class gained only another knot despite three diesel engines and shafts, and submerged speed remained much lower. The class still saw useful service: one boat damaged two German capital ships, while another was credited with sinking U-99 on the surface. J6 was the only wartime loss, sunk by a British Q-ship, and six of the seven boats survived the war. Yet a 19-knot surface maximum could not reliably accompany 21-knot dreadnoughts or faster battlecruisers. Those figures expose why a useful operational concept became a propulsion challenge that diesel technology could not yet solve.
Chapter · 16:41-22:45
Swordfish and the J class reveal the diesel speed ceiling
Swordfish reached about 18 knots and the three-shaft J class about 19, still too slow to accompany 21-knot dreadnoughts and faster battlecruisers.