Interwar diesel technology could improve, but not solve, the fleet submarine

Could the K-class fleet-submarine idea become viable with interwar or Second World War technology?

Larger diesel-electric designs derived from the Parthian or Rainbow classes could approach the role, but sustained fleet speed would still demand more fuel, more displacement, or at-sea replenishment.

The Parthian and Rainbow classes provide the closest conventional starting points. They could reach about 17.5 knots on the surface and offered long cruising range at economical speed, but maximum speed could not be maintained indefinitely. Enlarging the hull could add fuel and machinery, while replenishment might extend operations. Even so, the result remains a very large submarine with a difficult compromise between surface fleet speed and underwater endurance.