A 1,500 km mini-sub is really a coastal submarine

How much would a World War II mini-submarine with four 533 mm torpedoes and 1,500 km range weigh?

It would be better described as a coastal submarine and might approach roughly 200 tons submerged, because the required range demands far more fuel and volume.

The Japanese Type A midget submarine had a much shorter underwater range and displaced roughly 47 tons submerged. Fuel, range, crew space, and torpedo arrangements would likely drive the proposed craft toward roughly 200 tons submerged, depending on speed and external or internal tubes. Four externally loaded 533 mm torpedoes avoid a reload room but still impose length, stability, and launch-system demands. A 1,500 km radius is the decisive requirement because it calls for far more fuel and habitability than a short-range harbor-attack craft.