In the Keynes Cruisers thread Peg Leg Pom mentions the historical use of submarine freighters in particular the IJAs' use of what they called a submergence transport vehicle. And he provided a link to the Wikipedia article on them which I'll include here.
https://en.wikipedia.org/wiki/Type_3_submergence_transport_vehicle
Of course it's an inefficient and desperate way to transport troops and supplies but it does provide an option. My question is how to make a more effective and efficient transport submarine for WW2 service. Not necessarily as a project only for the Japanese but for any potential user. An engineering and naval design question.
Using the Japanese transport submarines as a comparison what could be changed to make a transport submarine more effective? Bigger cargo capacity? Simpler and cheaper engineering?
If you're going to build these vessels make them more of a transport and less as a submarine.
Some ideas I have. Why build a sub with a deep diving capacity? That's important to evade depth charging but is it needed on a boat that's not attracting attention? How deep does the sub need to cruise to avoid being seen by aircraft?
If built to only submerge 20 or 30 feet that greatly reduces the pressure on the hull permitting a thinner and lighter structure. Also I think it would allow the use of larger hatches so larger cargo can be loaded.
The sub would be powered by diesel electric engines but wouldn't need a large battery suite as almost all the running would be on the diesels. This is done by using the schnorkel all the time during the day. At night you can run on the surface. A watch must be kept during the barely submerged daylight cruising by using a periscope designed for all-sky viewing like a fish-eye lens to watch for airplanes and ships.
If a potential threat is spotted the submarine goes to the small back-up battery and retracts the schnorkel and periscope. It submerges down to 30 feet and idles on battery to wait out the threat. If a submarine is not doing anything to attract attention they tend to go unnoticed.
The only armament carried would be anti-aircraft guns for protection if surprised on the surface at night. No torpedo tubes. That uses space and weight. The sub is not suited for that purpose anyway.
Using these ideas a transport submarine about the size of the USS Argonaut could be constructed that could carry 1000 tons of cargo at 10 knots with a range of 2000 miles. Maybe.
If detected it's true these vessels would be very vulnerable to attack. But than so are freighters. And freighters can't hide beneath the waves. If you've lost control of the skies and the sea lanes than a transport submarine might be a option.
How practical does this seem?