Hi Jens. That's what I was thinking. We were sailing last weekend on San Francisco Bay with a friend and his 26 foot boat has a little two cylinder diesel motor. My thought was to use a pressure vessel with a bolt-on end cap dome, and install a small diesel motor, generator, and fuel tank in it. Make the "unit" completely separate from the cabin hull with a "disk type" thru-hull for cable and controls. The unit could provide power for running electric motors OR have a propeller shaft coming out the back end ( OR both.)
It would need to be neutrally buoyant as a unit so the sub could dive even if it was removed. This "power module" could easily be bolted to the sub and removed as a unit if it needed repair or maintenance. Cooling would be an issue but if the motor was water cooled, just a water pump and plumbing would be needed.....no radiator.
The small two cylinder diesel on my friend's boat pushed his boat at about 5 knots at 3/4 speed and he said it will run for days on a tank of fuel. I think the motor brand was a "Universal". Probably made in China but a quick search on Google would show what's available in a compact motor that was water cooled.
I designed my sub to accept a small 20 horse outboard on the rear transom plate for long distance surface transit. The cables and wires will pass through a small electrical thru-hull, and for steering the motor has a link to the electric thrusters/rudders linkage. The outboard is small enough to be put inside the sub if needed but most likely will be loaded onto the surface boat when diving.
A permanent diesel electric "module" seems like a fairly simple thing to fabricate, and the sub could still be used with or without it.
Frank D.