A couple of points need to be addressed:
Diesel Electric propulsion: A diesel engine turns a generator to create electricity. The engine itself is not directly connected to the propeller shaft. The electricity created by the generator is then sent to high speed electric motors that are connected to the shaft through reduction gears. These same generator engines also charge the battery.
Advantages:
1. The engine does not have to be aligned with the propeller shaft, allowing a more efficient engine room arrangement.
2. Complex, damage prone, and hard to maintain clutches are eliminated from the drive train.
3. The engines can be run at their most fuel efficient speed, as speed is regulated by sending more or less current to the motors.
4. Constant speed operation greatly reduces wear and tear on the engines.
5. Generally, a much greater amount of electricity can be produced, allowing for greater hotel loads (better habitability) and more powerful electronics (i.e. radar and sonar).
Disadvantages:
1. Generally requires a larger submarine
Towards the end of the war, the U.S. replaced the high speed motors with slow speed models, allowing the elimination of reduction gears, which were very sensitive to depth charge damage.
Diesel Direct propulsion: A diesel engine is connected via a clutch to a combination generator/motor, which is directly connected to the propeller shaft via reduction gears. For surface propulsion the diesel is directly turning the shaft through the clutch. Some arrangements have a 2nd clutch aft of the generator/motor so the engine can charge the battery without providing propulsion. For submerged propulsion, the engine is secured and declutched from the shaft. Electricity from the battery is fed to the generator/motor which turns the shaft.
Advantages:
1. Due to the generator and motor being the same unit, you can have a smaller submarine.
Disadvantages:
1. Generally, the reverse of all the advantages of Diesel Electric.
2. There is no transmission or reverse gear in the drive train, so in order to back down, the engine has to be declutched and current has to be sent the the other side of the motor from the battery. Greatly slows and complicates maneuvering orders, especially in critical situations in harbors.
3. Because the engine is directly connected to the propeller shaft,
you can only have two engines in your submarine! You can add some additional generator only engines for charging or electrical loads, but these can't be used for propulsion. On two separate occasions, the U.S. Navy experimented with connecting two diesels in tandem at the crankshaft, but these experiments were dismal failures. It is virtually impossible to precisely match the speeds of the two engines. Any mismatch is speed results in incredible torsional vibrations in the crankshafts and eventually broken crankshafts or engine mounts.
As for air conditioning: The major advantage of A/C on the fleet boats was not in providing crew comfort (which it did), but in greatly reducing humidity. Condensation in a submerged submarine is a huge problem. The condensate drips into electrical circuits and causes fires. The degree of mechanical reliability this gave to the Fleet Boats in incalculable.
This is a interesting discussion, but with no good answer. I think of it in this way: Which is the better airplane, a P-51 Mustang fighter, or a C-47 Dakota cargo plane? The answer is both. The P-51 is faster, more maneuverable, and carries a heavy armament. The C-47 has a much greater range and payload. The same line of thought applies to submarines. The Fleet Boat and the Type VII or IX were designed with completely different operating parameters and requirements. Both excelled in their respective areas, but can not be directly compared.
Dave
www.pigboats.com