Why 2 screws?
 

Why do the WWI era subs use 2 shafts with 2 screws instead of a single screw on a single shaft like a modern sub? It seems to me that one larger screw would push a lot harder than those 2 little ones. Was is an issue with the screw breaching the water while the sub was running on the surface?

I'd say because of the stern torps and the rudders. just a guess though.:hmm:

Going out on a limb here, as I'm hypothesising:

WWII-era boats were, effectively, surface vessels designed to be able to submerge. As such their design was affected by that paradigm. Multiple screws provide redundancy in case of damage or failure, and for the diesels it enabled recharging batteries and still having propulsion.

Modern submarines are designed, keel-out, as a submerged vessel that, due to necessity, periodically has to come to the surface. And its design reflects that (like having planes on the conning tower rather than the bow). It's designed to maximize thrust and minimize drag, which I think a single-screw design helps. Plus having a single screw cuts in half noise production from cavitation (I understand that military marine screws require a LOT of design and testing to keep cavitation to the strictest minimum).

As I said, I'm only conjecturing here.

You know I didnt even think of the rudders being there. Those 2 screws just look so small and ineffecient.

Darn good conjecture! U-boat. Essentially a boat that goes under for short period of time. I makes a U, submerge down go forward suface up...U. Therefore, U-boat. A true submarine is just that, made to be under the water and surface once in a while. Yes, two shafts, one to charge when the clutch is engaged to the generators and one to propel the boat. The schnorkel design created a true German SUBMARINE because it did not have to surface until food or fuel was consumed and drained.

It also added to manouverability. By running one screw fwd and one bwd you could swing the boat around even faster in case of emergency.

The two screws where there for redundancy and chargeing the batteries. Modern subs have done away with multiple screws because of cavitation. The larger the screw, the slower it can turn to produce a given speed, thereby reducing noise.

In correct the russian Delta IV and Typhoon's Oscars and remaining Yankees still use twin screw system, none have stern tubes but it does give redundancy if one is damaged, also because the vessels are so larger one screw just wouldnt propel the 48,000 tonne typhoon at 25knots.

the WWII era submarines were not built for silence untill the last part of the war when we began to understand the full idea and concept of submarines, which are stealthy undetectable platforms that should remain submerged constantly.

Also the hull shape of WWII submarines deny any possible use of a single screw nearly all of the submarines that use single screw configoration have an albacore shaped hull.

Not exactly, but perhaps you were joking.

HEHE!

also note the concept of using more than 4 blades on one screw would have been to risky and out of the ordinary even the first nukes like the novembers nautilus and seawolf all had 2 screws.