Das Boot or the American Fleet Boat

[Catfish]

Hello,
there are a lot of differences, despite the outer appearance being a bit (only a bit!) similar, due to the hydrodynamic hull when running surfaced.

You already mentioned some:
- speed
US Fleet boats were faster, if only 2-4 knots - but not all types.

torpedo stocks
- Much more torpedoes in the US subs, if not in the sugar boats

- aa protection
Depends on the types - the later VIIC, IXC and D types had heavy AA guns, including the 4x "whirlwind" type. And then there were the "Flak traps". A broken IXB unable to dive fought off more than 4 planes and a blimp for almost a day, in the carribean - with stock AA.

- dive time
German subs were a bit faster, i think 25-30 seconds for the VII types, and 35 - 40 for the bigger IX ones. Well trained crew assumed.

- maneouvrability
don't know whether the sims reflect the real thing, however it was said the german types were extarordinarily manoeuverable - maybe not the massive "Milk cow" types, and surely not the later type XXI which was a pig when it came to docking.


Two of the biggest differences were the propulsion system including Diesel types, and the dive depth.

Modern US fleet boats had Diesel-electric propulsion, meaning that they were running on electric engines all the time, being charged and powered by several Diesel engines which had not to be built in-line with the propellor shafts, but could be "scattered" all over the boat. Merely powerplants with wires to the electric engines, buffered by batteries. I was astonished to read that the US "Diesels" used in US boats were of the older two-stroke type.
When the batteries were loaded, one or more Diesels would then charge the batteries, resulting in drastically reduced speed, often turning off one of the propshafts and electric engines, entirely.

German boats' Diesels were coupled directly to the propshafts in a line, via a clutch system that after some time and wear caused slip, and had to be re-adjusted after some months.
The electric engines were again in line like with the US boats, but they could also be used as dynamoes/generators, in switching them to load the batteries.
So when the Diesels were running, one or two of the electric motors/generators could be clutched to the propshaft(s), thus charging the batteries, which only reduced the speed from 18 to 16 knots, instead of going down to 10 or 12 like with the US subs, at full speed.
The electric engines could also be switched additionally to the running Diesels, which would provide a top speed of 20 knots, if not for long because of rapidly draining the batteries.
This was shown in the film "Das Boot", when the captain ordered "Dreimal Wahnsinnige" to get near the coast after being detected, at Gibraltar.
"Ahead Flank" would be "Äußerste Kraft voraus", or "AK" in german, while this other command mentioned would urge the machine personell to get every horsepower out of the engines for the utmost speed, regardless wear or efficiency.

Again, the U-boat being driven by only one Diesel engine while the other would be maintained, or repaired, or just switched off to spare fuel, both electric engines were able to lad the batteries, since the turning screw of the unclutched repaired Diesel would load the generators via the turning propshaft - so one from the running Diesel, and the other by the - if slowly - turning screw being driven by the "slipstream".

Later IXC and D types had additional special belt-drive electric engines for silent running, apart from their stock inline electrical ones.
They also had two separated (not in-line with the propshaft) Diesels only for charging thr batteries.

Both US and german propulsion systems had their advantages, but in my opinion the german system was more flexible to use, under the circumstances experienced.


Regarding dive depth the german subs were able to dive much deeper than their british or US adversaries.
The VIIB was already able to dive to 220 meters (appx. 660 feet), or "140 meters plus A". "A" meant "Achtzig", to hide the capabilities of the boats in telegrams.

The later VIIC/41 and 42s were able to reach 300 meters, or 900 feet roughly, and one VIIC reached 360 (if by error of a sticking depth gauge), and survived - if with bent propshafts and the whole inner pressure cylinder bent and out of shape.

Don't know how deep fleet boats would be able to go, but i heard something of 450 to 500 feet (?), which would be 170 meters.


Another difference were the radar capabilities, and while later U-boats had and used radar, it was not as efficient and well-developed as in the late US silent service.


Greetings,
Catfish