Bismarck electric propulsion: what if

Discussions about the history of the ship, technical details, etc.

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Bismarck electric propulsion: what if

Post by johnmk » Thu Jun 23, 2016 7:42 am

Hello folks,

Does anybody know some specifics of Bismarck's proposed turbo-electric propulsion system? I'm aware that it was seriously considered, and in fact, more than considered, it was selected by Raeder and almost finalized, and then abandoned at the last minute. Here's my initial salvo of questions:

1) How much more fuel efficient would the turbo-electric transmission have been at cruising speed and at other speeds? I think U.S. technology by the 1930's decreased the cruising speed efficiency delta between geared & electric drive from where it was 20 years prior, so I'd expect a U.S. turbo-electric plant to be only slightly more fuel efficient than a double-reduction geared transmission (DRG). At say 15 knots, perhaps 10-20% lower fuel consumption on a U.S. ship with a then-modern 1930's TE transmission. At full speed, I would expect the TE plant to be perhaps 5-10% less efficient than a DRG transmission. I'm picking these numbers not quite out of a thin air, but via Gestalt, synthesizing everything I've read by Friedman and others, so I could be wrong, but probably not by much. Perhaps the German figures would be similar, perhaps not. Does anyone have any information or educated guesses?

2) How would the (at least slight) extra fuel efficiency of electric drive have influenced Bismarck & Tirpitz's tactical and strategic disposition?

3) Would induction or synchronous motors have been employed? I've read that one of the reasons (perhaps the main one) TE was abandoned (for the Bismarck class) in favor of geared transmission was because swift reversal couldn't be obtained with the planned implementation of electric drive (the motors would have been made by a consortium led by Siemens). Knowing that fast, responsive motor reversal was one of the hallmark, touted features of the American electric drive system, I'm perplexed. Could it be that synchronous motors were planned, instead of the American-preferred but possibly heavier induction motors? Are synchronous motors more efficient than induction motors, and if so -- by how much? Do synchronous motors sacrifice low-speed torque in these types of applications, versus induction motors?

4) Would a single, large electric motor have been chosen per shaft, or two medium-sized electric motors connected in series, as was done in U.S. experience with the 45000SHP shafts of Lexington-class battlecruisers/carriers? My personal guess would be something along the lines of the U.S. Lexington-class arrangement, so as to preserve sufficient width for the torpedo defense system. Then again, that does take more hull length, and the unique three-shaft arrangement employed by the Germans was already very conservative in its use of stern profile so maybe a solitary large motor per shaft would have fit just fine.

5) How heavy would electric drive have been vs. the geared transmission? I recall reading somewhere that the difference was around 500 tons, but I don't remember where I read that. IF that's right, that's about 1% of Bismarck's weight. Even if it's just 1%, that doesn't mean it wasn't important. I know Hitler and Raeder were trying to keep at least some upper limit on Bismarck's displacement (45000 tons?), and that meant everything was evaluated very closely for opportunity cost. I'm sure the thinking was: 500 tons can be spent on extra armor protecting the turrets, or magazines, or machinery -- OR spent to procure additional cruising economy, and thus operational range. Of course, electric drive was chosen anyway despite that trade-off, only abandoned at the last minute according to some accounts.

6) What was the planned trade-off for that extra weight? Did the electric drive plant take extra internal hull length, as it did in American experience? Would this have resulted in a lengthened Bismarck with a somewhat higher hull speed, or would hull length have been preserved and some internal trade-off made?

Is there anything else I should ask? This is a fascinating subject and virtually any aspect, event tangential, would probably interest me. The game of trade-offs is a fascinating one, no doubt about it. Much like chess: every move leads to another, and another.

If you can answer any of my questions, or have something else to add, please do so!