Electric-drive BB's - tactical use of astern?

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TheMarcksPlan
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Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 29 Aug 2020 20:50

Wondering whether the astern-power capabilities of electro-BB's were ever intended for tactical use. Was it possible and/or intended, for example, simply to reverse direction (at somewhat-lower speed) rather than executing a full 180-degree turn? I could see this being useful under certain battle conditions. A full turn is time-consuming and the guns are mostly useless during it, whereas it's probably feasible to program a firing solution during slowing/acceleration on a constant (then instantaneously flipped) ship bearing.

Given that not all "Standards" were electro-drive, I could also see the tactical development not having been considered for the battle line.

Likewise, could an electro-BB use reverse on one outer shaft to decrease its tactical turn radius? I could see problems caused by asymmetric propeller thrust after the execution of such a movement (prior to full-ahead being regained).
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Re: Electric-drive BB's - tactical use of astern?

Post by OpanaPointer » 29 Aug 2020 22:07

"Full Emergency Astern" is an option on all USN ships. How much that is varies by ship design.
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Re: Electric-drive BB's - tactical use of astern?

Post by Andy H » 29 Aug 2020 22:38

Hi

I wonder what the structural implications would be for a BB with a Transom stern operating to your suggestion?

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Re: Electric-drive BB's - tactical use of astern?

Post by OpanaPointer » 29 Aug 2020 22:59

Andy H wrote:
29 Aug 2020 22:38
Hi

I wonder what the structural implications would be for a BB with a Transom stern operating to your suggestion?

Regards

Andy H
Builder's trials are usually the last adjudicator for such issues. If the design can't handle the theoretical limits then a few pen-and-ink changes need to be made.
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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 30 Aug 2020 00:25

OpanaPointer wrote:
29 Aug 2020 22:07
"Full Emergency Astern" is an option on all USN ships. How much that is varies by ship design.
Yes but the reverse turbines weren't designed for anything like tactical speed IIRC. The e-BB's would lose only a couple knots at full astern.
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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 30 Aug 2020 00:38

Andy H wrote:
29 Aug 2020 22:38
Hi

I wonder what the structural implications would be for a BB with a Transom stern operating to your suggestion?

Regards

Andy H
Interesting. In terms of the bending moment created by a long-wave I don't see any difference. There's definitely a sea-keeping difference that would restrict the option to lower sea-states. Probably a panting* (I think that's the term for "coke-bottling") structural issue as well - the bow is configured to absorb and dissipate compressive forces along the length axis and the stern probably less so. That too would limit you to certain sea states.

*https://en.wikipedia.org/wiki/Panting_( ... struction)
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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 30 Aug 2020 05:12

TheMarcksPlan" wrote:In terms of the bending moment created by a long-wave I don't see any difference.
Well I checked an engineering textbook and there is a difference for "ship girder" strength (that which resists the hogging/sagging tendency) but it's small enough normally not to be taken into account:

Image

Sterns are not usually built with consideration (c) in mind. Probably the ~2kn lost going astern is sufficient to obviate reinforcement of the ship girder for when a 21kn ship crests a wave, suspending its forward end briefly.
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Re: Electric-drive BB's - tactical use of astern?

Post by OpanaPointer » 30 Aug 2020 12:28

TheMarcksPlan wrote:
30 Aug 2020 00:25
OpanaPointer wrote:
29 Aug 2020 22:07
"Full Emergency Astern" is an option on all USN ships. How much that is varies by ship design.
Yes but the reverse turbines weren't designed for anything like tactical speed IIRC. The e-BB's would lose only a couple knots at full astern.
I'm not sure what you meant there. Doesn't seem to have congruence with what I said.
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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 30 Aug 2020 17:40

OpanaPointer wrote:
30 Aug 2020 12:28
TheMarcksPlan wrote:
30 Aug 2020 00:25
OpanaPointer wrote:
29 Aug 2020 22:07
"Full Emergency Astern" is an option on all USN ships. How much that is varies by ship design.
Yes but the reverse turbines weren't designed for anything like tactical speed IIRC. The e-BB's would lose only a couple knots at full astern.
I'm not sure what you meant there. Doesn't seem to have congruence with what I said.
What speed was possible with the reverse turbines? I'm saying emergency power astern for non-electric BB's wasn't fast enough to merit any consideration for tactical use, whereas for e-BB's it was. The e-BB's could do around 19kn IIRC.
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Re: Electric-drive BB's - tactical use of astern?

Post by OpanaPointer » 30 Aug 2020 18:15

Sorry, having an off day.
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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 30 Aug 2020 18:27

OpanaPointer wrote:
30 Aug 2020 18:15
I'm not aware that there was a separate reverse turbine. (Could be the morphine, of course.)
Yes, one of the benefits of e-drive is elimination of the reverse turbine.

For many reasons it would be impractical to design a main turbine for reverse. The basic physics requires expansion of volume along the length axis; reverse would violate that and compress the steam rather than expanding and cooling it (how most air compressors work but they are powered rather than powering). The airfoils (each turbine has thousands) are optimized for uni-directional flow. A running turbine creates an axial force in the direction of flow, due primarily to the airfoils having extension along that length axis. Turbines need to be anchored at the end to combat this flow; usually the other (entry stage) end has a somewhat elastic grounding (what engineering textbooks call a "rocker") so that the entry stage end doesn't experience high tension forces. The expansion of the turbine under heat is another reason not to have it anchored at both ends.

I think later BB's substituted the reverse turbine for a gearing arrangement? That's also expensive/heavy though.
Sorry, having an off day.
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Re: Electric-drive BB's - tactical use of astern?

Post by OldBill » 31 Aug 2020 02:41

Wont they have to come to a full stop, then begin going astern? That makes a gunnery solution easier for the enemy doesn't it? Additionally this would seem to have plenty of potential to entirely disrupt the battleline.

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Re: Electric-drive BB's - tactical use of astern?

Post by Thumpalumpacus » 31 Aug 2020 03:59

33,000 tons of ship has a mind of its own. Not something I'd pull in a fleet engagement, you'll get broken divisions and slowing to a stop before reversing. Nice target you've got there.

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Re: Electric-drive BB's - tactical use of astern?

Post by TheMarcksPlan » 31 Aug 2020 05:43

OldBill wrote:
31 Aug 2020 02:41
Wont they have to come to a full stop, then begin going astern? That makes a gunnery solution easier for the enemy doesn't it? Additionally this would seem to have plenty of potential to entirely disrupt the battleline.
The engines would be reversed while still travelling full ahead. There would be negative acceleration to slow the forward velocity, an instantaneous moment of 0 velocity, then acceleration towards full speed astern. To create a firing solution against such a target would require detecting the deceleration curve and integrating it across projectile flight time. To model deceleration would require knowing something about thrust lapse rate of the propellers, thrust/weight ratio of the ship, etc. It'd require iterative calculations to fit a model on the data, which would need be sub-second level reporting of changing speed with sub-second level accuracy. While that's mathematically possible it's almost certainly beyond a tactically-useful capability of old-timey computers quickly to generate a solution. And in any case far harder than a firing solution against a ship assumed to be travelling at constant speed.

You're right about disruption of the battleline - that's why the OP says the non-electric Standards might have confounded tactical use of astern speeds.

OTOH there were 6 e-BB's including the last five commissioned before WW2. As these ships were the most powerful ships in the interwar US fleet, it wouldn't be crazy to have segregated them tactically into their own division of the battle fleet. Accompanying DD/Cruisers were fast enough to adjust to whatever hijinks these BB's pulled at 19-21kn.
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Re: Electric-drive BB's - tactical use of astern?

Post by paulrward » 31 Aug 2020 19:18

Hello All;

Mr. TheMarcksPlan noted :
OTOH there were 6 e-BB's including the last five commissioned before WW2.
As these ships were the most powerful ships in the interwar US fleet.......
Actually, by the time W2 had broken out, it was down to five e-BBs - The New Mexico,
in her 1931-1933 major overhaul had her turboalternators and motors removed and
replaced with standard type geared turbines. As a result, during the Fleet Problems
of 1936, she showed a clean pair of heels to all the other battleships, exceeding 21 knots.

Respectfully :

Paul R. Ward
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