Doubts about three shafts stern of Bismarck

[ontheslipway]

Sorry, wasn't trying to be unfriendly...

But the 3 out of 4 argument is a classic non-argument. Here is a link to a site with some argumental fallacies to help you spot weak arguments!

Logical Fallacies

It's a bit like the Japanese car industry. The Americans build big inefficient cars, the Japanese build small efficient cars. Well, 250 million Americans can't be wrong? They are!

[marcelo_malara]

In this case Bismarck design is more efficient than the four shaft-four propeller arrange in the KGV Class because each of Bismarck´s shafts is transmitting 50,056 (nominal) hp while the KGV´s shafts are transmiting 28,250 hp each.

What Karl is trying to say is not that the British arrangement is less efficient, but that they are "underusing" the capabilities of the shafts I believe. That´s right Karl?
Respect to the number of shafts and redundancy, I recently discovered that Nelson had only two shafts. Taking into account the null antecedents of shaft and engine damage in battle, and as posted not being any limit in the power a shaft can transmit, why didn´t any navy adopt a high power two shaft arrangement?

[Karl Heidenreich]

Hi Marcelo,

Yep. As a matter of fact the final output of a Bismarck Class BB with the same shaft arragement of the KGV Class (four) will be 37,542 hp per shaft. That means that each shaft will be subjected to less torque (kg-m) than a three shaft arragement. If each shaft of the Bismarck works with a total of 50,056 hp each then there is a nominal difference of 12,514 hp per shaft.
I know that´s not even the unit system and method (because I´m using final hp output on one hand and the torque on the other) to compare the efficiency of so big a powerplant, but gives an idea of the design philosophy that each country applied. It will be great if someone knows the torque per shaft of those two BBs to continue the thread.
Very best regards.

[ontheslipway]

That means that each shaft will be subjected to less torque (kg-m) than a three shaft arragement

Power is torque times rpm (/ 60 *2pi). This means that more power does not automatically equal more torque. You need both the power output and shaft rpm to determine torque [Nm]. Most WWII BB's had comparable rmp's though.

[Karl Heidenreich]

I believe that in a KGV 200 rpm equals 27 knots. Is it right?
So, an Iowa BB, or the Hood or another 4 shaft vessel have to reach (more or less) the 200 rpms to achieve the 27 knots, is that it?
But, that doesn´t necesarily apply to a 3 shaft BB like Bismarck. There must be a relationship between the overall thrust needed to reach certain speed and the means to obtain it. If we don´t contemplate the propeller design by itself, probably a 4 shaft vessel needs less rpm than a 3 shaft one because the 4 shaft thrust, at the same rpm, produces more thrust than 3 shafts.

[Bgile]

4 shafts at the same rpm doesn't necessarily produce less thrust than 3 shafts. It depends on the size of the blades and their pitch. For example, US submarines, which have one shaft, at one time used 5 bladed screws. When they switched to 7 bladed screws with less pitch they cavitated less, but developed less thrust resulting in slower speed on the same shaft horsepower.

In the era of the battleship, they would design whatever screw they needed to most efficiently transfer shaft horsepower to the water. Presumably a 3 shaft arrangement would require larger screws than that of a 4 shaft arrangement with the same horsepower, and that might make the screws hit the bottom at a deeper draft. Of course, I believe Bismarck was designed to have a shallower draft than contemporaries, having to do with canals and ports she would have to use.

[marcelo_malara]

Karl, I compiled this data for Hood and Bismarck, (roughly) equivalente ships:

Bismarck:
3-bladed, 4.8 m diameter propellers, 270 rpm for 150000 hp, 30 kt

Hood:
3-bladed, 4.57 m diameter propellers, 206 rpm for 151000 hp, 32 kt

It seems that the 3 shaft arrangement needs a little larger screw and 30% more revs.

[Bgile]

There is another aspect to this.

First, the German arrangement placed the center engine room further forward than the wing engine rooms. This meant that the outboard shafts were relatively short, which is a good thing from a damage control standpoint.

The problem with only 3 engine rooms is that you need physically larger engines, with bigger individual compartments both in width and especially in height. This can present a potential waste of space and compartmentation, especially when you consider that the nearby boiler rooms, aux gens, etc don’t need to be that high.

On PoW and many other designs, there were 4 engine rooms, with two of them located much further forward in the ship than the other two. This was to make it less likely that the whole powerplant would be disabled at once. The forward engine rooms powered the outboard shafts. When a torpedo hit the port outboard shaft, it warped, damaging the shaft seals and flooding all the compartments along it’s length. This problem was compounded by some design deficiencies and errors in drill which caused even more progressive flooding.

The Midway class carriers and the Montana BB design mitigated the long shaft problem to a certain degree by powering the center shafts from the forward engine rooms, thereby protecting the longest shafts from torpedo hits.

[Karl Heidenreich]

I believe we are reaching something today. But first let me clear a misunderstanding.

Bgile wrote:

4 shafts at the same rpm doesn't necessarily produce less thrust than 3 shafts.

I was stating the contrary, so, agreeing with you 100%:

If we don´t contemplate the propeller design by itself, probably a 4 shaft vessel needs less rpm than a 3 shaft one because the 4 shaft thrust, at the same rpm, produces more thrust than 3 shafts.

But it is Marcelo who really get it:

Bismarck:
3-bladed, 4.8 m diameter propellers, 270 rpm for 150000 hp, 30 kt

Hood:
3-bladed, 4.57 m diameter propellers, 206 rpm for 151000 hp, 32 kt

It seems that the 3 shaft arrangement needs a little larger screw and 30% more revs.

There it is! The Bismarck 3 shaft arragement design needs bigger propellers and more rpms to achieve less speed than the Hood. There we have a relationship between the final output (hp), rpms and the thrust. The last one is also a matter of the propeller design, which I excluded but Bgile put again in the topic. At doing so it explains a lot of things.
But Bgile puts another thing in the argument:

The problem with only 3 engine rooms is that you need physically larger engines, with bigger individual compartments both in width and especially in height. This can present a potential waste of space and compartmentation, especially when you consider that the nearby boiler rooms, aux gens, etc don’t need to be that high.

On PoW and many other designs, there were 4 engine rooms, with two of them located much further forward in the ship than the other two. This was to make it less likely that the whole powerplant would be disabled at once. The forward engine rooms powered the outboard shafts. When a torpedo hit the port outboard shaft, it warped, damaging the shaft seals and flooding all the compartments along it’s length. This problem was compounded by some design deficiencies and errors in drill which caused even more progressive flooding.

So, there a lot of things in favour and contrary to the three shaft arragement.
First: the Bismarck design uses an efficient arragement (in comparison to other contemporary designs) because it can carry more displacement to higher speeds using one shaft less.
Second: to achieve this the Bismarck´s propeller design calls for bigger screws and more rpms.
Third: the three shafts must endure greater stress in Bismarck than those of the 4 shaft Hood.
Fourth: There are inconveniencies associated to a three shaft design related to space and protection. But, if we want too, we can find problems with the four shaft design also.
At the end, all this had an effect in Bismarck´s fate? I believe it does. But that doesn´t implies a flaw in the design, it was just a design concept like any other. We can´t guarantee that a four shaft Hood, Yamato or Iowa would behave better than Bismarck with the same damage. Neither is a problem with the rudders, I believe, because in a lucky hit against one of two rudders (or against only one rudder) the ship would be fatally wounded also.
What do you think?

[Bgile]

I agree. A hit on a rudder in particular is very hard to deal with, and also pretty unusual. It depends to a certain extent on whether you have nearby ships which can assist you. Even then, if a rudder is jammed by serious battle damage it would be hard to even tow it.

[ontheslipway]

A few comments on the above. A 3 shaft ship does not need bigger propellers or higher rpm than a 4 shaft ship per se. Though larger propellers are often desirable, you can use smaller and more highly loaded props (less efficient). The rpm is preferably lower to reduce friction resistance, but the most important parameter pitch. With pitch you can match the required trust and torque to the engine rpm. Note that with a given pitch you can also sail at half the rpm on a three shaft ship!

Well, the battleships did have an awful lot of power to deliver with relatively small propellers, so you automatically end up with high rpm & pitch propellers. Choosing your diameter as large as possible is not only good for your efficiency, it means a reduction in either pitch or rpm. Note that the rpm is determined by your powerplant and reduction gear.

That Bismarck needs more power than Hood to gain a lesser speed may have something to do with overall resistance? Hood is a very slender ship meaning she probably had less wave resistance. Not necessarily of course. So:

4 shafts at the same rpm doesn't necessarily produce less thrust than 3 shafts.

No, depends heavily on the pitch on the speed of the ship! You devide the trust over more shafts, so with 4 props you have a lower pitch than a 3 shaft ship. If the efficiency is higher or lower depends greatly on its operating point. Could be more, could be less.

The three shafts must endure greater stress in Bismarck than those of the 4 shaft Hood

This depends on the thickness of the shaft. Note that a lower rpm at the same power means the shaft has to resist more torque.

For example, US submarines, which have one shaft, at one time used 5 bladed screws. When they switched to 7 bladed screws with less pitch they cavitated less, but developed less thrust resulting in slower speed on the same shaft horsepower.

Though 5 bladed propellers are slightly more efficient than 7 bladed propellers (if you have unrestrained diameter), the trust depends heavily on the pitch and loading distribution. Note that you have a lot of freedom with your design. Propellers that are designed to cavitate less almost always do worse efficiency wise. Getting a prop to be efficient is relatively easy, managing cavitation is the difficult part.

Feel free to argue, but I have 3 years experience in propeller design and I teach the stuff at University level

[Bgile]

LOL, no way I'm going to argue with someone who knows more about the subject than I do by a wide margin! Besides, what you say makes sense to me!

I served on submarines and I knew the 7-bladed screw was designed with reduction of cavitation a high priority, accepting whatever loss of efficiency that entailed. Obviously it was a tradeoff because the ship had to be able to go reasonably fast as well.

[marcelo_malara]

I would now compare the Machinery Rooms volumes of Hood and Bismarck:

Bismarck:

..............Aft ER...........Fw. ER
Height:......8.8 m...........8.8 m
Lenght:..14.02 m.......14.02 m
Widht:....13.75 m........11.27m
Volume:.1696 m3.......1391 m3
Total volume: 1696 + (1391*2)= 4478 m3

Hood:

..............Aft ER.........Md. ER.......Fw. ER
Height:........9 m............9 m..............9 m
Lenght:.12.75 m.....12.15 m.......13.35 m
Widht:........21 m.....21.90 m.......24.30 m
Volume:.2409 m3...2394 m3.......2919 m3
Total volume: 7722 m3

Bear in mind that Bismarck´s powerplant was a high pressure one (55 kg/cm2 against Hood´s 15 kg/cm2).

[Bgile]

I was going to ask why a tanker would have one screw while an aircraft carrier of the same displacement would have 4, but I think I can guess.

The merchant is designed for minimum operational cost. The military vessel is designed with redundancy in mind, so it can keep operating after being damaged.

[Bgile]

I would now compare the Machinery Rooms volumes of Hood and Bismarck:

Bismarck:

..............Aft ER...........Fw. ER
Height:......8.8 m...........8.8 m
Lenght:..14.02 m.......14.02 m
Widht:....13.75 m........11.27m
Volume:.1696 m3.......1391 m3
Total volume: 1696 + (1391*2)= 4478 m3

Hood:

..............Aft ER.........Md. ER.......Fw. ER
Height:........9 m............9 m..............9 m
Lenght:.12.75 m.....12.15 m.......13.35 m
Widht:........21 m.....21.90 m.......24.30 m
Volume:.2409 m3...2394 m3.......2919 m3
Total volume: 7722 m3

Bear in mind that Bismarck´s powerplant was a high pressure one (55 kg/cm2 against Hood´s 15 kg/cm2).

I am confused. Bismarck had 3 engine rooms and Hood had 4. Also, did hood's engine rooms include boilers? Bismarck's didn't. How about turbine generators?