Doubts about three shafts stern of Bismarck

[Karl Heidenreich]

Now I fell embarased. It´s like debating gravity with von Braun without knowing he is him.
But now that we are at it let´s continue because we are learning.
So, the main topic here is if the three shaft arragement was fatal to the Bismarck when she was hit by the Swordfish?
The second topic that developed is about the arragement by itself: was a good one or wasn´t that good. Then it came all this thing about hp, torque, stress, thrust and pitch (the latter I didn´t take into account )
The thing is:
a. The three shaft arragement was fatal? Well, I guess it was but it didn´t rule out that a four shaft arragement would be better. With such a devastating hit that destroyed one rudder and jammed the another one (and that destroyed the central propeller) we can extrapolate what would happen to a four shaft BB. The damage to the rudders (or rudder) will be the same, that doesn´t change with the number of propellers. What would have happened with the propellers in a 4 shaft BB? Probably one (or the two) from the impact side would be destroyed or heavily damaged. So the Bismarck fatal history would have been more or less the same.
b. The design. From what emerges of all this is that it´s a relative question in which many variables takes place. The question would be the following: Did Bismarck three shaft design was, at the end, more efficient than that of her contemporaries? I believe that it was better than that of the KGVs by merely a numeric comparison. But it wasn´t better than the Hood´s. All right, maybe Hood´s hull is another factor in all this. But the truth must lie somewhere around here. Do you agree?

[Bgile]

Karl,

I think all we have said is that there are many tradeoffs in designing a warship, and we probably don’t know which is superior.

My own bias is in favor of the newer American ships for the following reasons:

1. The rudders were further apart, making it less likely that both would be crippled at the same time.

2. They had hull strakes which were very strong structural members enclosing one shaft on each side. This would tend to shield the other side from damage.

3. Unlike Bismarck, longitudinal bulkheads continued into the stern area, making that part of the ship more rigid and stronger. Bismarck’s stern came off the rest of the ship after sinking, and as I understand it, it was welded on. Other German warships showed similar weakness there.

Unfortunately I don’t know very much about countries designs from that point of view.

[ontheslipway]

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.

Owners preferably have a 1 cylinder engine... there is growing criticism due to safety converns on the single engine single propeller ships. Fortunately, container ships are becoming so large that both engine and propeller are stretched to their limits (construction wise).

It´s like debating gravity with von Braun without knowing he is him

Well, fortunately it isn't very difficult! A few basic tricks you learn and you'll be able to discuss pitch and rpm...

As far as 4 shaft ship are concerned, the South Dakota had the outher shafts in protecion bossings with the inner shafts open. Probably not the best way to go efficiency or vibration wise (poor propeller inflow to all propellers) but should offer some degree of protection.

If you want to know if 3 or 4 shafts is better from an efficiency point of view: this is very very difficult to say. Both have their merits. Adding a shaft means a rather radical change of hull afterbody... this may influence ship resistance much more than the benefits of lower propeller loading (and so forth). I've tried to find a few nice articles on the subject but they are rather scarce!

I think the more important factor is engine room layout and general ship subdivision. Trying to desing around a rudder hit is just tricky. Just look at what happened at PoW on December 10th 1941....

[marcelo_malara]

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?

Bismarck had three engine rooms, two forward side by side, driving the outer propellers, and one abaft, driving the center one. Unfortunately, the AOTS volume doesn´t describe the engine room in detail to show if it contains other equipement aside from turbines and condensers.
Hood had three engine rooms too, the forward one driving both outer propellers, one abaft driving the inner port and one abaft this driving the inner starboard. The engine rooms contains only the turbines, condensers and pumps, nothing bulky.
That brings to me one further question: Bismarck´s forward engine rooms are side by side, divided by a longitudinal bulkhead. In case one of them floods, 1400 tons of water will be on one side of the ship. How many degrees would the ship list with this asymetrical flooding? The British were very carefull of not dividing large compartments longitudinally to avoid the risk of capsize, as happened to the three cruisers of the Cressy class torpedoed in the same attack in WWI.