# Economical Steaming of Four-screw Ships



## blueprint2002 (Sep 7, 2011)

Some SN members may recall the (RN) cruisers, battleships and fleet carriers of WW2 vintage, and earlier, many of which were powered by four sets of steam turbines, driving four screws. In the case of the cruisers, steam conditions were 350 or 400 psi, 700F, if memory serves.
Fuel consumption was not, perhaps, of very great import at the time, except for the limits it set on the ship’s endurance. The division of first-stage nozzles into separate groups, each providing appropriate steps in turbine power, was the only regular means the crew had to operate as efficiently as possible.
However, at least one commercial steam ship operator (Canadian Pacific) went a step further in pursuit of fuel savings. The 1931-built “Empress of Britain”, intended for winter use as a cruise ship, had provision for the use of only two of the four shafts, under these conditions. Apparently, the two outer propellers were removed, and the ship operated on the inner shafts alone, at the corresponding reduced speed, naturally with the associated boilers shut down.
While it is difficult to conceive of a warship being operated in a similar manner, even in peacetime, the general idea does seem to have some promise, which could hardly have escaped the attention of those responsible. A practical way of achieving something similar, without compromising the ability to revert immediately to normal operation, would seem to be to simply trail one pair of shafts, outer or inner, under vacuum.
This would naturally necessitate the provision of gland-sealing steam, running air ejector(s) and extraction pump(s), and possibly main circulator(s) if scoop effect were absent or insufficient. Needless to say, the Forced Lubrication pump(s) would be essential; it does seem, though, that only one each of all these auxiliaries would be adequate for both trailing sets.
With only half the usual shaft power available, at least two-thirds the ship speed should be achieved, even allowing for the extra drag of the trailing shafts. And the use of a wisp of “trailing steam” on the inactive turbines might minimise this extra drag as well (as done with the main circulators of some ships).
There does not seem to be any record, in the archives of the “Journal of Naval Engineering”, of any such trials, but this could be on account of the operational importance, at the time, of the results of such trials. Seventy or more years later, such considerations are probably irrelevant, so I am hoping that someone who knows will be able to enlighten the rest of us. Alternatively, is there a fallacy in my reasoning above?

NB This was originally posted in the Royal Navy Forum. I'm repeating it here so as to reach a wider audience.


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## Varley (Oct 1, 2006)

Your comment on forced lubrication sparks a memory. It was not that uncommon to trail a shaft on the GTVs with the trailed shaft spun-up after declutching. Not so good if also slow steaming. We 'ran' a shaft bearing between Bremerhaven and Greenock on Eurofreighter doing just that.

(Eurofreighter had been retrofitted with one 'big bertha' prop to make that shaft more efficient in single running mode. Somwhere there is a photograph of her in drydock clearly showing the difference in size).


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## BDWilky (May 28, 2021)

blueprint2002 said:


> Some SN members may recall the (RN) cruisers, battleships and fleet carriers of WW2 vintage, and earlier, many of which were powered by four sets of steam turbines, driving four screws. In the case of the cruisers, steam conditions were 350 or 400 psi, 700F, if memory serves.
> Fuel consumption was not, perhaps, of very great import at the time, except for the limits it set on the ship’s endurance. The division of first-stage nozzles into separate groups, each providing appropriate steps in turbine power, was the only regular means the crew had to operate as efficiently as possible.
> However, at least one commercial steam ship operator (Canadian Pacific) went a step further in pursuit of fuel savings. The 1931-built “Empress of Britain”, intended for winter use as a cruise ship, had provision for the use of only two of the four shafts, under these conditions. Apparently, the two outer propellers were removed, and the ship operated on the inner shafts alone, at the corresponding reduced speed, naturally with the associated boilers shut down.
> While it is difficult to conceive of a warship being operated in a similar manner, even in peacetime, the general idea does seem to have some promise, which could hardly have escaped the attention of those responsible. A practical way of achieving something similar, without compromising the ability to revert immediately to normal operation, would seem to be to simply trail one pair of shafts, outer or inner, under vacuum.
> ...


I served around 2½ years on HMS Blake, a cruiser very similar to HMS Belfast (Belfast's engine rooms looked very similar on my visit). I qualified for my propulsion unit ticket on Blake, and knew her well.

Blake had 4 shafts, each served by a set of turbines and a boiler. Normal configuration was isolated units, rather than cross connected. I don't ever recall trailing shafts as an economical steaming measure, though we probably did at some point for damage control exercise.
When we wanted economical steaming, and if we were assured on no need to manoeuvre, we would use 'cruising turbines'. The cruising turbines were normally out of the main engine steam flow, and de-clutched from the mechanical transmission system.

The cruising turbines also had nozzles grouped in sets of (if my memory serves) 13, 9, 7, and maybe another. The aim, as you say, was to open nozzle groups seletively to maximise steam pressure at the nozzles.
Exhaust steam from the cruising turbine would pass to the inlet of the HP turbine, and then to the LP turbine (which also included the Astern turbine).
The cruising turbines were a fraction of the size of the main HP turbines. Still, we were able to cruise at near 50% of full speed owing to the cubic relationship between ship power and speed.
With the cruising turbines, the steam was used so efficiently that at the last stages of the LP turbine, just before going down to the condenser, the steam was cold!
I would remove lagging pads on the turbine to prove this to disbelieving young stokers.

I loved steam turbines, I thought it a great pity that steam technology could not have been updated. Never had the same affection for gas turbines or diesel engines.

The vacuum gauges for the main condenser were widely disbelieved because they sometimes showed a vacuum of 31 inches or more. I noticed that whenever a poor vacuum was indicated, for which we were scurrying around increasing gland steam and any other tricks we could think of, the ship was moving around quite a lot. Of course, our vacuum gauges were an effective barometer, reflecting atmospheric pressure. I confirmed this with ship's meteorological records. Not to be relied on as such at higher power, of course.

Happy days.


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## Varley (Oct 1, 2006)

WRT last paragraph #3. I remember seeing a barometer in the control room of ? and thought that was for just such a correction. Another steam man (also a CEGB man) that vacuum good be 'too good' - I vaguely recall something to do with increasing the velocity of the flow somewhere towards the end of the turbine 'train'.


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## kewl dude (Jun 1, 2008)

re: Varley keyed: Eurofreighter had been retrofitted with one 'big bertha' prop to make that shaft more efficient in single running mode. Somewhere there is a photograph of her in drydock clearly showing the difference in size.

Attached: Big Bertha.jpg


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## waldziu (Jun 17, 2008)

In the late '60s, I was a stoker on the Carysfort which also had cruising turbines.


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