3-shaft propulsion

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

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marcelo_malara
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Re: 3-shaft propulsion

Post by marcelo_malara » Sat Aug 10, 2019 5:08 pm

Thanks Marc! Incredible source! So basically is like Byron said, two turbines in tandem in each shaft. I think that technically it is more demanding, you have to perfectly align not one but two turbines to each shaft.


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Re: 3-shaft propulsion

Post by culverin » Sat Aug 10, 2019 9:35 pm

Previously the 3 shaft Illustrious class carriers were mentioned and the attendant savings in weight over 4 shafts.

This issue had been discussed by the Admiralty Board and the Naval Architects long before when the design for Ark Royal was being formulated. The requirement was for the ship to attain 30 knots deep and dirty 6 months tropical. With approx 102,000 shp 4 shafts were preferred then after water tank tests 3 were proven to attain the 30 knots with the attendant reduction in uptakes and ventilation needed to run under her double hangars to the starboard side. All this to be within the treaty limits in force. Also the rudders could be reduced and the Ark had just the 1, unlike Bismarck's pair.

The next Illustrious class followed the same machinery arrangement as the Ark with an increase in shp to allow 31 knots. Still within treaty limits which itself was remarkable and much overlooked by most. As you all know their design was entirely new with a single deck armoured hangar. However, after the 4th ship was ordered a further pair were to be constructed, Implacable and Indefatigable which differed sufficiently from Illustrious and with the increase in displacement the more widely accepted 4 shafts arrangement was adopted.

So ended the 3 shaft RN Carrier comprising the 5 ships Ark Royal, Illustrious, Formidable, Victorious and Indomitable.

Until CVA-01 in the 1960's of course.
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Re: 3-shaft propulsion

Post by Byron Angel » Sun Aug 11, 2019 2:21 am

OK Marcelo,
Found it –

CB1516(A)
REPORTS ON INTERNED GERMAN VESSELS
Part 1 – Constructive Details and Machinery

"Main Machinery – These ships appear to be very well built, and from the manner in which the ships are subdivided they would be very difficult to completely put out of action. They are all three-shaft turbine ships with H.P. and L.P. (ahead and astern) on each shaft with W.T, bulkheads between each set; also transverse bulkheads between H.P. and L.P. giving six separate compartments for main engines.
<snip>
The main machinery appears to be Parsons reaction turbines with impulse wheels on H.P., and when cruising the centre H.P. takes steam first, which then passes to both potr and starboard H.P., the centre L.P. being shut off."

Hope this helps.

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Re: 3-shaft propulsion

Post by marcelo_malara » Sun Aug 11, 2019 5:23 pm

Thanks Byron. As a side note, in Argentina we had two Dreadnoughts built in US in 1910, they had three shafts, I am trying to find out how the turbines were disposed, I had no plans but a photo of a cutaway model extant in the Naval Museum don´t show turbines in tandem.

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Re: 3-shaft propulsion

Post by Bill Jurens » Wed Aug 14, 2019 12:24 am

There is an interesting article in the Journal of the American Society of Naval Engineers for May 1901, translated from the German periodical Marine Rundschau. Author unknown. This discusses the advantages of three-shaft vs two shaft propulsion in some detail, coming down generally in favor of three shafts and suggesting that a major reason for adopting three-shaft propulsion in the German navy was because it made it much easier to navigate the Kiel Canal.

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Re: 3-shaft propulsion

Post by Thorsten Wahl » Wed Aug 14, 2019 8:19 am

the advantage was denominated by the Germans
3 Welleneffiziens1.jpg
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with ~48% screw-efficiency compared to ~42% screw-efficiency for a 4 screw ship. As i wrote earlier this advantage is in line with british publications to the same theme in "The engineeer"

That means a 3 screw ship requires about 6 per Cent less enginepower for the same speed.
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Re: 3-shaft propulsion

Post by Thorsten Wahl » Wed Aug 14, 2019 8:33 am

i made a spreadsheet based on development of RPM to achieve certain speed normalised with RPM required to achieve 15 kn beeing 100%
-Data according source mentioned
-For the US data i used the average RPM number from the FTP 218 spreadsheets
-I interpolated RPM numbers for uneven speeds for Howe
-The 30kn datapoints for Bismarck/Tirpitz are based on 270 RPM
In reality Tirpitz achieved 30,15 kn with 270 RPM and Bismarck 30,05 kn with 270 RPM during test trials


speed.gif
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Meine Herren, es kann ein siebenjähriger, es kann ein dreißigjähriger Krieg werden – und wehe dem, der zuerst die Lunte in das Pulverfaß schleudert!

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Re: 3-shaft propulsion

Post by marcelo_malara » Wed Aug 14, 2019 11:06 pm

The hard fact is that were far fewer three shafts warships than two or four shafted ones.

I suspect that three shafts without gearing was a difficult proposition (because of the HP/LP tandem arrangement). Once the designers got used to 2 or 4 shafts, with the appearance of the gearing it would have been difficult for them to go to three shafts.

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Re: 3-shaft propulsion

Post by Bill Jurens » Thu Aug 15, 2019 5:25 pm

For some reason the number three does not seem to work well regarding propulsion systems. Number of three-legged animals? Pretty small, perhaps non-existent. Number of three-engined airplanes? Not many left of those. Number of three-wheeled automobiles? Again, not too many. Number of three-engined ships? They seem to have more-or-less died off, too.

This suggests the presence of some underlying 'natural law', but I haven't seen it articulated as such anywhere...

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Re: 3-shaft propulsion

Post by marcelo_malara » Thu Aug 15, 2019 7:00 pm

I took a look to the book "Distinguished liners" (a compilation of articles from the The Shipbuilder magazine related to passenger ships). It seems there were more cases of three shaft passenger ships than warships. The arrangements named are:

-two reciprocating machines in wing shafts and a LP turbine in the centre (Titanic for example)
-one HP turbine in the centre and two LP ones on the wings

There is also an interesting 4 shaft arrangement, one HP, one MP and two LP turbines.

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Re: 3-shaft propulsion

Post by marcelo_malara » Fri Aug 16, 2019 12:28 am

Guys, I think that we have to go back to pre-Dreadnoughts days to understand this. I took a look at the Jane´s Warships of WWI. In those days the triple expansion reciprocating engine reigned supreme. Battleships displaced around 15000 t, and the machinery developed about 15000 hp. This was achieved with 2 or 3 shafts. I found that British battleships were all two shafted, but French, Russians and Americans had a mix of 2 or 3 shafts battleships.

Then came the Dreadnought and turbine revolution. In a turbine set you must have a HP and LP spool. If you wish to continue using two shafts the natural tendency would be to drive one shaft with the HP turbine and the other with the LP one. In spite of having two shafts that is not redundant. The steam needs first to go to the HP turbine and then pass to the LP one and to the condenser. Any damage to one of the three elements would seriously impair the ship. For proper damage resistance you have to at least duplicate the configuration. And that would need four shafts.

The Germans uniquely manage to continue using 3 shafts as in some of theirs pre-Dreadnoughts. A HP and two LP turbines driving one shaft each was used in passenger vessels, but that had only one HP turbine to receive the steam from the boilers, so it is out of the question. The only alternative is using one HP and one LP turbine on each shaft. With no gearing, the two turbines must be mounted on the same shaft. Here is where I believe complications starts. The two turbines turn at exactly the same speed, that would (I believe) provoke torsion in the shaft because of the difference in torque between the turbines. Moreover, if you wish to separate the HP and LP turbines in different compartments (something readily done with 4 shafts) you have to provide the bulkhead with a passage for the shaft and a gland to waterproof it, plus a passage for the steam interconnection.

I just think that the designers didn´t want to overcome this difficulties, and went directly to four shafts.

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