Why was Rodney and Nelson so slow?

Propulsion systems, machinery, turbines, boilers, propellers, fuel consumption, etc.
paul.mercer
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Why was Rodney and Nelson so slow?

Post by paul.mercer » Mon Mar 02, 2015 10:59 pm

Gentlemen,
During and shortly after WW1 the RN produced the Queen Elizabeth and the R class battleships both capable of around 24-25 Knots, so why on earth did they build Rodney and Nelson which were even slower? I realise they were 'Treaty' ships but the RN must have known that speeds would normally as development progressed. Both ships were extremely long which would suggest that they would have a long waterline length which is normal to produce speed, was it just underpowered engines that made them so slow?

MikeBrough
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Re: Why was Rodney and Nelson so slow?

Post by MikeBrough » Tue Mar 03, 2015 9:18 pm

The post-Jutland lesson, as interpreted by the RN, was that protection and firepower were more important than speed. This led to the design of the G3/N3 classes as the best-protected and hardest-hitting warships in the world.

Then came Washington. Something had to give and that something was the machinery. Still, the hull form was designed to give the best speed possible for the horsepower installed.

So, in answer to your question, yes!

Given the size of the ships, though, I wonder whether the machinery could have been replaced, wartime demands permitting. Or would the limited citadel length have prevented that?

MikeBrough
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Re: Why was Rodney and Nelson so slow?

Post by MikeBrough » Wed Mar 04, 2015 8:29 am

I hate to reply to my own post but I posted in haste.

The Nelsons had the same speed as the aspirational N3s and were a couple of knots faster than the previous Revenge class. The slightly faster Queen Elizabeths had originally been planned at the 'standard' 21 knots and only rose to 24 due to the weight savings realised by ditching a turret.

Basically, The Nelsons were seen as being 'fast enough'. Their near-contemporaries were all of roughly the same speeds (although the Japanese did tend to build faster ships than everyone else) and nothing new was expected to be built for 10 years or so.

I suspect that, if more weight had been available under the WT, the Nelsons would have been up-armoured rather than up-engined.

Andy H
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Re: Why was Rodney and Nelson so slow?

Post by Andy H » Sat Aug 01, 2015 11:38 pm

Hi

There speed was contemporary against others of its ilk when built.
Also the speed to some extent was sacrificed to give greater range, something not warranted in some navies, with lesser Empires to govern or defend.

Regards

Andy H

northcape
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Re: Why was Rodney and Nelson so slow?

Post by northcape » Sun Aug 02, 2015 11:52 am

45,000 SHP - what can one expect for 35,000 tons

but still, "the captains of the gate!"

Byron Angel
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Re: Why was Rodney and Nelson so slow?

Post by Byron Angel » Sun Aug 02, 2015 1:52 pm

NELSON was 150ft shorter than HOOD with about 1/3 the horsepower, from which can be inferred that the overriding design criterion was to limit overall size and tonnage.

B

Maciej
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Re: Why was Rodney and Nelson so slow?

Post by Maciej » Wed May 11, 2016 6:42 am

I think better question was "why Nelsons were so fast" ;)

But seriously
Nelsons were more ore less direct response to Colorado and Nagato.
After WNT Britain did not need extra battlecruisers. Only ships of that class were in Japan – 4 Kongos, and Britain had Hood, Repeir, Refit and Tiger.
Hood stronger than any of Kongo ( first unmodernized version ), Tiger on pair.
Armour of Repeir and Refit were to be upgraded, so after that upgrade no worse than Kongo.
Problem was with those 16” battleships in USA and Japan.
On paper, no existing British battleship could much them ( now I don’t want to start discussion about difference between “paper” and “real” capabilities, so say only “paper” capabilities )
New 16” battleships were needed.
British knew that max speed of Colorado is ~21 knots.
They “knew” that max speed of Nagato was ~21 knots, possibly 22, not more.
So 23.5 for Nelsons guarantee some speed advantage over those most important “enemies”.
Clearly in WWII it was a bit too slow, but that time Nelsons were not so “new”.

In case of reconstruction.
Many things were considered.
After remedy of most urgent early problems, priority of upgrading ships were:
1. Add deck armour forward of the citadel ( done on Nelson, not on Rodney )
2. Add light AA armament ( done on both )
3. Upgrade AA fire control ( done on Nelson, not on Rodney )
4. Add extension of belt armour to protect against diving shells ( not done ).

And than in very late 30thies it was real need to repair they electrical systems.
This alone was expected to take ~9 months if not more. So there were some conceptions of larger reconstruction including new machinery.
New machinery was considered to gain ~2 knots extra speed. So max speed with new machinery will be something like 26 knots.
In my opinion more was impossible.
Power/Speed curve shows clearly that in low speeds Nelsons required very low power. Lowest of all British WWII era battleships.
But over ~20 knots it started to rise very sharply. Somewhere around 23 knots Nelsons required more power than KGV to gain the same speed.
Curve ends at this point, but tendency is quite visible. Possibly over 26 knots it should be nearly vertical, so even very large increase of power will result marginal increase in speed ( 2 shaft machinery limited possibility of waste increase of power anyway, and putting more shafts was problematic at best )

So in time, Nelsons were not so “slow”.

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