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Old 01-18-2023, 05:51 AM   #1
Marasmusine
 
Join Date: Feb 2005
Location: Doncaster, UK
Default [Vehicles] Marine Steam Engines

Hello all, time for my annual Vehicle-related post.

This time I'm looking at marine steam engines. It seems to me that their power-to-weight ratio is much lower than those used for locomotives.

For example, the RMS Titanic's multi-expansion steam engines produced a total of 22,000 kW with a weight of 5,890 tons.
(calculated: 2 engines, 720 tons per engine, 195 tons for each engine bedplate; 29 boilers at 91 tons each, each with 48.5 tons of water - this doesn't include the weight of the steam turbine.)
So about 4 kW per ton.

My estimate for a LNER Class A4 locomotive engine is 75 tons (75-percent of the weight of the vehicle), generating 3,000 to 4,000 kW
So about 40 to 50 kW per ton (this is also about the power-to-weight given for a "triple-expansion steam engine" in GURPS Vehicles).

I wonder why the marine engine has less power-to-weight by an order of magnitude. More fuel efficient? Cheaper per ton?
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Old 01-18-2023, 07:07 AM   #2
mlangsdorf
 
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Default Re: [Vehicles] Marine Steam Engines

A couple of quibbles first:
* the Titanic's boilers also fed its steam turbine, so you're overestimating the weight by a bit. Actual weight efficiency was closer to 4.5 kW, which doesn't negate your point.
* The LNER Class A4 was a mid-1930s design compared to the Titanic's roughly 1909 design, and engine efficiency increased rapidly in the early 20th century. A GWR 4300 class locomotive from the same time as the Titanic weighed about 60 tons and delivered (maybe, sources are hard to find) 800 kW, for about 18 kW per ton, so about four times as efficient as the Titanic.
* Closer to the time period of the LNER A4, a Liberty ship's 140 ton triple-expansion engine produced 2,500 kw, or about 18 kw/ton.

Quibbles aside, it seems there was a 2x to 4x efficiency difference between marine engines and land engines. The reasons seem to be that marine engines were optimized for reliability and reduced cost at the expense of weight, while land engines prioritized lower weight.

Reliability seems like a major issue. The Titanic's engines needed to run continuously for a week or more. Locomotive engines, AFAICT, rarely ran for more than a day or two continuously - the Orient Express is a 28 hour route. An easy way to improve reliability is to overbuild the components, which increases weight.
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Old 01-18-2023, 07:27 AM   #3
Marasmusine
 
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Default Re: [Vehicles] Marine Steam Engines

Okay, that's interesting, thanks.
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Old 01-18-2023, 07:50 AM   #4
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Default Re: [Vehicles] Marine Steam Engines

Fuel efficiency seems to be a high priority also. You can easily refuel on land but in the mid-Atlantic or mid-Pacific there's no place to refuel.
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Old 01-18-2023, 08:20 AM   #5
johndallman
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Default Re: [Vehicles] Marine Steam Engines

Quote:
Originally Posted by mlangsdorf View Post
Quibbles aside, it seems there was a 2x to 4x efficiency difference between marine engines and land engines. The reasons seem to be that marine engines were optimized for reliability and reduced cost at the expense of weight, while land engines prioritized lower weight.
Two of the reasons for that were:

Railroad engines must be small enough to fit the "loading gauge" for the railway lines they run on.

Marine engines need to condense their steam back to fresh water, using "condensers" cooled by sea water. The condensers take up substantial weight and space, but carrying enough fresh water to run for weeks without condensers is impossible. Land vehicles can usually refill with fresh water every few hours, and don't need condensers. Steam power stations use condensers, because those allow greater fuel economy, by retaining some of the heat.
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Old 01-18-2023, 10:03 AM   #6
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Default Re: [Vehicles] Marine Steam Engines

Quote:
Originally Posted by mlangsdorf View Post
A couple of quibbles first:
* the Titanic's boilers also fed its steam turbine, so you're overestimating the weight by a bit. Actual weight efficiency was closer to 4.5 kW, which doesn't negate your point.
* The LNER Class A4 was a mid-1930s design compared to the Titanic's roughly 1909 design, and engine efficiency increased rapidly in the early 20th century. A GWR 4300 class locomotive from the same time as the Titanic weighed about 60 tons and delivered (maybe, sources are hard to find) 800 kW, for about 18 kW per ton, so about four times as efficient as the Titanic.
* Closer to the time period of the LNER A4, a Liberty ship's 140 ton triple-expansion engine produced 2,500 kw, or about 18 kw/ton.
And note that the Liberty ship's steam plant was far from state of the art at the time (intentionally - this made it much easier to find crew capable of maintaining it).

Quote:
Quibbles aside, it seems there was a 2x to 4x efficiency difference between marine engines and land engines. The reasons seem to be that marine engines were optimized for reliability and reduced cost at the expense of weight, while land engines prioritized lower weight.
Land engines also didn't much care about water efficiency, and nor did they need to recycle water, and thus didn't have condensation plants, etc. The weight of a marine plant might also include the evaporators to create more clean water as well.
Quote:
Reliability seems like a major issue. The Titanic's engines needed to run continuously for a week or more. Locomotive engines, AFAICT, rarely ran for more than a day or two continuously - the Orient Express is a 28 hour route. An easy way to improve reliability is to overbuild the components, which increases weight.
This is also important, especially for a passenger liner like Titanic, which was designed to run at or near full speed for the entire trans-Atlantic trip, something reciprocating engines were not noted for, as a rule.

EDIT: When it comes to Vehicles, there's no by-the-book way to really describe this difference, as it abstracts away water consumption, etc. You'll get some of the effect because steam plants on ships will need the extra volume for 'long occupancy' access space. Arguably steam trains don't actually have any access space, because the engine is effectively exposed. That makes the whole thing smaller and thus it will require less frame, so the overall plant will be lighter.
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Last edited by Rupert; 01-18-2023 at 10:10 AM.
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Old 01-18-2023, 10:18 AM   #7
mlangsdorf
 
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Default Re: [Vehicles] Marine Steam Engines

Quote:
Originally Posted by Tinman View Post
Fuel efficiency seems to be a high priority also. You can easily refuel on land but in the mid-Atlantic or mid-Pacific there's no place to refuel.
Do you have a cite on that? The Titanic consumed ~600 tons coal/day, which works out to about 1,300 kw/ton of coal in an hour. A GWR 4300 had a 7 ton fuel capacity - which I think is the bunker, not the tender - but I can't figure out the burn rate so that doesn't help much.

I'm not saying you're wrong, and certainly fuel efficiency and operating range were a design consideration of steamer ships. But John Dallman's point that ships required steam condensers while trains just periodically refilled from water towers would explain a lot of the power-per-weight difference for marine engines.
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Old 01-19-2023, 08:01 PM   #8
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Default Re: [Vehicles] Marine Steam Engines

The water capacity of railway engines was also an important factor. In 1941 the Germans discovered that the water capacity of Soviet steam engines was much greater than that of German equipment, forcing the Germans to not only change the rail gauge but build a new set of water towers on the Soviet railways which they were using. They also, of course, became targets for Soviet partisans. {From the book "Engines of War")

Also, "lagging", or the insulation of engine components (against heat loss) improved significantly between c. 1880 and 1920, improving the fuel efficiency of triple expansion engines considerably. See D.K. Brown's series on marine design and construction.
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Old 01-19-2023, 11:22 PM   #9
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Default Re: [Vehicles] Marine Steam Engines

Quote:
Originally Posted by mlangsdorf View Post
Do you have a cite on that?
No. Just going off of logistics & reasoning. (Though this topic would be a great question for Drachinifel. A naval warfare youtube channel.)

Whether it's fuel, fresh water or whatever, you've got to take it with you. Looking at territories around the world that were naval resupply depots for both the UK & the USA and their distances from other bases shows the range of ships. In between their launch point & destination there was nowhere to really resupply.
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Old 01-20-2023, 10:39 AM   #10
mlangsdorf
 
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Default Re: [Vehicles] Marine Steam Engines

Quote:
Originally Posted by fredtheobviouspseudonym View Post
Also, "lagging", or the insulation of engine components (against heat loss) improved significantly between c. 1880 and 1920, improving the fuel efficiency of triple expansion engines considerably. See D.K. Brown's series on marine design and construction.
There's only 1 instance of the word "lagging" that Kindle search finds in my copies of The Grand Fleet and Warrior to Dreadnought, and it's in reference to boiler design on the Argus. I'm not saying you're wrong, but is there a different DK Brown series on marine design and construction?
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