Power plants in large ships.

[enginesrus]

I've wondered why there are still very large marine diesel engines that are used in large ships? Is the power density better for their size and weight, than multiple smaller engines? I realize with multiple smaller engines that the transmission system or electrification of it adds more complexity. Since the larger engines are very efficient I suppose that plays into it.

 

[itsmoked]

It depends on the application and probably more than just how cheaply can one power a ship.

I believe it's the pretty good match of engine to screw. When the engines get enormous they turn slowly just as the screw needs too. That said I believe all cruise ships (and they're really big) are all using azipods now and so are filled with banks of generators all feeding a bus that the electric props are powered by. It provides very high reliability because if you can run some generators you can make some headway so being dead-in-the-water is pretty rare.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

For US military ships, an engine runs a generator which drives the electric motor driving the screws. That used to be different, with the turbine driving the screws and the generator. But, there are supposedly tactical advantages for having an electric drive and allowing the engine to run at its maximum efficiency, without having to throttle down or run through a transmission to drive the screws.

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[EdStainless]

Large naval ships still drive the screws from the turbines through reduction gears.
There are not many 260,000HP electric drive systems out there.
Remember that the aircraft carriers are the fastest ships in the navy.
The drive shafts are nearly 6' in diameter and under full power they twist 1.5 times (after all it is 3million ft-lb of torque).

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P.E. Metallurgy, Plymouth Tube

 

[enginesrus]

Carrier ? I'd like to see one at full throttle. Those turbines are steam I'd guess??

 

[BlackJackJacques]

Prime mover size and number are typically dictated by tradeoffs considering manning, redundancy, operations, economics, plant architecture, as well as fleet issues (maintenance) and tradition.

An integrated plant (e.g., propulsion & ship service, etc) for a ship employing electric propulsion will be different for a split plant arrangement, or for a plant employing electric power take-offs from a mechanical shaft, and all that will be different for a combatant versus a cruise v. commercial vessel. Redundancy and continuity of mission services will, of course, have more weight in a combatant than in others. However, all that being said, traditions and subjective considerations sometimes factor in.

With respect to space, weight efficiency, having one larger plant most often requires less space/weight than having multiple smaller plants for a given shaft horsepower especially when considering the support services needed. For instance, consider the support services required for a diesel engine in the way of lube oil, cooling water, air, exhaust, monitoring, and control, etc. Space and weight are more important in a ship than in a landlocked structure and thus given more importance in optimizing a plant.