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I've seen, in some marine engine project guides, arrangements where you have two medium-speed engines coupled to one propeller through a gearbox instead of one larger engine running that propeller. I've also seen the twin-in/single-out approach done with electric motors in Diesel-electric vessels.

My question is, why do some vessels (assuming twin-screw) use this arrangement instead of two larger prime movers?

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Four into two arrangement can help with mid speed fuel economy, but that often isn't enough by itself to justify the complexity. Sometimes, the the general arrangement of things is better. The two engines need not be the same. For instance, near shore in areas with emissions restrictions, one of the engines can be emissions compliant, perhaps running dual fuels with the second being a big, dumb bunker diesel. Or commonly, one is a diesel and the other is a GTE (as is the case with the totally botched Freedom class LCS.)

Military vessels such as the DDG-51 (4 GTEs into 2 shafts) will do this to achieve a high sprint speed as well as an efficient cruise speed using one of the pair. It also gives them redundancy. With turbines, you need a lot of gearing anyway, so the combining gears don't add much to what is needed to gear a GTE down to shaft speed.

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Using two can make low to mid propulsion more efficient with full power available with both.

Otherwise one single could be running out of its efficiency band if the load required is low.

Case in point was when the aerodynamic roofs were added to some trucks the fuel consumption became worse as the combination of the load and throttle position took the engine out of its efficient design point.

As another example, many production processes that require a lot of heat are supplied from multiple boilers say 4 and any 3 out of the 4 are capable. The 4th is spare for maintenance or breakdown.

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  • $\begingroup$ But that just begs the question why don't you just use one larger engine if ganging two smaller engines together is more efficient than ganging two larger ones. Unless you are suggesting to just run one engine when you need low power (which wasn't made explicit in your answer). $\endgroup$
    – DKNguyen
    Aug 7, 2022 at 19:16
  • $\begingroup$ @DKNguyen Even power factor control units apply the same concept with several capacitors, of different sizes, being combined in configurations to match the reactive load - check it out, lots of fun calculations… $\endgroup$
    – Solar Mike
    Aug 7, 2022 at 21:46
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There can be man reasons for using two smaller instead of on larger engine, as already said in the answers above. I would like to summarize, to add:

  1. Redundancy could be a reason.
  2. On a Roll-On, Roll-Off ferry vessel also the deck hight in the engine room could be a limitation.
  3. If expected running regular and longer time in part load, this could also be a reason, as already expressed in one answer above. By doing so, the efficiency of a single diesel engine is higher than of a larger one in part load.

To stay with on instead of two propellers (with two engines) gives in general a higher propeller efficiency.

I worked for more than 15 years for a ship yard as Head of Mechnical Design, where built very large yachts. Here one argument for a propulsion concept could also be: The owner just want it. :) So there are not in any case only logical arguments we would agree to. But for two engines on a single gear box speak a lot of reasons. This is not and unusual and exotic solution.

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  • $\begingroup$ If you aren't already a member, I invite you to join Boat Design Net. We have a good crew of international talent covering all aspects of boats, ships, yachts, and marine engineering works. boatdesign.net/forums $\endgroup$
    – Phil Sweet
    Apr 20 at 16:52

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