Low RPM Engine vs High RPM Engine

Discussion in 'Diesel Engines' started by Leopard, Jan 15, 2023.

  1. Leopard
    Joined: Nov 2021
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    Location: Belgium

    Leopard Junior Member

    Hi,
    What are the advantages and disadvantages of using High RPM Engine over Low RPM Engine?

    Let's say for a passenger cruise vessel, I need propeller RPM 800, and Engine RPM is 2400, so I am using Gear box with 3:1
    Now if I use an Engine of same power with 1200 RPM with Gear box with 1.5:1, is there any problem? Won't the performance be the same?
     
  2. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    Most low RPM engine are heavy and robust and is designed to operate at high or continuos duty. High RPM engines are small, light and limited duty.

    Consider a fishing trawler that runs almost continously running at full power at heavy loads and a yacht that sits in the dock 10% if a year.

    Google Marine Diesel Engine Ratings as they have a lengthy explanation. Some have duty ratings up to six categories.
     
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  3. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    As well as what rxcomposite replied, "low RPM" is relative to who you are asking. Below are the terms generally used with marine diesel engines.

    Low-Speed Diesel (LSD): These are large (1+ m piston diameter) house sized long stroke (2-3m) engines directly connected to the prop shaft. Having a power upwards of 20,000 shp they are the prime moves of large cargo ships. Electrical power is not provided by LSD's as they turn too slow, so ships will have one or more smaller engines to provide power.

    Medium Speed Diesels (MSD): These are moderate sized (think diesel locomotive sized) engines rated for ~2500-5000 shp continuous. Engines of this class are generally run in groups and power most passenger ships as well as provide electrical power to the vessel.

    High Speed Diesels (HSD): these are small automotive sized diesels that power many small vessels and most yachts. These engines tend to be less than 2,500 shp maximum rating, but as rxcomposite says, their continuous rating is much less. Because electrical power generation can draw so much torque from the engine, often separate very small generators are fitted depending on vessel hotel loads.

    But to answer the original question.... While power is interchangeable through gearing other factors are not. Generally, the lower the speed, the more efficient the engine is. Conversely, the higher the speed the lighter the engine installation is. Additionally, engine RPM is only tenuously related to operating torque per stroke; so you will need to make sure that the engine selected can support the required torque at the operational rpm and shp required by the propulsor.

    Suffice it to say, engine and gearing selection has to look at the whole mission statement for the vessel.
     
  4. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Weight.

    My grandfather had an old 200 hp 1200 rpm cat. Darn thing had been running longer than I'd been alive when it was finally pulled. We dropped in a 350hp at 1800 rpm modern diesel, stern of the boat was 4-5 inches higher with that monster yellow block out of the bilge. On really big cargo boats it is a small percentage of weigh on smaller vessels it's just hauling ballast.
     

  5. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    As JE has noted, the engine has to be fitted to a marine gear (with the exeption of the direct drive). These are mostly the small diesel engines available commercially.

    Here is the link from Cummins for a more expanded definitions. Marine Ratings and Definitions | Cummins Inc. https://www.cummins.com/engines/marine-ratings

    Attached is the typical application based on duty rating on a particular "small" engine model from CAT. Note that it is possible to get all types of ratings from a single block. For it to be continuous duty, it is derated in HP and RPM. They adjust the governor to limit the RPM and the block is much heavier with thicker walls. What is not shown is the weight. The continuous duty can be twice as heavy from the pleasure duty as all components are beefed up all while keeping the same outside dimensions.

    If you are leaning towards high speed engines, try Volvo Penta. They especialise in High Speed Marine diesels.
     

    Attached Files:

    Last edited: Jan 16, 2023
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