Twin engine CPP

Discussion in 'Powerboats' started by marcoh, Sep 18, 2020.

  1. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re the worry about having a main engine and a wing engine, the Nordhavn 'Dirona' has been around the world very happily with this set-up - here is a good article on their website where they describe having to use their wing engine out in the middle of the South Atlantic so that they could do an oil change.

    To Change or Not to Change? That is the Question.

    And here they talk about the oil change -

    Oil Change at Sea

    Here is a well reasoned argument for a single engine rather than twin -

    Single or Twin Engines? - Knots and Boats
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  2. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    How does the engine cost vs power scale? Do a pair of 160s, for example, cost more or less than a 320? How about hull costs? The engine beds for a 320 will be more than the beds for a 160. Installation and replacement costs are also non-linear. Components that can be man-handled are cheaper to replace than parts that need some sort of lifting assembly. And is the main plus wing so very much cheaper than a pair that it justifies carrying parts for 2 unrelated engines? I don't know how his prospective boat will handle, but for mine, twin engines gives such a massive boost in maneuverability that I'll gladly pay the increased cost. Hardly a large issue in a slow, heavy, long distance cruiser I know, but I see a lot of bow thrusters out there. They aren't free, and they sure don't help mileage.

    Not nit picking, but factors only matter if you can do numerical analysis, and I don't know the numbers.
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  3. missinginaction
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    missinginaction Senior Member

    Well maintained engines very rarely fail. Electronics fail. Wiring can break. Nuts and bolts can corrode. There are lots of things that can cause problems but a failure of a major internal marine engine component? Extremely rare.

    I can see why people like twins, especially when docking or in close quarters. That said, I wouldn't trade my single for a twin, ever. Especially with a wing engine on boats like the Nordhaven or an auxiliary outboard on a little boat like mine, you have complete redundancy. I've never had a problem but with the auxiliary and a portable Honda generator I can run indefinitely without the main engine and still have all the electrical power I have with the main running. My speed is reduced to hull speed with the auxiliary but I can move and that's what counts.

    You know what else I like about being a single? It keeps me at anchor or tied to the dock when I see other boats going out in dangerous conditions. I'm always thinking about how I'll get back to the dock.

    It's also a lot "greener" than running two large engines. Not that a boat's ever going to be very green.

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  4. DogCavalry
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    DogCavalry I aim to misbehave.

    Well, Bajansailor, that is a solid argument in favor of a single engine installation.
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  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Aaahh.. that old chestnut.
    There are of course many pro's and con's to this and often it becomes a subjective choice rather than a technical one.

    However, you need to look at the rating of the engine.

    Is the engine 'designed' to run at low rpms for extended periods? Generally not - that is not how a desiel engine likes to be at around a minimum of 75-80% of load most of its life. Any value below this, causes issues with the engine and increases maintenance costs. Also 2 smaller engines when you shut down one..and run on the single engine, you'll find, in general, the difference in fuel consumption, again, comes down to the rating used. So need to compare apples with apples... oh and you have spare apples if the smaller one trips over i.e. redundancy! :D

    So, before anyone attempts to answer this with a simple one liner .... one must first look at the rating of the engines as the initial basis of comparison. And then also, the nominal annual hours of usage, and hence what the effect is of running at low rpms for slow speed/manoeuvring is on the overall life of the engine.

    Adding a CPP helps to "flatten this curve" of what you want from your boat and through-life-costs.
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  6. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    The greenest engine is the one running at it's ideal rpm. Any engine run at WOT is puking out far more emissions than it would running at 85% of full power.

  7. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    Here's another question: since folks are explaining about how you don't need two engines, because you can get by with 3, main engine, wing engine, and bow thruster, how much does that cost?

    And we've gone off topic, since the op was asking about fpp vs cpp. Cpp takes it.
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