Tug boat and barge engine power calculation

Discussion in 'Diesel Engines' started by yodani, Aug 18, 2014.

  1. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member


    I agree with mik that a higher Hp engine will consume more fuel to develop the power required. In going for a higher Hp, you will have higher torque, thus you can swing a larger propeller. A larger slower turning prop will have more efficiency. This will in turn decrease the need for higher Hp (and less fuel consumption). Note that in your Calc sheet, the prop efficiency is only 55%. 60 or even 70% is possible.

    You need to recalculate based on new power, new transmission, prop dia, pitch required. The propeller demand curve is shown in the engine data sheet. I havent read all the fine print but some manufacturer will state this as Shaft Hp after losses in the bearing and transmission.

    There are two possibilies that is probable with your old system.
    1. Your tug is pulling a weight more than it is designed for. The clue is that you have stated that the engine rpm is around "1,300". Is this less than the engine specifications? If it is less, that means your engine is not developing its full power potential and is laboring under load. It may also be that your engine is old and needs overhauling.
    2. You have too much propeller slip. The prop is just spinning without doing a good job of pulling. Note that slip is different from efficiency and for this type of boat, slip is around 45%

    What you can do is measure the amount of tow load (the barge or the hotel barge) with a borrowed load cell. If you cannot obtain one, measure the height of the hotel above waterline, the draft, and the estimated displacement and then we can use the theoretical formula to estimate towrope Hp.

    Given that you have the engine curve and ratings, it will be easy to plot the graph where your new engine will be happiest. Dave Gerr's book has some pretty decent power estimation and the more sophisticated Bp curve method of powering. I am sure Mik would be happy to chase some numbers for you.;)
  2. Mik the stick
    Joined: Dec 2012
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    Mik the stick Senior Member

    I will help any way I can. I thought you may have had a wrong size prop did a quick estimate and 80cms prop seem perfect for your 150hp engine.
  3. parkland
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    parkland Senior Member

    I don't believe this is true, IMHO.
    A larger engine running lower RPM might be more fuel efficient than a smaller engine running higher RPM.


    Of course the boat version of a 're-gear' is a different prop.

  4. parkland
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    parkland Senior Member

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