Trawler CG

Discussion in 'Powerboats' started by Easy Rider, Mar 5, 2011.

  1. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    Regarding a discussion on Trawler Forum we'd like to know where the CG is on a GB, CHb, MT or basically a typical trawler yacht.
    1. Would one raise or lower the CG by installing lighter engines in a twin engine or single engined trawler?
    2. Would a stock single engined 36 GB have a lower CG than a twin?
    I'm thinking a single engine GB 36 would be a bit more seaworthy (generally)
    than the twin.
    Any comments to take home would be appreciated.
     
  2. El Sea
    Joined: Aug 2006
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    El Sea Junior Member

    I have a single engine in our 44' Thompson, however our hull is hard chine rather that the round bottom of the TT's you mentioned.

    I would think the CG can be altered simply by the volume you maintain in your tanks.
     
  3. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The location of the CG is certainly important for a sea boat ,

    BUT

    to survive the boat must be built for ocean operation.

    This makes the boat about 3X as expensive to build and purchase , so few stock boats are ever built for ocean service.

    "Trawler" is a nice shippy sounding name but few will cross much of anything.

    Most are stock hulls that have "trawlery" styling for the pilot house , instead of "motor yacht".

    Take a look at a Coastie 44 ft rescue boat , observe just the marine windows in the pilot house , then look at most any stock "trawler, you will get the idea.


    FF
     
  4. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Easy,

    First I hesitate to answer because I don't know what you might be trying to "prove" with the information........:p

    With a bunch of caveats about not guaranteeing the accuracy of the statements that follow, and no conclusions can be drawn, etc.......

    I assume you are asking about VCG (vertical center of gravity)?

    The only way to accurately ascertain the VCG of any boat is to do an inclining experiment in the water......estimates are just that....estimates......and only good for general comments.......they are of no use (due to garbage in) in specific comparisons.....

    For a modern motoryacht with single level deck house the typical VCG height is about 1/2 freeboard (WL to main deck level) height. For a yacht with a multi level house the VCG height is closer to main deck level.......these are vast generalizations and subject to changes depending the specific build and design........I think it would be closer to the main deck height in a (traditional) Grand Banks as they have rather low freeboard.

    Question 1.....

    You will either raise, or lower, the overall VCG by installing lighter engines depending on whether the (wet weight) engines CG is above or below the vessel's overall VCG.......Lighter engines above VCG will lower VCG, lighter engines below VCG will raise VCG......

    Question 2....

    We can't say because we don't have any specifics..... are the engines CG above or below the vessel VCG? Are the twins double the weight of the single or are they less? Are the twins mounted higher than the single or at the same height? What is the height difference in feet? What is the weight difference in pounds? Where is the boat's VCG? (in one or the other configuration)

    As a measure of what the difference might be, I ran the attached (very simple) spreadsheet........Note that the twin engines weigh 2000 pounds more and are mounted 6" higher, but only raise VCG by less than 1", hardly noticeable......

    View attachment GB36weights.xls
     
  5. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    Tad,
    Thank you very much.
    Quick question.
    Is the "main deck" ...
    A. Same as the fore deck?
    B. Same as upper salon deck?
    C. Same as side decks and aft cockpit deck?

    Easy
     
  6. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    I think in most Grand Banks models the fore deck and side and aft decks are all one level, which would be the main deck level, upper deck is just that, the upper deck (Flying Bridge).

    GB47.jpg
     

  7. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Thank's TAD. I think the fore deck is 12 to 15" above the side decks/aft decks.
    I'm not sure but for our discussion it may be close enough. I must admit I'm surprised the CG is that high. I was guessing it to be a few inches above the WL. Sometimes I love being wrong as I get smarter. Knowledgable would be a better word.

    Thanks
    Easy
     
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