Motors too far forward

Discussion in 'Stability' started by Aristos, May 7, 2019.

  1. Aristos
    Joined: May 2019
    Posts: 2
    Likes: 0, Points: 1
    Location: Western Australia

    Aristos New Member

    I purchased a 43” outsider motor boat which had been extended to 48”. It has 2 x 3208 Cats 300hp a side. When the boat was extended the owners did not move the engine and propellers back. Boat is bow heavy and difficult to get up on the plane. Previous owner had 800 kg of lead ingots placed in the aft. I removed the ingots because motors not reaching full rpm under load, short 200rpm. It would be a too costly exercise to move engines back.....not sure if there is any other action to take to try to balance the boat.
     
  2. TANSL
    Joined: Sep 2011
    Posts: 5,725
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    Location: Spain

    TANSL Senior Member

    It would be prudent to make some calculations before starting an expensive work, without guarantees that it is the solution. Do you have, by chance, or could you get a body lines plan for your boat?
    Would it be a solution to return to 43"?
     
  3. Magnus W
    Joined: Nov 2017
    Posts: 135
    Likes: 6, Points: 18
    Location: Sweden

    Magnus W Senior Member

    Do you know that the engines reached full rpm before the extension and does it reach it now sans the 800 kg? And how does a stock boat run, well balanced?

    In such a big boat I guess you have a few batteries and perhaps also a genset. Moving mass is often better than adding it, do a quick estimation of stuff you have in the boat that doesn't change weight (not like fuel- or water tanks) and see what you end up with.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    I'm having trouble seeing how extending the hull has made it more difficult to plane, unless the addition is unusually heavy, and creating a stern down trim. It is usually weight added aft that creates these kinds of problems, it might be that the weight added aft is greater than the buoyancy provided by it. You have a boat that is heavier with the same power, more wetted area, you say bow heavy now, that in itself usually assists the attitude of the boat running up to speed, but will limit top speed. Is the problem a loss of top speed, or a loss of speed at cruise rpm setting ?
     
  5. Aristos
    Joined: May 2019
    Posts: 2
    Likes: 0, Points: 1
    Location: Western Australia

    Aristos New Member

    Thanks for the replies, no changing back and as I mentioned s costly exercise to also move engines. It’s a planning hull. I just have to move some weight around where possible. Engine reaches flight revs ok. After calculating I could replace props with a lager diameter might help the speed slightly but weight is too far forward and I have to live with it!!
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Likes: 273, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The boat may just be running flatter than what you were used to previously, naturally you can't expect to go faster with more weight, more wetted area, but the same engines. Has the boat became "wet", or bogging in down-hill running ? I don't understand how it would be more difficult to plane, though, by having a flatter trim angle.
     

  7. kapnD
    Joined: Jan 2003
    Posts: 507
    Likes: 37, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    What I’m not getting about this thread is how a boat that’s been stretched and is still only four feet long can carry two 3208s and a ton of ballast!
    Could Aussie inches. Be that much bigger than they are in the USA?
    On a serious note, we don’t know how much horsepower the 3208s make, but if they are the basic 210 hp models it will always struggle to plane for lack of ponies.
     
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