A question of attitude

Discussion in 'Boat Design' started by Twizzle, Apr 7, 2011.

  1. Twizzle
    Joined: Apr 2011
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    Twizzle New Member

    Apologies if this is a dumb question from an ignorant newbie.
    My boat is a GRP river cruiser, 12m x 4.2m, 0.9m draft and fairly flat hull shape, pointy at the front and squarish at the stern and with with a long central keel. Approx. 7 tons. Her attitude is noticeably bows-down by something like 50mm, whether by design fault or change since she was built (she has a different engine, but it's in the same place slightly aft of the mid - nothing else significant that I can think of). She's not loaded with anything much at the bows, or forward of her mid-point. Tanks are located one on each side, in line with the engine).
    The situation means that water collects in 'downhill' places when it ought to run aft and away.
    Is there any rule-of-thumb calculation for how much weight I'd need to add at the stern (it would be possible, I have some unused below-decks/waterline locker space running across the stern) to make a correction?
    Is this actually feasible and if so, what's the best/cheapest way to do it?

    Thanks for any collective wisdom.
     
  2. bntii
    Joined: Jun 2006
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    bntii Senior Member

    Get a friend or two to walk aft to see how much weight is needed.

    Do you have a picture of the boat?
     
  3. Twizzle
    Joined: Apr 2011
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    Twizzle New Member

    Thanks. That has been suggested, but we've had maybe six at a time so far on the aft deck, with very little difference. Is the hull just too buoyant at her aft end for any practical remedy?
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Without more information about the design and pictures, it would be just speculation, but most boats need trimming ballast, particularly if major changes are made, such as engine swaps. I have a 11.9 m x 3.2 m x .08 m power yacht that carries about a ton of trimming ballast. These are 7 pound (3.2 kilo) bricks of lead, stacked in her bilge. These bricks had to be redistributed when I put the new diesel in her.
     
  5. ekamarine
    Joined: Nov 2008
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    ekamarine Junior Member

    I agree with PAR about ballast. Lead nuggets are always an easy and cheap way, if someone is not lazy to redistribute those every time )))
    I didn't understand one thing, do you have his trim at all conditions? or at a specific loading?
     
  6. Frosty

    Frosty Previous Member

    Personally I wouldnt be wanting to ballast a river cruiser. 6 men already is 1000lbs.

    What kind of a job is it to get those tanks aft,--like at the transom.

    I know what you mean about bow down boats --just don't look right.

    Only other thing would be to consider a bulbous bow to give some bouancy.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Well, again without pictures or a make, model and year it's still just speculation, but a bulb nose on the bow is a pretty ridiculous way of approaching a trim issue.

    It's very rare to have a trim issue on a boat that once sat on her lines, that can't be corrected with some minor ballasting. Sure moving tanks and accommodations is an option, but a wheel barrow full of lead bricks is a lot easier to work with, trust me.

    Then again, was the LWL repainted wrong the last time it was out? I've seen this happen more then once, where someone got creative with the bottom paint and screwed up the actual location of the LWL, so someone took a guess, which worked out to look like the boat was out of trim.
     
  8. Twizzle
    Joined: Apr 2011
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    Twizzle New Member

    I have considered some of these (not the nose buoyancy one though :eek:).
    I think she's bows-down because she looks it and because of water collection where it didn't oughter. The problem is not serious enough to merit extensive engineering. How much do lead nuggets cost?
     

  9. Frosty

    Frosty Previous Member

    Thats about the only use a bulbous bow is good for under 60feet. Crowther cats use a bulbous bow for static trim. Lifting the bow by 50mm will be easier than forcing down the stern down by 50mm

    If the boat is collecting water forward how can that be from the water line painted incorrectly?
     
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