Center of balance?

Discussion in 'Hydrodynamics and Aerodynamics' started by nine mile, May 9, 2014.

  1. nine mile
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    nine mile Junior Member

    Not that I would understand any formula but just for the sake of Knowing built a cradle to match the hull of my prodject Delmar and plan on balancing the boat with and without the engine. Wondering if it was even considered at all. At some point it is beneficial,right? Boat will be Weighed also and is 16`.
     
  2. nine mile
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    nine mile Junior Member

    Early 60`s Glaspar Delmar
     

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  3. PAR
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    PAR Yacht Designer/Builder

    Without the engine will not offer much useful information.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think it makes sense to be too pedantic about where the "centre of balance" ( do you mean centre of gravity ? ) might be in a small boat like this, that will change quite a bit with different load conditions anyway, the main thing is how it actually performs on the water with testing, it will tell you by it's behaviour if there is a problem, such as porpoising, e.g.
     
  5. nine mile
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    nine mile Junior Member

    I stand corrected, centre of gravity is more like it. Been said I have too much time on my hands and we are talking about a "hole in the water", just want to get it right and have fun doing it.
     
  6. rwatson
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    rwatson Senior Member

    'Balancing' the boat on land is a bit useless, its the 'balance' in the water that counts, especially without a crew.

    There is big difference of a hull, fully loaded, floating on its waterline versus teetering on a 'seesaw' on land.

    If you need to analyze the displacement of the hull - you can get a copy of Freeship software, or do like Mr E said, and test in under operation,
     
  7. PAR
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    PAR Yacht Designer/Builder

    Actually finding the CG on land can be beneficial if you've made significant changes. It'll give you an idea as to where you might need to move things to get a better CG location. I do this with conversions often enough, say an inboard to outboard or I/O. There's a desirable range I look for and if it's much off this, I consider what I need to move (tanks, batteries, etc.) to make it hit about where I think it needs to be.
     
  8. nine mile
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    nine mile Junior Member

    Moving the oversized engine forward is a significant change and a lake test is out of the question for the time being. I imagined somewhere in the range of 2/3 rds. of the way back.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Right, well it seems I have not been paying attention ! You have cut into the back of the boat to create this well, not added pods either side of the engine ? That is a little different ! You are possibly going to find she does squat a little, even with the engine weight going forward, but until you run it on the water, it is mainly guesswork. My feeling is you will not need to move weight aft, probably the opposite. But it may not be much different.
     
  10. jarmo.hakkinen
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    jarmo.hakkinen Junior Member

    Donald Blount has a little program in his site, that will give you an estimate of the possibility of dynamic instability. Try a few numbers in it, and when in safe area, you will know where the CG should be. http://www.dlba-inc.com/dlba_interactive.htm .
     
  11. nine mile
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    nine mile Junior Member

    Getting back to the engine height, it is difficult to line up the anti-vent plate as the height changes as it is trimmed out. Seems like the plate should be paralllel to the water. Not knowing the angle the boat will be at confuses me. Thinking the same, that the boat will squat, I left the motor high so the plate is slightly higher than the bottom.
     
  12. PAR
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    PAR Yacht Designer/Builder

    There's just going to be some "playing" with it on the water, that you can't anticipate on the hard. Without a full set of hydros on the boat, you're just guessing, so splash her, make adjustments and hope for the best. You've removed some volume aft and moved the engine and probably other stuff, so unless you've kept a solid track of what has been removed, moved, the replacement weights and locations and other seemingly tedious stuff, you're just going to have to get her wet an find out.
     
  13. nine mile
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    nine mile Junior Member

    Tell me more about hydros if you will, thanks.
     

  14. TANSL
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    TANSL Senior Member

    Maybe I interpret things wrong but I think yes, you should study a bit about "hydros" because if you do not know the center of buoyancy, and some other things, will not help much to know the center of gravity.
     
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