Buoyancy and Floatation calcs

Discussion in 'Stability' started by Mr.Design, Jun 5, 2007.

  1. Mr.Design
    Joined: Apr 2007
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    Mr.Design Junior Member

    Hey guys,

    I need some help calculating the buoyancy and flotation for my fishing boat project. I don't know what formulas to use and how to go about doing the calcs.

    Any suggestions will be highly appreciated!!:)

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  2. PI Design
    Joined: Oct 2006
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    PI Design Senior Member

    Isn't that fairly fundamental to the design???

    Anyway, your boat will need to displace (as in literally move aside) a weight of water equal to the boats weight. So first you need to know how much your boats weighs.

    Assuming your boat is intended for use in sea water, divide this weight (in kg) by 1.025. The answer is the volume of the hull that will be underwater. So now you need to calculate what the volume of your hull is at a variety of different draughts. If you have not done your design on a computer there is no quick, easy and accurate way of doing this (unless your boat is a very very simple shape). The standard way is too difficult to describe here, but involves using Simpson's Rule on the curve of areas (if you have produced one), but any method you like will do. If you have some maths knowledge you can figure out ways of approximating the volume.
    Your boat will sit at the draught which gives the volume you calculated earlier.
    volume = length * beam*draught*block coefficient
    As a very crude first go, if you estimate the block coefficient as 0.5 then:
    draught = 2*volume/(length *beam)

    Hope that helps a little. Is this a college project?
  3. raw
    Joined: Jul 2006
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    raw Senior Member

    USL Section 10 Appendix N.,

    from the nmsc website. Use Google. Viola!
  4. extremejet
    Joined: Jun 2007
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    extremejet New Member

    If your talking about positive flotation, try this out:
    (sourced from www.mast.tas.gov.au)

    Positive Buoyancy

    Boats fitted or built with positive buoyancy will keep afloat when filled with water. Small boats with no buoyancy fitted or negative buoyancy can fill with water and sink very quickly. If a boat with any sort of buoyancy swamps with water, it should still stay afloat. This can give those on board time to reach life saving equipment, put on their life jackets, if they are not already wearing them, and attempt to bail the water out. A floating boat is an identifiable target for search and rescue efforts.

    MAST trials show that boats that have underfloor buoyancy were not likely to remain upright but would float either upside down or bow up depending on the weight of the outboard and the buoyancy fitted in the stern of the boat.

    Flotation material added at the sides, as far aft and as high as possible (up under the gunwales) will under normal conditions, make the boat float level. Even if the boat is swamped, it will stay upright and sit lower in the water, even to where the deck is level with the surface of the sea or surrounding water. If the occupants are tipped out they can get back into the upright boat more easily.

    Type of Flotation
    Isothane (green) and Isothex (orange) Polyurethane Forms are available from:
    - RMAX, Launceston
    - The Fibreglass Shop - Hobart

    Amount of Flotation
    A simple calculation is available for owners of trailer boats to check the required amount of flotation in your boat. This is a guide only.

    Aluminium, GRP and Steel vessels:

    1.2 x (M x K + F)
    1000 – D M = Mass of the hull and deck
    K = Alum 0.62, GRP 0.375, Steel 0.87
    Timber vessels F = Mass of machinery and fittings
    D = Density of buoyancy material (40kg/m3)
    1.2 x F
    1000 - D

    Example for an aluminium vessel When: M = 240 kg (hull and deck mass)
    K = Aluminium 0.62
    1.2 x (240 x O.62 + 80) F = 80 kg (machinery)
    1000 – 40 D = 40 kg/m3

    Required flotation = 0.286 m3
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  5. Guillermo
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    Guillermo Ingeniero Naval

  6. Trevlyns
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    Trevlyns Senior Citizen/Member

    Or, of course, look up Simpsons Rule on google
  7. Student101
    Joined: Oct 2011
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    Student101 New Member

    Firstly, i apologize if i should not be bumping old posts, but this particular post had so much pertinent information to my question;

    How do you find out how much a simply rectangular vessel sinks into sea water when it is 142 m long x 18.5 m wide weighing 22000 long tons?

    PI Design, your first reply gave me much to think about, but i tried implementing some of what you said and seem to be going round in circles... In doing the math it seems that my vessel is going to sink in about 10m?! that can't be right, can it? I've become so frustrated with what first seemed like a really interesting college project, (i'm designing a harbour for my Civils BEng) it just feels as though i'm getting nowhere. Any help would be massively appreciated!!

    Many thanks!

  8. JRD
    Joined: May 2010
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    JRD Senior Member

    Work out the volume of water weighing 22,000LT, keep in mind the density of sea water higher than fresh water. Vol=mass/density. Also weight must be total loaded weight - ship + payload
    The volume you have worked out is the displacement volume, which is: the volume of water dispaced by the boat (obviously) and further is the volume of the boat below the water line. To work out the sinkage for a simple flat bottomed rectangular vessel, divide the volume by the water plane area (142x18.5) and you will have the sinkage. I get 8.3m draft, if you want to make it float higher, you will need to make it longer or wider.... and not increase the weight.
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