Calculating hull weight from materials and design parameters

Discussion in 'Boat Design' started by JonathanCole, Jan 4, 2006.

  1. JonathanCole
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    JonathanCole imagineer

    Does anyone know of a way to calculate the weight of a boat by using the materials and their dimensions (thickness, etc.) along with design parameters such as L, B, Cp, etc.? Such a program could be used to compare a design's weight if made with different materials.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    As a start, it might be easier to just use the surface area, multiply by the nominal thickness and then use the density of the material to work out the weight.

    There are some formulae around for estimating the weight of larger ships (using, for example, CB, draft, and legal requirements on freeboard), and recently some work has been done on finding formulae for aluminium catamarans, but I don't think that there is much for small vessels.

    Leo.
     
  3. Vincent G
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    Vincent G Junior Member

    Dear Mr Lazauskas,

    your comment on formulae corresponding to aluminium catamarans is interresting. Would you be kind enough to supply any link to the corresponding reference ?

    Best regards

    Vincent
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Have a look at:

    http://www.cyberiad.net/library/pdf/llmsc.pdf

    The references are in the bibliography. I used a modified formula for the generalised trimarans I was investigating. The formulae are really only applicable to ships rather than boats and bathtubs.

    Cheers,
    Leo.
     
  5. Vincent G
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    Vincent G Junior Member

    I downloaded the link and I have found the corresponding references.
    I will also read your thesis!
    Thanks for the link and quick answer.

    Regards

    Vincent
     
  6. JonathanCole
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    JonathanCole imagineer

    It is pretty amazing that there would not be well developed means for calculating the weight of a boat when you consider how much a part of displacement the vessel's own weight is. Also when you consider how long people have been building boats and ships. I guess it proves that there is still work left to do. Since boat design is an iterative spiral, the need to pin down the weight (at least to reasonable approximations) would be useful. Sounds like a good capstone or thesis project for a mathematician or naval architect.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    This type of naval architectural research has been going on for a long time. There are a couple of fairly well known formulae, but they are mostly for large steel ships. There are a few papers in Trans. RINA if you are keen.

    Vessels come in so many different shapes and sizes that it is unreasonable to expect a simple formula to apply to all of them. It's the same with drag. You can't expect a single simple equation to apply to the wave drag of monohulls, cats, planing hulls and hovercraft.

    Then there is the problem of what wave (and other) loads the vessel will be subject to. And on and on it goes, complication upon complication. Better to be a mathematician and deal with nice, clean problems! ;)

    Cheers,
    Leo.
     
  8. JonathanCole
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    JonathanCole imagineer

    But Leo, how can you compare the complexity of drag which is totally dynamic, with weight which is totally static? If nothing else I would have thought it could be accomplished statistically.
     
  9. jehardiman
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    jehardiman Senior Member

    People,

    Calculating weights for any vessel is not hard, just tedious. One must only be complete and precise.

    There is a well-developed way, it is called a weight and moment form and uses complex mathematics called arithmetic.

    And another thing, while it is true that when at equilibrium that weight is equal to displacement * density, displacement is a function of hull geometry and attitude with respect to the free surface, it is not a function of weight. Indeed, in most cases weight is adjusted to bring about a given still water displacement, rather than the other way around.

    While there are many “rules of thumb” for estimating weights and centers for preliminary design, they all have errors and cannot be relied upon. In the second and third crank through you will need to get out the pencil and paper and start working them out. By the final turn through the spiral, 90% of the math is doing the weights.

    Tedious,

    Simple,

    And within the grasp of any 4th grader.
     
  10. Karsten
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    Karsten Senior Member

    Another problem are manufacturing and material variations. The weight of foam core is about +- 10% and the weight of a layer of fibreglass will depend on the operator if it's hand laminated. If you want to know the weight accurately the person who is going to laminate the hull should make a small panel exactly the way you intend to laminate the hull and measure its weight. If it's foam sandwich there is even a difference in weight between a flat panel and a curved panel. Curved panels need more glue to stick the foam to the fibreglass and if the core is cut to enable bending the cuts have to be filled with resin as well.
     
  11. Willallison
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    Willallison Senior Member

    Most of the programs being used these days to generate hull forms will have an input for the desnity, or mass per unit area, of the various surfaces. With this done, the mass and centroid numbers become little more than the press of a button.
    Now it depends to what extent you model your boat, but at the least you should have figures for the shell....
    Beyond that, as jehardiman says... simply a matter of tedious arithmetic (though excell at least takes some of the pain out of it)
     
  12. rxcomposite
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    rxcomposite Senior Member

    Yes its possible. As Willalison says, its tedious. It just have to be systematic. I do use a spreadsheet pattern or series of spreadsheets that is interrelated and calculates from basic parameters, to hull design, bill of materials and even cost analysis.

    I designed it due to necessity of repetitive work.

    I was wondering if some of the builders out there is using a similar system and would like to hear from them. I beleive there is always a better system. Mine takes about 3 workdays to complete.
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Required hull strength, in part, depends on the dynamic loads to which it will be subject. Sure, you can try a statistical method, but there are many components made of different materials, and no method will give you reliable weight estimates. As someone else here suggested, there is no accurate substitute to adding up all the component weights (and moments).

    Good luck!
    Leo.
     
  14. rxcomposite
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    rxcomposite Senior Member

    Jonathan,
    I will send you my design spiral spreadsheet. The spreadsheet will calculate the weight, cost, bill of materials, labor hours. We will assume a hull thickness first but the sheet will give you a fairly good cost approximation of your project. I will throw in production cost also so you would know how much it would cost to produce a series of boats. 90 to 95% accuracy.

    I will simplify the spreadsheet so that you can easily enter data. The final stage would be final hull design and structural analysis. This will pin down the cost but you dont need it yet.

    You will have to work on metric system though and have to know the cost of materials in your area.

    Good luck in your project.
     

  15. oceanmaster66
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    oceanmaster66 Junior Member

    Would it be possible to email me your spreadsheet also ? I'm working on a project right now and your spreadsheet sounds like it would be very helpful. Thank you.
     
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