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Thread: How to Calculate Displacement, Tonnage

  1. #1
    Join Date
    Dec 2007
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    Default How to Calculate Displacement, Tonnage

    What is the difference in the measurements "displacement, net tonnage, and gross tonnage"

    Can someone tell me how to calculate, from a boat's lines drawings, the displacement of the design? How about the other measurements - net and gross tonnage?

    Thanks for any response - kenjio30

  2. #2
    Join Date
    Oct 2008
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    Walney, near Cumbria UK
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    Default Re: How to Calculate Displacement, Tonnage

    Quote Originally Posted by kenjio30 View Post
    What is the difference in the measurements "displacement, net tonnage, and gross tonnage"

    Can someone tell me how to calculate, from a boat's lines drawings, the displacement of the design? How about the other measurements - net and gross tonnage?

    Thanks for any response - kenjio30
    The boats displacement is her weight, you can calculate from the lines plan, it is simple math but very tedious to do. Here is a thread that discusses how. http://www.boatdesign.net/forums/boa...ment-1661.html

    Net and Gross tonnage are an approximate measure of intenal volume in units of 100 foot^3. They are used for calculating harbour dues charged on commercial shipping. Why do you need these tonnage measures? There are several obsoleted measures of tonnage for yachts, the most famous being Thames Tonnage, was this what you needed?
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
    Join Date
    Jun 2000
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    Bridgewater NS Canada
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    Default Re: How to Calculate Displacement, Tonnage

    Displacement is the mass of the volume of water that a hull displaces when floating. It is normally calculated from a linesplan by measuring the area of each section up to the designe waterline (using a planimeter in the old days, via computer aids these days) and, using the sectional area data, calculating the volume using a formula such as Simpson's 1st or 2nd Rule, then multiplying the volume units by the mass of a cubic unit of whatever water (salt, fresh, brackish, etc.) the vessel will float in.

    Gross and net tonnage are measurements of a vessel's interior volume - very broadly, gross being the total volume and net being the cargo-carrying volume - and is used for registration and the levying of fees. It has nothing to do with how much the boat actually weighs. There are varying formulas for calculating tonnage, changing from country to country, though the final results are pretty much the same. Basically, a tonnage surveyor such as myself will inspect the vessel & drawings, determine which areas are exempt from tonnage measurement, calculate the internal volume of the hull, and put this figure through a series of formulas to come up with the gross & net tonnage figures for that particular vessel.
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