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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.

*Hope for the best, but plan for the worst.*