Hull Displacement and Block Coefficient
A boat's displacement -- what it weighs, since by Archimedes it floats on the weight of water it pushes aside. The immersed L x B x draft box is filled only partway by the real hull shape, that fraction being the block coefficient Cb (~0.35-0.45 fine/planing, 0.40-0.60 full/work hull). Displacement volume = LWL x BWL x draft x Cb; weight = volume x water density (64.0 lb/ft^3 seawater, 62.4 fresh); long tons = weight/2,240. A 30 ft waterline, 10 ft beam, 4 ft draft hull at Cb 0.5 in seawater displaces 600 ft^3 = 38,400 lb = 17.1 long tons; fresh water floats it a touch deeper. A first-order block estimate for sizing ground tackle, a trailer, or a lift; the lines drawing integrated by Simpson's rule (or the builder's displacement table), the loaded trim, and the naval architect's hydrostatics govern the real value.
Formula and source
displacement_ft3 = LWL x BWL x draft x Cb; displacement_lb = displacement_ft3 x water_density_pcf (64.0 seawater, 62.4 fresh); displacement_long_tons = displacement_lb / 2240.
Hull displacement by Archimedes' principle and the block coefficient (naval-architecture hydrostatics), by name; the lines drawing (Simpson's rule) and the naval architect's hydrostatics govern the exact value.
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