Small Boat Capacity Calculator

It's an empirical freeboard proxy: length × beam approximates the deck area and (loosely) the waterplane whose immersion absorbs load, and 15 ft² per person was calibrated so typical small hulls retain workable freeboard with that many seated adults aboard. It's deliberately crude — the actual capacity plate is computed by the builder from displacement, freeboard and flotation tests, and overrides the folk formula wherever they disagree.

Depends on jurisdiction: many US states make exceeding capacity or horsepower ratings a citable offense outright; federally it governs builders rather than operators, but overloading can constitute negligent operation anywhere — and insurers and admiralty courts treat the plate as the standard of care after a swamping. Practically: the plate is the design envelope; outside it you're the test pilot.

Freeboard is the budget and waves spend it: a loaded 14-footer with 12 inches of freeboard meets a 18-inch powerboat wake... arithmetic complete. Loading also raises the center of gravity (especially the standing-passenger crime) and slows the hull's response, while following seas board over a squatting stern. The seamanship rule: in anything but flat water, mentally derate the plate by a person or two — the formula's authors assumed you would.

A different formula family (based on length × transom-width factors, 33 CFR 183 Subpart D) targeting a different failure: over-powered small hulls become unstable at speed — chine-walking, hooking in turns, transom stress. The persons number protects against sinking; the HP number against flipping at 35 mph. Both ride on the same plate because both are envelope edges the builder certified.