GM from Roll Period Calculator

Calm water, no mooring-line interference (slack lines, beam to the dock), crew weight rhythmically on one rail to build a few degrees of roll, then hands off and time 4–5 complete cycles (out AND back = one cycle) with a stopwatch; divide for the period. Repeat twice. The big error sources: lines snubbing the roll, shallow water under the keel, and counting half-cycles.

The distance from the center of gravity G up to the metacenter M — the virtual pivot the hull rolls about at small angles. Big GM = strong spring (the righting lever GZ ≈ GM·sinθ initially): the boat snaps upright. Small GM = weak spring: slow, lazy rolling. Negative GM = the spring pushes the WRONG way and the boat lolls to an angle. GM is initial stability only — large-angle survival lives in the full GZ curve.

Because the period IS the measurement: T ∝ B/√GM, so a lengthening roll period means GM is shrinking — weight has crept upward (gear on deck, ice aloft, a new flybridge, slack free-surface water sloshing in tanks). Fishing-vessel casualty reports repeat the story: the boat 'rolled easier' for weeks before the capsize. The stopwatch hears what comfort disguises.

No — excessive GM makes a brutal roller: the snap accelerations strain rigs, cargo lashings and crews (container ships deliberately manage GM down with ballast for this reason). Too little courts capsize and loll. Vessels have design GM ranges; the rolling-period test's job is detecting DRIFT from normal — your boat's healthy period, learned in calm times, is the baseline that makes the test diagnostic.