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Seamanship

GM from rolling period

A quick check of the ship's stability at sea: the metacentric height (GM) estimated from the time she takes to roll, by the formula in the IMO Intact Stability Code.

Find
m
s

Full roll, port–starboard–port; time several and average

Or give draught and length below

m
m

Fill in the fields to see the result.

How it works

T = 2 C B / √GM, so GM = (2 C B / T)², where T is the full rolling period in seconds, B the beam and GM the metacentric height in metres. The Intact Stability Code (2008, Part B) gives C = 0.373 + 0.023 (B / d) − 0.043 (L / 100), with d the mean draught and L the waterline length; C is often about 0.35–0.45.

A long, slow roll means a small GM (tender ship); a short, quick roll a large GM (stiff ship). Time the roll in calm water, without forced rolling by waves, over several rolls. The result is only an estimate: free surfaces, deck cargo and ice change it, and the ship's approved stability booklet and loading computer govern.

For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.

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