Ship Stability Basics Complete Guide 2026

KG, KM, and GM

KG is the height of the ship’s center of gravity above the keel — it moves as cargo is loaded, ballast is shifted, or fuel is consumed, and calculating it accurately for the current loading condition is the officer’s starting point. KM is the height of the metacenter above the keel, a value that comes from the ship’s hydrostatic tables and depends mainly on the vessel’s draft and form, not on how it’s loaded. GM, the metacentric height, is simply KM minus KG, and it has to stay positive — a positive GM means the center of gravity sits below the metacenter, so when the ship heels, the buoyant force acts to right it back upright. SOLAS sets a minimum GM of around 0.15m for most cargo ships, and it’s checked against the ship’s actual loading condition, not just assumed from a generic stability booklet figure.

A ship with too little GM is described as “tender” — it rolls slowly and heavily, taking a long time to return upright, uncomfortable and potentially dangerous in a seaway. A ship with excessive GM is “stiff” — it snaps back too quickly, which sounds safer but actually causes high accelerations that stress the hull, cargo lashings, and crew, and is its own operational problem, particularly on container ships where excessive GM has been linked to lashing failures and cargo loss overboard.

GZ and the righting arm curve

Where GM describes stability at small angles near upright, GZ (the righting arm, or righting lever) describes the actual restoring force at any given angle of heel, from 0 all the way to the angle where the ship would capsize. Plotted across the full range, this becomes the GZ curve — a graph every stability booklet includes — and its shape tells officers far more than GM alone: how much righting energy is available at moderate heel angles, where the maximum righting arm occurs, and the angle of vanishing stability beyond which the ship would no longer right itself. Two ships can have identical GM values but very different GZ curves, which is why regulatory stability criteria (the IMO’s Intact Stability Code) set minimum requirements on both the GM and specific points along the GZ curve, not GM in isolation.

Free surface effect

A tank that’s completely full or completely empty doesn’t affect stability calculations beyond its weight and position. A tank that’s partially filled — “slack” — does, because the liquid inside is free to shift toward the low side as the vessel heels, effectively moving weight in the direction of the heel rather than staying fixed. This “free surface effect” reduces the ship’s effective GM, sometimes significantly on a large partially-filled tank, and stability calculations apply a free surface correction to account for it. This is the direct, practical reason ballast and fuel tanks are managed to be kept either pressed up or run dry whenever possible rather than left at intermediate levels, particularly when stability margins are already tight.

Why officers calculate this before every departure

Loading condition changes every voyage — different cargo quantities, different tank levels, different consumption of fuel and fresh water en route — so a GM and GZ curve calculated for the last voyage doesn’t apply to the next one. Before departure, the officer works out the current KG from the loading plan, checks it against the ship’s hydrostatic tables for the corresponding draft to get GM, applies free surface corrections for any slack tanks, and confirms the resulting figures meet the minimum criteria in the stability booklet. Getting this wrong isn’t a paperwork failure — inadequate stability has been a direct or contributing cause in a significant share of vessel capsizing incidents, which is why it’s checked before every single departure, not periodically.

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