Ship Metacentric Height (GM) & Stability Calculator
Calculate ship transverse metacentric height (GM, BM, KM), free surface correction, natural roll period, and compliance with the IMO 2008 Intact Stability Code.
Hull Dimensions & Hydrostatics
Vertical Center of Gravity & Free Surface
Metacentric Height & Intact Stability
| Displacement (Δ / Volume ∇): | -- t |
| Center of Buoyancy (KB): | -- m |
| Transverse Metacentric Radius (BM): | -- m |
| Transverse Metacenter Above Keel (KM): | -- m |
| Solid Metacentric Height (GM solid): | -- m |
| Natural Roll Period (Tφ): | -- sec |
| Initial Righting Arm GZ (at 10° heel): | -- m |
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Frequently Asked Questions
What is the minimum Metacentric Height (GM) required by IMO regulations?
Under the IMO 2008 Intact Stability Code (Resolution MSC.267(85), Part A, §2.2.4), the initial fluid metacentric height (GM0) corrected for free surface effects shall not be less than 0.15 meters (approx 0.49 ft) for standard cargo and passenger vessels under all loading conditions.
What is the difference between a "stiff" ship and a "tender" ship?
A stiff ship has a large GM (e.g. > 2.0 m) with great righting energy, which rights the ship rapidly, causing violent short-period rolling accelerations that damage cargo lashings and cause seasickness. A tender ship has a small GM (e.g. 0.15 - 0.40 m), resulting in a long, slow roll period that is gentle on passengers but lacks reserve righting arm against heavy seas and gusting beam winds.
How does the Free Surface Effect (FSC) degrade metacentric stability?
When a tank is partially filled (slack), liquid shifts to the low side as the ship heels. This lateral liquid movement shifts the overall center of gravity from G to G1, effectively creating a virtual upward rise in the vessel's center of gravity (delta KG = i / V, where i is the transverse moment of inertia of the liquid surface). This directly reduces effective GM: GM_fluid = GM_solid - FSC.