Ship GM Tool: Metacentric Height & Free Surface Correction

GM • KM − KG • Free Surface Moment • Slack Tanks • Virtual Rise of G • Weight Changes • TCG • Stability Margin

Ship GM Tool: Metacentric Height & Free Surface Correction

Calculate uncorrected and free-surface-corrected GM, corrected KG, and margin to the vessel’s approved minimum GM. Simple mode gives a fast stability screen from displacement, KM, KG and free-surface effect. Advanced adds slack-tank FSM calculations, added/removed weights, transverse and vertical moments, virtual rise of G, estimated list angle, GM sensitivity and a visual stability dashboard.

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Need much more depth? Click Advanced above.Advanced adds slack-tank rows, approved or rectangular-screening FSM, added/removed weights, VCG/TCG moments, corrected displacement/KG, virtual KG effects, estimated small-angle list and sensitivity analysis.

Fast6 primary controls

Core answerCorrected GM

Vessel-specificApproved minimum entered by user

Fast GM & Free-Surface Screen

Use KM and KG from the vessel’s approved stability data for the same loading condition. The tool does not supply a universal minimum GM.

Displacementtonnes.

KMm above keel.

KGm above keel before free-surface correction.

Free-surface input

Total free-surface momentt·m. FSC = FSM ÷ displacement.

Approved minimum GMm from approved booklet/loading instrument for this condition.

Base Loading Condition

Base displacementt before weight adjustments below.

KM at resulting conditionm. Enter from approved hydrostatics/loading computer at the resulting displacement/draft.

Base KGm before listed weight adjustments and free surface.

Base TCGm. Positive starboard, negative port.

Approved minimum GMm for the specific loading condition.

Other virtual rise of Gm. Optional additional virtual KG correction not otherwise modeled.

Added / Removed Weight Register

Use this to model cargo, ballast, stores or other discrete weight changes. Removal is handled with negative weight moment internally.

Action	Weight t	VCG m	TCG m

Slack-Tank / Free-Surface Register

Prefer approved FSM values from the stability booklet/loading computer. Rectangular screening uses FSM = ρLB³/(12n²), where n is the number of equal effective longitudinal subdivisions. It is not suitable for irregular tanks unless specifically justified.

Tank type	Method	Fill %	Approved FSM t·m	Length m	Breadth m	ρ t/m³	Subdivisions n

Stability Readiness

KM / hydrostatic basis verifiedKM corresponds to the resulting displacement/draft. Base KG verifiedLightship/cargo/ballast/consumable moments are current. All slack tanks consideredFree surfaces that can exist simultaneously are reviewed. Approved FSM used where availableGeometry screening is not replacing vessel-specific data unnecessarily. Minimum GM is vessel-specificEntered from approved stability documentation or instrument. Added / removed weights verifiedVCG, TCG and sign of weight changes are correct. Damage / grain / special-code criteria reviewedAdditional criteria are handled separately where applicable. Approved loading instrument / booklet checkedThis screen is reconciled against the controlling stability calculation.

Stability limitation: positive GM alone does not prove compliance with intact or damage stability requirements. The vessel’s approved stability booklet/loading instrument controls and may require GZ-area, maximum-GZ-angle, weather criterion, damage, grain, lifting, icing or other checks. Free-surface treatment also depends on actual tank geometry and the approved stability method. This calculator is a planning/cross-check tool.

Current methodology context: the IMO 2008 Intact Stability Code requires initial GM and the righting-lever curve to be corrected for free-surface effects in loading conditions. It states that free-surface effects should generally be considered when tank filling is below 98%, subject to the Code’s detailed provisions and exceptions. The Code also requires stability information for the master and does not establish one universal GM value suitable for every ship and condition. This tool therefore uses vessel-specific KM/KG and a user-entered approved minimum GM, with direct approved FSM data preferred over generic tank geometry.

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