Transverse wind acts against the vessel’s above-water lateral profile, comprising topsides, accommodation superstructures, and deck container stacks. The resultant aerodynamic thrust acts through the center of lateral windage above the waterline, while hydrodynamic drag acts upon the underwater hull (governed by the vessel’s Block Coefficient Cb).

Fig. 1. The force couple formed by lateral wind pressure above water and hydrodynamic resistance below the waterline.

These opposing forces form a heeling couple with a vertical lever arm separating the center of windage from the center of lateral underwater resistance. The ship heels until its hydrostatic righting moment balances the wind moment. Under the IMO Weather Criterion (Code on Intact Stability, 2008), every merchant vessel must prove its ability to survive a sudden gust of wind occurring at the maximum roll amplitude toward the windward side.

When heavy project cargo or containers are handled using the ship’s own deck cranes or derricks, a hazardous shift occurs in the overall Center of Gravity of the ship-cargo system.

Fig. 2. The weight of suspended cargo acts through the crane jib head, elevating and shifting the ship’s Center of Gravity.

Governing Physical Law: The instant cargo is hoisted clear of the quay, its effective center of gravity transfers immediately to the crane jib head (boom tip), regardless of how close to the water or deck the load hangs. This creates a severe compound hazard:
• The vessel’s center of gravity shifts upward toward the boom head, decreasing metacentric height (GM);
• As the crane slews outboard, the center of gravity shifts athwartships, generating a large heeling moment toward the lifting side. Handling procedures should always adhere to guidelines set in Shipboard Crane Operator Certification.

When a vessel executes a rapid turn at sea speed, centrifugal acceleration induces an outward heeling couple (see rudder steering principles in Helmsman Watch & Steering Orders).

Fig. 3. Centrifugal force acting at the center of gravity heels the vessel outward away from the center of turning.

The horizontal centrifugal force acts through the vessel’s Center of Gravity (G), directed away from the turning center. Simultaneously, centripetal hydrodynamic water pressure acts against the submerged bilge on the inward side. This generates an upsetting moment that heels the ship away from the direction of the turn (e.g., turning hard to starboard causes an outward heel to port). The heeling lever is directly proportional to the square of the ship’s speed (V²) and inversely proportional to the turning circle radius (R).

When a vessel strands or contacts an underwater obstruction off-center, an upward reaction force (R) develops at the point of impact.

Fig. 4. Grounding reaction force R acts like a removed bottom weight, causing a virtual rise in the Center of Gravity.

The upward reaction force R reduces the net buoyant force supporting the displacement to (Δ − R). In hydrostatic terms, this reaction behaves like the removal of an equivalent weight from the keel line, causing a severe virtual rise in the Center of Gravity. As the tide falls, R increases, accelerating loss of stability and causing the vessel to capsize over the grounding pivot. Navigators should monitor shallow margins using our UKC Calculator Online.

During alongside operations, excessive tension on mooring lines caused by passing wash or heavy tidal streams can impart steady heeling angles.

A far more critical risk exists for escort tugs. When a towing line leads broad on the beam of a tug while moving ahead, the transverse towline pull creates an extreme heeling moment known as girting (tripping). The towing point is situated high above the water, while the hydrodynamic resistance of the tug’s hull opposes the motion below, capsizing the tug within seconds unless the towline is tripped immediately. Mooring crews should review standard setups in our Mooring Operations Simulator Online.

Transferring liquids between wing tanks or the physical shifting of solid cargo transverse to the ship’s centerline displaces the overall Center of Gravity from point G to G₁.

Fig. 5. Transverse weight shift creates a permanent static list, reducing available dynamic righting reserves.

A horizontal separation is created between G₁ and the initial Center of Buoyancy B. The vessel rolls until the newly submerged volume shifts the Center of Buoyancy to B₁, aligning it vertically below G₁. In severe seas, shifting of bulk dry cargoes (grain, iron ore fines) produces a permanent list, dangerously degrading dynamic righting energy on the listed side.

Partially filled (slack) ballast, fuel, or liquid cargo tanks represent one of the most common causes of stability degradation at sea.

Fig. 6. Fluid shift inside a slack tank moves the effective center of gravity to virtual point Gv.

As the ship rolls, fluid inside a slack tank surges to the low side (volume wedge A-T moves to A₁-T₁). The local center of mass of the fluid shifts from g to g₁. Consequently, the combined gravitational vector of the ship acts through a virtual Center of Gravity G_v, located significantly above the physical center G. The effective metacentric height shrinks to G_vM, reducing the righting arm to G_vZ_v and lowering resistance against capsizing. Tanker cargo distribution can be verified beforehand using Tanker Stowage Plan Software.