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Containership operation: ships motion in a seaway

Ships are affected by movement in six degrees of freedom: rolling, pitching, heaving, swaying, surging and yawing. Of these, rolling, pitching and heaving generate the greatest forces in heavy weather. This may also indicate that the aft sections of larger container vessels are subject to abnormal dynamic load conditions generated by slamming. This may cause containers to “jump” out of their automatic locks. Container locks are type and batch-approved by the Classification Societies.

Major acceleration occurs fore and aft at high levels. The transverse acceleration increases by increased metacentric heights GM. Correct stowing of containers keeps the stability of the ship within appropriate limits; not too low but not too high either. In container ships with wide beams or in partly loaded ships, the GM may be large, perhaps even exceeding 4-5 m, which will lead to severe rolling in heavy seas and bad weather.
CMA CGM Lamartine at sea passage
Fig: CMA CGM Lamartine at sea passage


Rolling affects container corner posts, twistlocks, hatch covers and the deck by inducing compression and tension forces in these areas

The motion also creates transverse racking forces which, if excessive, may distort the walls and ends of container frames . Deck cargo racking forces are resisted primarily by lashing rods and turnbuckles .

A tipping moment may also occur which, in extreme conditions, could cause the stack to topple over .

Although the effects of rolling are resisted by the vessel’s securing arrangements, the system itself is designed to operate within specific parameters. Classification society limits typically allow for a maximum roll amplitude of between 22º and 30º.


The forces created by pitching are similar to those caused by rolling, but act on the sides of the container longitudinally rather than transversely. A longitudinal racking force is generally less than its transverse equivalent. However, towards the bow and the stern the compressive forces due to pitching can be high.


This force is induced by pitching, and varies according to the motion of the ship’s deck. Heaving increases the compression and tension forces acting on container corner posts and twistlocks.

Wind force

In adverse weather the outboard container stacks, and any others which are partially exposed, may be subjected to wind pressure.

The degree of force depends on the velocity and direction of the wind, and the profile of the stacks affected. The higher the stacks, the greater the surface area and, consequently, the amount of force generated. In extreme conditions the wind, acting on the surface area of a single 40 foot container, may produce a transverse force of approximately 3.6 tonnes. Given that the effect is cumulative, the transverse force induced by a five tier stack of containers could be as high as 18 tonnes. All forces caused by the wind are in addition to those produced by the motion of the ship.

Ship resistance

The motion of a ship through water requires energy to overcome resistance, i.e. the force working against movement. As the resistance of a full-scale ship cannot be measured directly the knowledge about the resistance of ships comes from model tests. The total resistance on calm water can be divided into three main components: frictional resistance, residual resistance and air resistance. The frictional resistance depends on the size of the wetted area. It represents often about 70-90% of the ship total resistance for low-speed ships (bulk carriers and tankers), and sometimes less than 40% for high-speed ships (containers and passenger ships).

Residual resistance comprises wave resistance that refers to the energy loss caused by waves created by the vessel and viscous pressure resistance. This residual resistance normally represents 10-25% of the total resistance for low-speed ships and up to 40-60% for high-speed ships. Air resistance normally represents about 2% of the total resistance, however, for loaded container ships in head wind, the air resistance can be as much as 10%.

During the operation of ship, the paint film on the hull breaks down. Erosion starts, and marine plants and barnacles, etc. grow on the surface of the hull. In addition, the propeller surface can become rough and foulded. The total resistance caused by fouling may increase by 25-50% throughout the lifetime of a ship. Resistance also increases because of sea, wind and current. The resistance when navigating in head-sea could perhaps increase by as much as 50-100% of the total ship resistance in calm weather.

Parametric roll resonance –

A ship in longitudinal seas experiences a completely different shape of the underwater volume as compared with the ship in calm water and in beam seas. The reduction of righting arm GZ at wave crest causes a larger heel under the action of wind and sea. The ship rights again, due to the increased righting arm GZ in the wave trough, when the wave passes the ship. The ship in a seaway behaves dynamically, i.e. she starts rolling, and passes the upright position when returning from the first large roll. If the time of the large roll to the opposite side coincides with a wave crest passing the vessel, then the ship ends up with another reduction of righting arm GZ, and consequently with larger roll. Roll amplification due to the “timing” of the restoring moment variation with the roll motion is called “parametric resonance”.

This resonance can cause the ship to roll to very large angles in a moderate sea, leading to cargo damage, loss of containers and, in extreme cases, capsizing of the ship.

Large containerships are prone to parametric rolling because the shape of the fore body and aft body are usually very different, leading to a variation of righting levers as wave crests and trough move alongside the ship. A small initiating force at the right time from the rudder, wind gusts and other influence can set the ship rolling to a large angle. Possible consequences on machinery operation of the ship heeling to very large angles include: loss of cooling water, loss of suction, exposure of lubricating oil sumps and, for resiliently-mounted engines, problems with connection of services – and hence shutdown of the main engine.

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