When cargo ships travel across the sea they are subjected to six types of ship motions due to wave action. Both shipping containers being carried by the ship and the cargo these containers hold are subject to the ship’s motions, which affect the ship, its containers and cargo in different ways.
There are six general types of ship motions
The six types of wave action motion are constantly applied to the ship in varying degrees as it travels across the water. This complex interaction of motions is why sailors often find it difficult to walk on a deck.
Ship motions occur along the vertical, transverse or longitudinal axis, or around the center of gravity for the ship. They are made up of three lateral movements and three rotations. The six ship motions are:
- Heaving: vertical movement
- Swaying: transverse movement
- Surging: longitudinal movement
- Rolling: longitudinal rotation
- Pitching: transverse rotation
- Yawing: vertical rotation
Cargo ship movement types
A special case of ship motion: slamming
Slamming is used to describe the motion where he bottom of the ship hull crashes into wave crests, then abruptly becomes immersed in the sea. Slamming occurs when ships travel through the water and are forced to move up and down in large waves. The bow of the ship rises from the water as it goes over the crest of a wave, then drops to impact on the water below.
Movements are more significant than rotations
In general the rotational motions, such yawing, pitching and rolling, do not affect the ship as severely as the lateral movements, i.e. heaving, swaying and surging. How the ship is affected by the rotational and lateral forces (accelerations) applied by these ship motions depends upon several major factors, including:
- Shape, size and weight of the ship
- Position of the center of gravity within the ship
- Position of the center buoyancy within the ship
- Maximum width at the waterline (beam) of the ship
Rotational motions are centrifugal forces (accelerations) that are directed outward from the center of gravity and buoyancy. The stresses and pressures that result from these forces are at a maximum at the ship extremities (e.g. bow, stern and outer walls) and diminish to zero as they move to the center point of rotation. Lateral movements typically apply stress and pressure at a constant rate along the length of ship or the section of ship to which they are applied.
Sea wave action that causes rotational movement does not move the ship as much the same wave action that generates lateral movements. The lateral forces from heaving, swaying, surging and slamming can produce extremely high stresses and pressures to a ship relative to rotational forces – so much so that their effects have to be taken into careful consideration when designing ships.
Heaving motion on a cargo ship
Heaving is upward and downward force (acceleration) applied along the ship’s vertical axis. This motion always occurs when there is wave action. Only in absolutely calm water (no waves) is there is no heaving.
Wave action causes buoyancy to vary along the ship as it travels across the sea through wave crests and troughs. Where there is a wave trough, the buoyancy along the ship at that point lowers and the ship moves downward. Where there is a wave crest, the ship buoyancy rises and moves upwards.
When the ship has a section of the hull on one side of a wave crest and the following section of hull on the other side, the heave movement will cause the ship to bend. This can result in considerable torsion force on the ship. The constant rising, lowering and bending (oscillation) along the ship can have a significant effect on the shipping containers and their cargo.
Heaving motion on a cargo ship
Surging and swaying motions on a cargo ship
Surging and swaying occurs when wave action produces forces that accelerate and decelerate the ship forwards and backwards (longitude axis) or side-to-side (traverse axis). Depending upon the orientation of ship within the wave action, these lateral movements may occur in both the transverse and longitudinal axis. This combined movement can twist the ship’s hull and result in a significant torsion force being applied.
Surging motions on a cargo ship
Swaying motions on a cargo ship
Yawing motion on a cargo ship
Yawing is when wave action rotates the ship around its vertical axis. Yawing is not a significant cause of cargo damage. When wave action is present it is not possible to maintain the ship on an absolutely straight course. Depending upon amount of wave action and rudder deflection, a ship will continually swing back and forth from its projected course, however this swinging is a mild force in comparison to those generated by heaving, swaying, surging and slamming.
Yawing motion on a cargo ship
Rolling and pitching motions on a cargo ship
Pitching is ship motion where the bow is lifted and stern lowered, and vice-versa. The length of the ship will dictate the maximum pitching angles that will occur in a given wave action. Ships that are short can have a typical pitch of five to eight degrees; very long ships will have pitch angles that are typically less than five degrees. Pitching angles of these sizes do not produce a significant impact on shipping containers and the cargo they hold, however steep tilting caused by rolling can result in cargo slippage within containers if they are not packed properly.
Rolling motion on a cargo ship
Rolling and pitching generate upward and downward forces (acceleration) tangential to the direction of rotation. Their magnitudes are inversely proportional to the square of the period of the rolling or pitching. If the period of the rolling or pitching is halved, the force is quadrupled. Conversely, if the period of the rolling or pitching is doubled, the force is quartered. The oscillations from rolling and pitching may be intensified when one is superimposed on the other.
Example of effect of pitching on a cargo ship
Consider a cargo ship that is 300 metres long. A stowed shipping container in the bow or stern of the ship will be approximately 140 metres from centre of rotation. A pitching angle of three degrees makes the shipping container move up to 30 metres for every pitching oscillation. From the horizon the container will rise 7.5 metres, lower 15 metres then, finally, rise 7.5 metres. Pressures on cargo will rise and fall inside the container during the upward and downward motion. Although the distance travelled by the shipping containers at the bow or stern seems large, the rotational forces that result are relatively small compared to the lateral motions being experienced from the wave action. This is because the rotations occur over long periods of time.
Pitching motion on a cargo ship
Importance of proper shipping container packing
When packing shipping containers it cannot be assumed that there will be calm weather and smooth seas. Methods for securing cargo for land transport are not adequate for sea freight. The packing and securing of shipping containers need to be carried out with these factors in mind.
Each type of ship motion causes different strains and stresses on the shipping container and the cargo packed within it. If the cargo becomes loose and is able to move inside the container, then damage is likely to occur to goods and the shipping container. Extreme weather may result in the cargo bursting through shipping container walls or doors. This can then cause damage to the ship or other shipping containers. It can even result in harming or killing some of the crew.
To find out more, read up our article on Avoiding sea freight cargo damage from improper shipping container packing.
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