The apparent period of wave is the time interval between the passage of two successive crests relative to a shipborne observer. | |
It is sometimes called period of encounter. |
Occurs when rolling or pitching period is equal or nearly equal to the apparent period of wave. | |
Synchronism may be synchronized rolling or synchronized pitching. |
Tendency of the bow plating and to a lesser extent the stern plating to work in and out when the ship is pitching. | |
Fore and aft regions of the vessel are extra strengthen by thicker plating, panting beams and stringers, reduced frame spacing in designed to withstand panting stress. |
Change of true wind direction to an anti-clockwise direction. |
Change of true wind to a clockwise direction. |
Occurs when vessel running before the sea. | |
Sea comes from the stern. | |
The ship encounters various dangerous phenomena. |
Occurs when vessel running before the sea. | |
Sea comes from the quarter. | |
The ship encounters various dangerous phenomena. |
In a following or quartering sea, following dangerous phenomenon may occur:
Combination of various dangerous phenomenon
Breaking of rising wave over the stern in poop deck area. | |
Develops when bad weather is directly from stern. | |
Vessels with less freeboard may suffer from popping. | |
Occurs when a vessel falls into the trough of a wave and does not rise with it. | |
It may occur if the vessel falls as the wave is rising. | |
Causes following wave to break over the stern or poop deck areas. |
May cause considerable damage to stern area. | |
Damage to propeller and rudder due to severe buffeting. | |
Engine room can be flooded if the openings which face aft are not properly secured. |
Occurs when velocity of sea is equal to or greater than ship's speed. | |
Alter course and head sea. |
Occurs when a ship situated on a stiff forefront of high wave in a following or quartering sea. | |
Vessel and waves have equal velocities. | |
Vessel may be accelerated. | |
Vessel rides on advancing wave slope. | |
This phenomenon is called surf riding. |
Vessel slewed violently (broach-to). | |
Vessel heeled over and swamped. |
Critical speed for surf riding is considered (1.8ÖL)/cos(180°-α) knots. | |
Surf riding/broaching-to may occur when angle of encounter 135°<α<225°. | |
To avoid surf riding, speed/course or both to be taken outside the dangerous region. |
May occur when a ship is surf ridden in a following or quartering sea. | |
The vessel is slewed violently. | |
Ship heels suddenly and unexpectedly to a large angle. |
Positive stability disappears to the existing angle of heel. | |
Vessel may cause a vessel to capsize due to sudden change of heel and heading. |
Reduce speed below 1.8ÖL knots. | |
A marginal zone (1.4ÖL to 1.8ÖL) below critical speed may cause a large surging motion (broach to). Speed to be reduced below 1.4ÖL in the case. |
Large rolling motions may be excited when natural rolling period of a ship coincides with the encounter wave period. | |
It may happen in following and quartering sea. | |
It happens when natural roll period is longer due to marginal transverse stability. |
Occurs in a following or quartering sea. | |
Occurs when period of encounter is approximately equal to the natural rolling period of the ship. | |
Occurs particularly if initial metacentric height height is small and natural roll period is very long. | |
Unstable and large amplitude roll motion takes place. | |
May occur in head and bow seas. |
Unstable and large rolling motion takes place. |
Reduction of speed. |
May occur in a following or quartering sea. |
Various detrimental factors may affect ship's dynamic behavior. |
These factors are: |
Additional heeling moment due to deck water. |
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Water shipping and trapped on deck. |
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Cargo shift. |
The factors may be occur with other dangerous phenomenon. |
They may create extremely dangerous combination to capsize the ship. |
Occurs when ship's speed component in the wave direction is nearly equal to the wave group velocity. | |
It is equal to the half of phase velocity of the dominant wave component. | |
The ship is attacked successively by high waves. | |
Expectable maximum wave height can reach almost twice of observed wave height. | |
May be evident when average wave length is larger than 0.8L, significant wave height is larger than 0.04L. |
Reduction of intact stability. | |
Synchronous rolling. | |
Parametric rolling. | |
Combination of various dangerous phenomena. | |
Vessel may capsize. |
Reduce ship speed to go out of dangerous zone. | |
Combination of appropriate speed reduction with slight course change. |