Definitions

Definitions

Apparent period

bullet The apparent period of wave is the time interval between the passage of two successive crests relative to a shipborne observer.
bullet It is sometimes called period of encounter.

 

 

 

 

Synchronism

bullet Occurs when rolling or pitching period is equal or nearly equal to the apparent period of wave.
bullet Synchronism may be synchronized rolling or synchronized pitching.

 

 

 

 

Panting

bullet Tendency of the bow plating and to a lesser extent the stern plating to work in and out when the ship is pitching.
bullet 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.

 

 

 

Backing

bullet Change of true wind direction to an anti-clockwise direction.

 

Veering

bullet Change of true wind to a clockwise direction.

 

 

 

 

Following and quartering seas

Following seas

bullet Occurs when vessel running before the sea.
bullet Sea comes from the stern.
bullet The ship encounters various dangerous phenomena.

 

Quartering seas

bullet Occurs when vessel running before the sea.
bullet Sea comes from the quarter.
bullet The ship encounters various dangerous phenomena.

 

In a following or quartering sea, following dangerous phenomenon may occur:

Pooping

Surf riding

Broach to

Synchronous rolling

Parametric rolling

Combination of various dangerous phenomenon

Successive wave attack
 


 

         

Pooping

bullet Breaking of rising wave over the stern in poop deck area.
bullet Develops when bad weather is directly from stern.
bullet Vessels with less freeboard may suffer from popping.
bullet Occurs when a vessel falls into the trough of a wave and does not rise with it.
bullet It may occur if the vessel falls as the wave is rising.
bullet Causes following wave to break over the stern or poop deck areas.

Result:

bullet May cause considerable damage to stern area.
bullet Damage to propeller and rudder due to severe buffeting.
bullet Engine room can be flooded if the openings which face aft are not properly secured.

Corrective actions:

bullet Occurs when velocity of sea is equal to or greater than ship's speed.
bullet Alter course and head sea.

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Surf riding

bullet Occurs when a ship situated on a stiff forefront of high wave in a following or quartering sea.
bullet Vessel and waves have equal velocities.
bullet Vessel may be accelerated.
bullet Vessel rides on advancing wave slope.
bullet This phenomenon is called surf riding.

Result:

bullet Vessel slewed violently (broach-to).
bullet Vessel heeled over and swamped.

Action:

bullet Critical speed for surf riding is considered (1.8L)/cos(180-α) knots.
bullet Surf riding/broaching-to may occur when angle of encounter 135<α<225.
bullet To avoid surf riding, speed/course or both to be taken outside the dangerous region.

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Broach to

bullet May occur when a ship is surf ridden in a following or quartering sea.
bullet The vessel is slewed violently.
bullet Ship heels suddenly and unexpectedly to a large angle.

Result:

bullet Positive stability disappears to the existing angle of heel.
bullet Vessel may cause a vessel to capsize due to sudden change of heel and heading.

Action:

bullet Reduce speed below 1.8L knots.
bullet A marginal zone (1.4L to 1.8L) below critical speed may cause a large surging motion (broach to). Speed to be reduced below 1.4L in the case.

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Synchronous rolling

bullet Large rolling motions may be excited when natural rolling period of a ship coincides with the encounter wave period.
bullet It may happen in following and quartering sea.
bullet It happens when natural roll period is longer due to marginal transverse stability.

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Parametric rolling

bullet Occurs in a following or quartering sea.
bullet Occurs when period of encounter is approximately equal to the natural rolling period of the ship.
bullet Occurs particularly if initial metacentric height height is small and natural roll period is very long.
bullet Unstable and large amplitude roll motion takes place.
bullet May occur in head and bow seas.

Result:

bullet Unstable and large rolling motion takes place.

Action:

bullet Reduction of speed.

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Combination of various dangerous phenomenon

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May occur in a following or quartering sea.

 

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Various detrimental factors may affect ship's dynamic behavior.

 

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These factors are:

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Additional heeling moment due to deck water.

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Water shipping and trapped on deck.

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Cargo shift.

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The factors may be occur with other dangerous phenomenon.

 

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They may create extremely dangerous combination to capsize the ship.

 

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Successive wave attack

bullet Occurs when ship's speed component in the wave direction is nearly equal to the wave group velocity.
bullet It is equal to the half of phase velocity of the dominant wave component.
bullet The ship is attacked successively by high waves.
bullet Expectable maximum wave height can reach almost twice of observed wave height.
bullet May be evident when average wave length is larger than 0.8L, significant wave height is larger than 0.04L.

Result:

bullet Reduction of intact stability.
bullet Synchronous rolling.
bullet Parametric rolling.
bullet Combination of various dangerous phenomena.
bullet Vessel may capsize.

Action:

bullet Reduce ship speed to go out of dangerous zone.
bullet Combination of appropriate speed reduction with slight course change.

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Updated: June 24, 2007
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