Definitions

Apparent period

	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.

Synchronism

	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.

Panting

	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.

Backing

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

Veering

Change of true wind to a clockwise direction.

Following and quartering seas

Following seas

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

Quartering seas

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

Pooping

	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.

Result:

	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.

Corrective actions:

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

Surf riding

	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.

Result:

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

Action:

	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.

Broach to

	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.

Result:

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

Action:

	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.

Synchronous rolling

	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.

Parametric rolling

	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.

Result:

Unstable and large rolling motion takes place.

Action:

Reduction of speed.

Combination of various dangerous phenomenon

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.
	Water shipping and trapped on deck.
	Cargo shift.

The factors may be occur with other dangerous phenomenon.

They may create extremely dangerous combination to capsize the ship.

Successive wave attack

	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.

Result:

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

Action:

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

Updated: June 24, 2007