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Table of Contents

SEAKEEPING

In this section the term seakeeping covers, in general, the behaviour and performance of a ship in a seaway including, in particular, ship motions and the sea states which cause them.

Added mass

[M]
The total hydrodynamic force, per unit acceleration, exerted on a ship or other body in phase with and proportional to the acceleration.

Added mass coefficient

(Aij)[-]
A non-dimensional coefficient expressing added mass in ith mode due to jth motion.

Amplitude

Extreme value of a sinusoidal quantity with respect to the mean value.

Angle of drift or sideslip

Angle of heel or list

Angle, leeway

Angle, pitch

Angle, roll

Angle of trim

Angle of wave direction

Angle of wave encounter

Angle, yaw

Apparent

Referring to wave characteristics, a visible property of an irregular wave record as distinguished from a property of the components waves. Thus, an apparent wave height is a particular peak-to-trough distance.

Auto correlation

The correlation between a random function of time, or space, and the same function shifted in time, or space, by a specified “lag” <m>tau</m>. The normalised auto correlation function is the auto covariance divided by the variance.

Bilge keel

See: Ship geometry section under Keel

Broaching

An involuntary and dangerous change of heading produced by a severe following sea.

Celerity

Coherency

A measured of the linear dependency of two random functions of time, or space, analogous to a correlation coefficient.

Coupling

Influence of one mode of motion on another mode of motion, for instance, coupling between heave and pitch.

Covariance

Average of squares of the deviations from the mean value.

Cross-correlation

The correlation between two random functions of time, or pace, with one shifted in relation to the other by a “lag” <m>tau</m>.

Damping

A characteristic property of a dynamic system, which dissipates energy and reduces the motion.

Damping coefficient

Ratio of damping force or moment amplitude as a function of frequency.

Drift

That motion, or component of motion, caused by some action other than that of the main propulsion devices of a ship, such as wind, waves, current and like. See also: sideslip

Drift or sideslip, angle of

(β)[-]
The horizontal angle between the instantaneous direction of motion of the centre of gravity of a ship and its longitudinal axis. It is positive in the positive sense of rotation about the vertical body’s axis.

Emergence

()[L]
The relative vertical distance of a part (usually the bow) of an oscillating ship above the water surface; opposite to submergence.

Factor, magnification

The ratio of the output amplitude at a certain frequency to the static response.

Factor, tuning

(Λ)[-]
Ratio of excitation frequency to natural frequency or ratio of natural period of a motion to period of encounter. The tuning factor in heave, pitch and roll have the symbol

<m> wedge_z=omega_E/omega_Z wedge_theta=omega_E/omega_theta wedge_varphi=omega_E / omega_varphi</m>

or

<m> wedge_z=T_Z / T_E wedge_theta=T_theta / T_E wedge_varphi=T_varphi / T_E</m>

respectively

Flare

Force, damping

A force which tends to reduce the motion and, if assumed to be linear, is proportional to the velocity.

Force exciting

A fluctuating external force that causes motion of body, as for instance, a ship when encountering a train of waves.

Force, restoring

A force tending to return a body to its initial condition when displaced by an external force.

Freeboard

Frequency

(f)[T-1]
The number of cycles occurring per unit of time.

Frequency, circular

(ω)[T-1]
In any cyclic motion, or in any periodic motion which may be represented by a cyclic motion, the circular frequency is the angular velocity. If ω is in radiant per second, then
<m>omega=2 pi/T</m> and <m>f= omega/{2 pi }</m>
where T is the period and f is the frequency.

Frequency of wave

(fW)[T-1]
The number per unit time of successive crests of a train of waves at a fixed angle of encounter, μ; the reciprocal of the wave period TW.

Frequency of wave encounter

(fE)[T-1]
The number per unit time of successive crests of a train of waves meeting a fixed point of a ship, at a fixed angle of encounter, μ; the reciprocal of the period of encounter TE. In deep water:
<m>f_E=f_W + {2 pi}/g Vf{2}under{W}cos mu</m>
where <m>f_W</m> is wave frequency and V ship speed

Frequency of wave encounter, circular

E)[T-1]
<m>omega_E={2 pi}/T_E=2 pi f_E</m>

Frequency, natural, of heave, pitch or roll of a ship

<m>f_Z, f_theta, or f_varphi T^-1</m>
The frequency of the periodic heaving, pitching or rolling motion of a ship.

Frequency, natural circular, of heave, pitch or roll

<m>(omega_Z, omega_theta or omega_varphi [T^-1])</m>
Frequencyral, matural circular, of heave, pitch or roll has the following definitions respectively:
<m>{2 pi}/T_Z , {2 pi}/T_theta</m>, and <m>{2 pi}/T_varphi</m>, where <m>T_Z , T_theta</m> and <m>T_varphi</m> are the natural periods

Green water

Water shipped on the deck of a ship in heavy seas, as distinct from spray.

Group velocity

The average rate of advance of the energy of a finite train of gravity waves.

Gyradius

(radius of gyration)
(kX, kXX, kY, kYY, kZ, kZZ)[L]
The square root of the ratio of mass moment of inertia (referred to body axes) to the mass of a body. See General Section for body axes under axes, co-ordinate

Harmonic

Sinusoidal, in referring to a function or motion.

Heading

(ψ)[ ]
The instantaneous direction of the projection of the forward longitudinal axis of a ship in a horizontal plane, defined by degrees of the compass or degrees azimuth.

Heaving

The vertical oscillatory motion of a specified point in a vessel, usually the centre of gravity. Although the heaving of a ship is a motion which is confined to operation in waves, it is possible with a high-speed planing craft for such motion to occur in calm water under some conditions. (See Porpoising)

Heave to

To maintain control of a ship, especially in extremely heavy weather, with minimum possible speed through the water.

Heel or list

A steady inclination of a ship about a longitudinal axis; to be distinguished from rolling, which is an oscillatory motion.

Heel or list, angle of

(φ)[-]
The angle, measured about a longitudinal axis, between a static inclined position of a ship and its normal upright position.

Hydroelasticity

Analogous to aeroelasticity. The study of the interaction between the inertial, hydrodynamic and elastic forces in a structure subjected to hydrodynamic loading. Divided into dynamic hydroelasticity, where these three forces are coexistent, or static hydroelasticity where inertial forces are absent.

Impact

The sudden contact of body or ship, or any part thereof, with the surface of a liquid.

Leeway

The down wind or down sea motion of a ship. More specifically, the lateral distance the ship has been forced off the desired path.

Leeway angle

List

See: heel

Long crested seas

A wave system in which all components advance in the same direction.

Lurch

A more or less isolated large roll amplitude.

Mass, added

Mass, added, coefficient

Moment, damping

A moment which tends to reduce the motion and, if assumed to be linear, is proportional to the angular velocity.

Moment, destabilising

A moment associated with a displacement from a position of equilibrium and tending to increase this displacement.

Moment, exciting

A fluctuating external moment that causes motion of a body or ship when encountering a train of waves.

Moments of inertia or roll, pitch and yaw moment of inertia

(IX ,IXX, IY ,IYY, IZ ,IZZ)
[L2M]
The summation of products of elementary masses and the squares of their distances from the respective body axes through the centre of gravity – equal to the mass times the square of the gyradius or radius of gyration. See General Section for body axes under axes, co-ordinate.

Moment, pitching

Exciting moment in pitch.

Moment, restoring or righting

A moment tending to return a body to its initial condition after being displaced by an external moment.

Moment, rolling

Exciting moment in roll. Moment, stabilising Moment associated with a displacement from a position of equilibrium and tending to decrease this displacement.

Moment, wave bending, horizontal or lateral

(MB3 or ML, formerly MBH)
[L2MT-2]
That part of the inertial lateral bending moment acting on a cross section of a hull which is caused by the action of waves and ship motions.

Moment, wave bending, vertical

(MB2 or MN , formerly MBV)[L2MT-2]
That part of the internal vertical bending moment acting on a cross section of a hull which is caused by the action of waves and ship motions.

Moment, wave, torsional

(MT or MB)[L2MT-2]
That part of the internal torsional or twisting moment acting on a cross section of a hull which is caused by the action of waves and ship motions.

Motions, ship

The all inclusive term to describe the various dynamic motions which may be made by a ship including the following which are defined separately:

  • Rolling, Pitching and Yawing (angular)
  • Heaving, Surging and Swaying (translatory)

These motions may occur while the ship is stationary in the water or travelling through it.

Natural period of motions: heave, pitch, roll

(TZ, Tθ, Tϕ)[Τ]
The time for one complete cycle of the motion resulting when a body or ship is displaced in calm water from its equilibrium position by an external force, then released.

Oscillator

A mechanism used to impose a controlled, known, oscillatory motion on a body. Also used to describe any oscillatory body.

Period

(T)[T]
The time for one complete cycle of a periodic quantity or phenomenon. (See also: natural period of motions).

Phase angle

(<m>varepsilon_l</m>)[-]
The angle between two vector representing sinusoidal quantities of the same frequency.

Phase response operator

Phase angle between output and input of a linear system performing forced motion, as a function of frequency.

Pitch angle

(θ)[-]
The angle, measured about the transverse body axis, between the instantaneous position of the longitudinal axis of a ship when pitching and its position of rest. (Positive bow up).

Pitching

The angular component of the oscillatory motion of a hull about a transverse axis. Although pitching of a ship is a motion confined to operation in waves, it is possible with a high-speed planing craft for such motions to occur in calm water under some conditions. (See: Porpoising)

Porpoising

The cyclic oscillation of a high-speed craft primarily in calm water in which heaving motion is combined with pitching motion. The motion is sustained by energy drawn from the thrust.

Pounding

Described broadly as impacting between a water surface and the side or bottom of a hull. Pounding can perhaps be differentiated from slamming in that the impact, while heavy, is not in the nature of a shock. (See: Slamming)

Power in waves, mean increase in

(PAW)[L2MT-3]
The mean increase in power in wind and waves as compared with the water at the same mean speed.

Pressure, impact

A local pressure experienced by a hull when subjected to impact with the water. Usually associated with slapping, slamming or pounding.

Radius of gyration

See: gyradius

Resistance in waves, mean increase in

(RAW)[LMT-2]
The mean increase in resistance in wind and waves as compared with the still water resistance at the same speed

Resonance

The dynamical condition of a simple, uncoupled system where the excitation frequency is equal to the natural frequency. Note: In a coupled system, the dynamic condition where the excitation frequency corresponds to the frequency of maximum response to unit exciting force over a range of frequencies.

Response

The reaction of the system to an excitation.

Response amplitude operator

The square of the ratio of response amplitude to excitation amplitude of a forced harmonic motion applied to a linear system, as a function of frequency.

Response function

A complex function of which the modulus is equal to the response amplitude operator and the argument is equal to the phase response operator.

Revolutions, mean increase in rate of, in waves

(nAW)[Revs. T-1]
The mean absolute increase in rate of revolution (usually per minute), as compared with those in smooth water, necessary to maintain speed in wind and waves.

Roll angle

(<m>phi</m>)[-]
The angle measured about the longitudinal body axis, between the instantaneous position of a ship when rolling (which see) and its normal upright position. (Positive starboard down).

Rolling

The angular component of the oscillatory motion of a hull about a longitudinal axis.

Sea direction

  • Beam sea - A condition in which a ship and waves, or the predominant wave components, advance at right angles, or nearly so.
  • Bow sea - A condition in which a ship and the waves, or the predominant wave components, advance at oblique angles. This condition covers the direction between a head sea and beam sea.
  • Following sea - A condition in which ship and the waves, or predominant wave components, advance in the same, or nearly the same direction.
  • Head sea - A condition in which a ship and the waves, or the predominant components, advance in opposite, or nearly opposite directions.
  • Quartering sea - A condition in which a ship and the waves, or the predominant wave components, advance at oblique angles. This condition covers the directions between a beam sea and a following sea.

Seakeeping

In general, a term covering the study of the behaviour and performance of ship in a seaway. As an adjective, a term signifying a ship’s ability to maintain normal functions at sea.

Seakindliness

The quality of behaving comfortably in a seaway; that property of ship which produces easy motions in a seaway.

Force, wave shearing, horizontal or lateral

(FL)[MLT-2]
That part of the inertial lateral shearing force acting on a cross section of a hull that is caused by the action of waves and ship motions.

Force, wave shearing, normal or vertical

(FN)[MLT-2]
That part of the inertial vertical shearing force acting on a cross section of a hull that is caused by the action of waves and ship motions.

Short-crested sea

An irregular wave system in which the components advance in various directions.

Sideslip, angle of

Significant wave height

Sinkage

The steady state lowering of a ship’s position of flotation in the water; to be distinguished from heaving, which is an oscillatory motion.

Slamming

A phenomenon described broadly as severe impacting between a water surface and the side or bottom of a hull where the impact causes a shock-like blow. (See also: pounding and whipping).

Slapping

A phenomenon described broadly as light impact between the water and the hull. A classification for impacts less severe than those associated with pounding. (See also: pounding).

Smith effect

The difference between actual pressure at a point under a wave profile and the static pressure corresponding to the actual distance below the surface.

Spectral density, one dimensional

(S(ω))
A function of frequency whose integral over any interval represent the energy contribution of all the component waves of a random function in that interval; the Fourier transform of the auto-covarince function.
<m>S_varsigma(omega)d omega = sum{d omega}{}{1/2 varsigma {2}under{An}}</m>
<m>S_theta(omega)d omega = sum{d omega}{}{1/2 theta varsigma {2}under{An}}</m>
etc. The subscript n denotes a particular component amplitude.

Spectral density, two dimensional

(S(ω,μ))
A function of frequency and wave direction whose integral over any interval represents the energy contribution of all the component waves of a random function in that interval.

Spectrum

  • Amplitude - A function of frequency whose integral over any interval represents the squared amplitude of a wave at the central frequency having the same energy as all the component waves in that interval.
  • Co-spectrum - The real part of a cross-spectrum (which see).
  • Cross-spectrum - A complex function of frequency expressing the mutual properties of two random functions; the Fourier transform of the cross-covariance function, The real part, or co-spectrum, indicates the relationship between in-phase frequency components; the imaginary part, or quadrature spectrum, indicates the relation between 90° out-of-phase frequency components.
  • Quadrature spectrum - The imaginary part of a cross-spectrum.

Speed loss

The decrease in speed, as compared with that in smooth water, caused directly by wind and waves at a constant setting of the main propulsion plant. Usually speed loss is determined at constant power (turbine plant) or constant torque (diesel plant).

Speed reduction

The decrease in speed, as compared with that in smooth water, caused mainly by reducing the setting of the main propulsion plant in order to minimise the adverse effects ion the ship of wind and waves.

Springing

The continuous ship-hull vibration induced by the non-impulsive hydrodynamic forces acting on the ship hull. In particular, the vibratory response of the ship hull girder to short waves with frequencies of encounter close to the lower structural modes of vibration of the ship. See also: whipping.

Stabiliser

Equipment to reduce the rolling (or pitching) motions of a ship.

Standard deviation

The square root of the average of the squares of the deviations from the mean value; the square root of the variance.

Steepness ratio, wave

Stiffness

The property of a ship that causes a short rolling period.

Submergence

()[L]
The relative vertical distance of a part (usually the bow) of an oscillating ship below the water surface; opposite to emergence.

Surging

The longitudinal oscillatory motion of a specified point in a ship, usually the centre of gravity (or origin of body axes).

Swaying

The transverse oscillatory motion of a specified point in the ship, usually the centre of gravity.

Thrust in waves, mean increase in

(TAW)[MLT-2]
The mean increase in thrust, as compared with that in smooth water, necessary to maintain speed in wind and waves.

Torque in waves, mean increase in

(QAW) [ML2T-2]
The mean increase in torque as compared with that in smooth water, necessary to maintain speed in wind and waves.

Transfer function

Transient

Irregular or non-harmonic, such as the free vibration of a damped mechanical system.

Trim

The steady-state longitudinal angular position of a ship; to be distinguished from pitching, which is an oscillatory motion.

Trim, angle of

(θ)[-]
The angle, measured about a horizontal axis, between the position of the longitudinal axis of a ship at rest and the horizontal plane.

Virtual mass

The combined effect of the mass of the ship and added mass corresponding to the hydrodynamic forces in phase with and proportional to the acceleration. (See also: Added mass.)

Wave

A disturbance of the surface of a fluid that usually progresses across the surface as the result of circular or other local motions of the fluid components. A standing wave is special case of a wave that does not advance.

  • Amplitude (<m>varsigma_A</m>)[L] - The radius of orbital motion of a surface wave particle, equal to one half of the wave height .
  • Components - The infinity of infinitesimal waves of different frequencies and directions which are found by spectral analysis to compose an irregular sea, or the large of finite wave used to approximate such an irregular sea.
  • Direction, angle of (μ)[-] - The angle between the direction of a component wave and the <m>x_0</m> axis.
  • Encounter, angle of (<m> mu </m>)[-] - The angle between the longitudinal axis of the ship and the direction of the wave encounter.
  • Encounter, period (TE)[T] - The time between successive crests of a train of waves passing a fixed point in a ship, at a fixed angle of encounter <m>mu </m>; the reciprocal of the frequency of encounter fE (which see).
  • Frequency (<m>f</m>)[T-1] The reciprocal of wave period = 1/T, or circular frequency = 2<m>pi </m>/T.
  • Height (HW)[L] - The vertical distance from wave crest to wave trough, or twice the wave amplitude of a harmonic wave.
  • Height, apparent (H<sub<WV</sub>)[L ] - The vertical distance between a successive crest and trough, estimated by visual observation.
  • Height, significant (HW1/3)- The average apparent height of the 1/3 highest waves in an irregular pattern.
  • Instantaneous elevation (η)[L] - The instantaneous elevation of a point in a wave system above the level of the undisturbed surface.
  • Length(LW, λ)[L] The horizontal distance between adjacent wave crests in the direction of advance.
  • Length, apparent(LWV)[L] - The horizontal distance between adjacent wave crests of an irregular sea in the direction of advance.
  • Number(<m>k</m>)[L-1]
    <m>K={2 pi}/lambda </m> or <m>{2 pi}/L_W</m>
  • Period (TW)[T] - The time between the passage of two successive wave crests passed a fixed point.
  • Period, apparent (TWV)[T] - The time elapsing between the occurrence of two successive crests of an irregular sea, or between two successive upward crossing of zero in a record, estimated by visual observation.
  • Profile - The elevation of the surface particles of a wave plotted as a function of space in a fixed time.
  • Slope of surface - The surface slope of a wave profile perpendicular to the crest in space co-ordinate. Maximum wave slope of a regular harmonic or trochoidal wave is π/2 x steepness ratio.
  • Speed celerity (cW)[LT-1] - The phase velocity of a surface gravity wave in deep water.
    <m>C_W=sqr{gL_W/{2 pi}}</m>
  • Steepness ratio - The ratio of wave height to length.
  • Train - A continuous sequence of wave crests and hollows.
  • Trochoidal - A profile closely approximating that of a regular surface gravity wave in a fluid; it can be geometrically constructed by tracing the path of a point on the radius of a circle as the circle rolls along the underside of a horizontal line.

Wave encounter period

(TE)[T]
The time between successive crests of a train of waves passing a fixed point in a ship, at a fixed angle of encounter μ; the reciprocal of the frequency of encounter <m>f_E</m>.

Wetness

The quality of a part of the ship, usually the weatherdeck forward, with respect to its liability of being wet as a result of motions of ship and waves.

Whipping

The transient ship-hull vibration which is induced by impulsive excitation forces. For example, fore-bottom slamming, bow-flare slamming, shipping of water and stern slamming. (See also: Springing).

Yaw, angle

(<m>X</m>)[-]
The angle, measured about the vertical body axis, between the instantaneous position of the longitudinal centreplane of a ship when yawing (which see) and its mean heading. (Positive bow to starboard).

Yawing

The angular component of the oscillatory motion of a hull about a vertical axis.

structured_dictionary/seakeeping.txt · Last modified: 2014/01/10 10:45 by ubuwiki
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