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Table of Contents

PERFORMANCE

This Section is concerned essentially with performance in the context of power required to propel a ship at a given speed and various factors and matters related thereto. The propelling device is generally understood to be a screw propeller.

Admiralty coefficient

A quasi-dimensionless coefficient used for assessing or comparing the performance of ship. Admiralty coefficient = <m>{Delta^{2/3} V^3}</m>/P,
where Δ is the displacement, V speed and P any corresponding power.

Advance, speed of

Air, still, resistance

Angle, rudder

Apparent slip ratio

Appendage scale effect factor

(β)[-]
A factor taking account of the effect of scale between model and ship on the resistance of appendages. It is defined by a factor β, where:
<m>R_APS/{1/2 {rho_S V{2}under{S} S_S}} = beta R_APM/{1/2 {rho_M V{2}under{M} S_M}}</m>
Where RAP is the appendage resistance (See: Resistance, appendages), ρ the fluid density, V the speed and S the wetted surface.

Approach run

Area, above-water projected

The area of the above-water hull, superstructure, deck erections, funnels, masts, and like, as projected onto either the vertical x-z or y-z plane of the ship. (See: General Section under Axes, co-ordinate).

Augment fraction, resistance

Brake power

Coefficient, Admiralty

Coefficient, quasi-propulsive

See: Efficiency, propulsive, and Efficiency, quasi-propulsive.

Coefficient, wind resistance

Correlation allowance, model-ship

(RA) [LMT-2]
This is the addition which has to be made to the resistance of the “smooth” ship, as predicted from the model results, to bring it into agreement with the actual ship performance determined from full scale trial or service result. The correlation allowance depends upon the method used to extrapolate the model results to the “smooth” ship, the ship length and type, the basic shell roughness of the newly-painted ship, fouling, weather conditions at the time the ship measurements were taken and scale effects on the factor making up the model and ship propulsive coefficients.

Correlation allowance coefficient

Correlation factor, ship-model, for propeller rate of evolution

(K2)[-]
The scale effect between the rate of propeller rotation of model nM and ship nS is defined by the factor K2, such that
<m>K_2 = eta_S/eta_M sqrt{lambda}</m> where λ is the scale factor.

Correlation factor, ship-model, for propul-sive or quasi-propulsive efficiency

(K1)[-]
The scale effect between the propulsive efficiencies of the model and ship is defined by the factor K1, such that
<m>K_2 = eta_DS/eta_DM </m>
where the efficiencies ηDS and ηDM for ship and model respectively are derived at corresponding speed and propeller loading.

Course made good

The mean direction which a ship moving. This is defined by degrees of the compass or degrees of azimuth in a horizontal plane.

Course measured

A straight measured course, which is used for speed trials of a ship. When such a course is one nautical mile in length it is often referred to as a measured mile.

Course steered

( )[-] The mean heading of a ship, defined by degrees of the compass or degrees of azimuth in a hori-zontal plane
Fig. 7-4

Course, original

(<m>psi_0</m>)[-]
The course at the beginning of a manoeuvring test, defined by degrees of the compass or degree of azimuth in a horizontal plane. (See Figure 7-1 and Figure 7-2).

Current, tidal

A current in the water caused by the tide and influenced by the coastline and contours of the seabed.

Current, wind

A surface or near-surface current in a body of water induced by wind.

Delivered power

Effective power

Effective wake fraction

See: Wake fraction, effective.

Efficiency, gearing

(<m>eta_G</m>)[-] The ratio of the power output to the power input of a set of reduction – or multiplying – gears between an engine and propulsion device:
<m>eta_G = P_S/P_B</m>
where PS and PB are the shaft and brake powers respectively (which see).

Efficiency, hull

<m>(eta_H)</m>[-]
The ratio between the useful work done on the ship and the work done by the propeller or other propulsion devices in a given time that is effective power PE and thrust power PT respective.
<m>eta_H = P_E/P_T = {R_T v}/TV_A = {1-t}/{1-w}</m> in Taylor notation or
<m>eta_H = (1+ W_F)(1-t)</m>
in Froude notation. Where RT is the total resistance, V the ship speed, T the propeller thrust and VA the speed of advance; t is the thrust deduction fraction; w and wF are the wake fractions according to Taylor and Froude respectively (which see).

which links for “which see” (Taylor, Froude) ?

Efficiency, mechanical

<m>(eta_M)</m>[-]
The ratio between the power output and the power input of any machinery installation.
<m>eta_M = P_S/P_1</m>
or
<m>eta_M = P_B/P_1</m>
where PS and PB are the shaft and brake powers respectively and PI is the indicated power (which see)

on pdf it links to “indicted power” but only “indicated power exsist”

Efficiency, propeller, behind hull

(<m>eta_B</m>)[-]
The ratio between the power PT, developed by the thrust of the propeller and the power PD absorbed by the propeller when operating behind a model or ship:
<m>eta_B = P_T/P_D = TV_A/{2pi Q_n} = eta_0 eta_R</m>
where T is the thrust, VA speed of advance, Q shaft torque and n rate of propeller rotation; η0 and ηR are the open water propeller and relative rotative efficiencies respectively.

Efficiency propeller, open water

(<m>eta_O</m>)[-]
The ratio between the power developed by the thrust of the propeller PT, and the power absorbed by the propeller PD when operating in open water with uniform inflow velocity VA:
<m>eta_0 = P_T/P_D = TV_A/{2pi Q_0 n}</m>
where T is the thrust, Q0 the torque in open water and n the rate of propeller rotation.

Efficiency, quasi-propulsive or quasi-propulsive coefficient

<m>eta_D</m>[-]
The ratio between the useful or effective power PE and the power delivered to the propeller or the propulsion device PD.
<m>eta_D = P_E/P_D=eta_0 eta_H eta_R</m>
where ηO, ηH and ηR are the open water propeller, hull and relative rotative efficiencies respectively

Efficiency, propulsive

<m>eta_P</m>[-]
The ratio between the useful or effective power PEand the brake power PB.
<m>eta_P = P_E/P_B=eta_0 eta_H eta_R eta_S eta_G</m>
where η0, ηH ηR ηS and ηG are the open water propeller, hull relative rotative shafting and gearing efficiencies respectively (which see).

Efficiency, relative rotative

<m>eta_R</m>[-]
The relative rotative efficiency is the ratio of the propeller efficiencies behind the hull and in open water, as already defined.
<m>eta_R = eta_B/eta_0</m>

Efficiency, shafting

<m>eta_S</m>[-]
The shafting efficiency is a measured of the power lost in shaft bearings and stern tube:
<m>eta_S = P_D/P_S</m>
where PD and PS are the delivered and shaft powers respectively (which see).

Factor, appendage scale effect

Factor, form

Factor, load

Factor, ship-model correlation

See: Correlation factor.

there are 2 “correlation factor”. Which one? now the link on the first one

Friction deduction force in self propulsion test

(FD)[LMT-2]
The towing force applied to a model to compensate for the increased specific frictional resistance of the model and to achieve the ship point of self-propulsion.

Form effect

The difference between the viscous resistance of a model or a ship and the two dimensional friction resistance of a flat plate of the same length and wetted area and at the same speed in a given fluid. The difference arises because of the augmented speed of flow around the ship form as compared with along a flat plate and the pressure resistance of viscous origin. See also: Form factor.

Form factor

(k)[-]
The ratio between the total viscous resistance coefficient of a model or a ship CV and the two dimensional frictional resistance coefficient of a flat place CF0 at the same free stream Reynolds number. It may be expressed in two ways, either:
<m>k={C_V-C_F0}/C_F0</m>
or
<m>k={C_V-C_F}/C_F</m>

Fraction overload

Fraction, resistance augment

Fraction, thrust deduction

See: Thrust deduction factor.

on document it links to “Thrust deduction faction” but this item does not exist, there' s only Thrust deduction factor, that's linked now

Fraction, wake

Fresh water, standard

Frictional wake

Gearing efficiency

Geosim

One of a series of models which differ in absolute size but are geometrically similar. It is a contraction of the expression “geometrically similar model” and was first used by Dr. E. V. Telfer.

Ground speed

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. See also Fig.24.

Hull efficiency

Hydraulically smooth surface

Indicated power

Load factor

(1+x)[-]
See: Power prediction factor.

Load fraction in power prediction

(x)[-]
<m>X= eta_D P_D/P_E -1</m>
where PD and PE are the delivered and effective powers respectively and ηD the quasi-propulsive efficiency (which see). See also: Power prediction factor.

Measured course

Mechanical efficiency

Mile, measured

Overload fraction

Potential wake

Power, brake

(PB)[L2MT-3]
The power measured at the engine coupling by means of mechanical, hydraulic or electrical brake.

Power, delivered

(PD)[L2MT-3]
The power delivered to the propeller:
<m>P_D={2pi etaQ}</m>

Power, effective

(PE)[L2MT-3]
The power required to tow a ship, usually without its propulsive device, at constant speed V in unlimited undisturbed water:
PE = RTV
The power may be for ship either with or without appendages. If the latter, it is usually known as the naked or bare hull, effective power.

Power, indicated

(PI)[L2MT-3]
The power developed in the cylinders of a reciprocating engine, either steam or diesel, as determined from the pressure measured by an indicator or similar device.

Power prediction factor

(1+x)[-]
A factor based on the correlation of ship and corresponding model data, which is introduced in estimating ship power to allow for the method of extrapolating model results to ship, scale effects on resistance and propulsion and the effects of hull roughness and weather conditions such that:
<m>P_D={P_E(1+x)}/eta_D</m>
where PD and PE are the delivered and effective powers respectively and ηD the quasi-propulsive efficiency (which see). The results of model propulsion experiments are analysed for a propeller loading equivalent to the power prediction factor. The factor (1+x) is sometimes known as the load factor and the factor x as the load fraction (which see).

Power, shaft

(PS)[L2MT-3]
The power delivered to the shafting system by the propelling machinery.

Power, thrust

(PT)[L2MT-3]
The power developed by the propeller thrust T, at the speed of advance VA:
PT=TVA

Power in waves, mean increase in

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

Propeller efficiency

Propulsive coefficient or efficiency

Quasi-propulsive coefficient or efficiency

Ratio, slip

Relative rotative efficiency

Relative wind

Resistance, appendages

(RAP)[LMT-2]
The increase in resistance relative to that of the naked, or bare hull resistance, caused by appendages such as bilge keels, rudders, bossings, struts, etc.

Resistance augment fraction

(a)[-]
The thrust T required to propel a model or ship at speed V is greater than the resistance RT of the hull when towed at the same speed. The increase (T-RT) is called the augment of resistance, and the resistance augment fraction is:
<m>a={T - T_T}/R_T</m>
<m>T=(1 + a)R_T</m>

Resistance coefficient, incremental, for model-ship correlation

(CA)[-]
The model-ship correlation allowance RA (which see) expressed in coefficient form:
<m>C_A=R_A/{1/2 rho V^2 S}</m>
where ρ is the water density, V speed and S wetted surface.

Resistance coefficient, wind

(CAA)[-]
The ratio between the air or wind resistance on a ship or body RAA, and the force corresponding to the dynamic pressure times a specified area. It is customy texsed it as :
<m>C_AA=R_AA/{1/2 rho V {2}under{R} S}</m>
Where A is the appropriate above water area of the ship, VR the relative wind velocity (which see) and ρ the air density.

Resistance, roughness

(RAR)[LMT-2]
The increase in resistance relative to the resistance of a hydraulically smooth hull due to the effect of roughness. The hull roughness may be of different types such as:

  • Structural roughness caused by method of shell construction, waviness of plating, scoops, valve openings etc.
  • Paint roughness depending on the type of paint as well as how it is applied.
  • Corrosion roughness due to breakdown of the paint film and corrosion of the shell plat-ing.
  • Fouling roughness caused by marine organisms depositing shell, grass etc.

Resistance, still air

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 mean speed.

Resistance, wind

(RAA)[LMT-2]
The fore and aft component of the resistance of above water form of a ship due to its motion relative to still air or wind. When there is no natural wind, this is called the still air resistance. See also: Resistance coefficient, wind.

Restricted water

Revolutions, rate of, mean in waves

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

Rough surface

Roughness allowance

Roughness, equivalent sand

(KS)[L]
Equivalent sand roughness is used as a convenient measure of the roughness of a surface and is determined by equating the frictional resistance of a surface of random roughness with that of a flat plate completely covered with sand grains of a sensibly uniform size as in Nikuradse’s experiments. It is the average diameter of the Nikuradse sand grains.

Roughness, height or magnitude

(k)[L]
A length dimension expressing the height of a roughness element on a surface exposed to liquid flow. It is often expressed as some form of average such as root mean square or mean apparent amplitude.

Roughness, resistance

Rudder angle

(δR)[-]
The angular displacement of a rudder about its stock relative to the neutral position and measured in a plane normal to the stock. See also: Manoeuvrability Section.

Rudder angle, ordered

(δRO)[-]
The ordered angle set on the steering control apparatus. This may differ from the rudder angle δR, depending on the lag and lost motion in the steering control and gear.

Run approach

The path taken by a ship when accelerating during the approach to a measured course to attain a steady speed corresponding to give engine setting.

Salt water, standard

Scale effect

The change in any force, moment or pressure coefficients, flow pattern, or the like, due to a change in absolute size between geometrically similar models, bodies or ships. These variations in performance due to differences in absolute size arise from the inability to satisfy simultaneously all the relevant laws of dynamical similarity (e.g. gravitational, viscous and surface tension).

Shaft power

Shafting efficiency

Shallow water

Slip ratio, apparent

(SA)[-]
This is similar to the real slip ratio (which see) except that the ship speed V is used instead of the speed of advance VA, that is:
<m>S_A={P_n-V}/P_n = 1 - V/P_n</m>

Slip ratio, real

(SR)[-]
This is defined by the ratio:
<m>S_R={P_n-V_A}/P_n = 1 - V_A/P_n</m>
where P is the nominal, geometrical pitch, or the effective pitch of the propeller (i.e. advance per revolution at zero thrust), VA is the speed of advance and n the rate of propeller rotation.

Smooth surface

Speed of advance of a propeller

(VA)[LT-1]
Speed of advance of a propeller in open water. When a propeller behind a ship or model is producing the same thrust at the same rate of rotation as in open water the corresponding speed VA determined from the open water propeller characteristic is termed the speed of advance of the propeller. This is usually less than the ship speed V. (See also: Wake fraction, effective). This is based on thrust identity. There is another corresponding speed based on torque identity.

Speed, corresponding

The speed of a ship VS related to that of a model VM , or vice-versa, according to Froude’s Law of comparison:
<m>V_S=V_M sqrt{lambda}</m>
where λ is the scale factor.

Speed, ground

The speed of a ship relative to the ground, that is the speed including the effects of tide and currents. When the ship is moving through still water the ground speed is the same as the true water speed.

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 on the ship of wind and waves.

Speed, true water

The speed of a ship relative to the surrounding water.

Still air resistance

Surface, rough

A surface marked by sensible or visible irregularities

Surface, smooth

A surface free from irregularities sensible to the touch or visible to the naked eye. A surface is called hydraulically smooth when there is no increase of resistance due to the surface irregularities.

Surface, wavy

A surface, which may be either smooth or rough, in which there are undulations of relatively large curvature.

Thrust deduction factor

(t)[-]
It is logical to view the effect of the propeller behind the hull as causing an increase in resistance- See: Resistance augment fraction. However, it is also common practice to look upon this increase in RT as a deduction from the thrust T available at the propeller, i.e. to assume that of the total thrust T only RT is available to overcome resistance. This “loss of thrust” (T - R,), expressed as a fraction of the thrust T, is called the thrust deduction fraction, t, where
<m>t={T-R_T}/T</m>
or
<m>R_T=(1-t)T</m>

Thrust power

Towing force, for model at ship-point of self propulsion

See: Force, model towing.

Force, model towing” item does not exist

Track

The path along which the centre of gravity of a ship is moving (See Fig. 24).

Trial, measured mile

A trial carried out on a measured mile course to determinate the performance characteristics of a ship, namely ship speed, corresponding rate of rotation of propeller shaft, power, and also thrust where practicable.

True wind direction or velocity

Wake

The wake is a term used to describe the motion imparted to the water by the passage of the ship’s hull. It is considered to be positive if its direction is the same as that of the ship.

Wake fraction

(w, wF)[-]
The difference between the ship speed V and the speed of advance VA is called the wake speed(V - VA). Froude expressed the wake speed at the position of the propeller as a fraction of the speed of advance, calling this ratio the wake fraction wF, such that
<m>w_F={V-V_A}/V_A</m> and VA=<m>V/{1+W_F}</m>
Taylor expressed the wake speed at the position of the propeller as a fraction of the ship speed, such that
<m>w={V-V_A}/V</m> and <m>V_A=V(1-w)</m>.

Wake fraction, torque

(wQ)[-]
A propeller will develop the same torque Q at the same revolutions per unit time, n, when working behind a hull advancing at speed V and in open water at a speed of advance VA. The torque wake fraction will then be
<m>w_Q={V-V_A}/V</m>
This depends on identity of torque.

Wake fraction, thrust

(wT)[-]
A propeller will develop the same thrust T at the same revolutions per unit time, n, when working behind a hull advancing at speed V and in open water at a speed of advance VA. The thrust wake fraction will then be
<m>w_T={V-V_A}/V</m>
This depends on identity of thrust.

Wake fraction, nominal

[-]
Wake fractions calculated from speed measured at the propeller position by Pitot tube, vane wheels, etc. in the absence of the propeller are called nominal wakes.

Wake, frictional

The component of the wake which results from the frictional action of the water when moving along the solid surface of a body or ship.

Wake, potential

The component of the wake due to the potential flow around a body or ship, with velocity and pressure relationship in accordance with Bernoulli’s Theorem.

Wake, wave or orbital

The component of the wake set up by the orbital motion in the waves created by a body or ship.

Water, restricted

A term describing a body of water in which the boundaries are close enough to the ship to affect its resistance, speed, attitude, manoeuvring, and other performance characteristics, as compared with the corresponding characteristics in an open, unlimited, body of water. Principally, “restricted” applies to the proximity of the water boundaries in a horizontal direction.

Water, shallow

A term describing a body of water in which the boundaries are closed enough to the ship in a vertical direction to affect its resistance, speed, attitude, manoeuvring, or other performance characteristics as compared with its corresponding characteristics in water of unlimited depth.

Water, standard fresh

Water having zero salinity and a temperature of 15°C (59°F) with:
density ρ = 999.00 kg/m3 (1.9384 lb s2/ft4.)
Kinematic viscosity ν = 1.13902 * 10-6 m2/s. (1.22603 10-5 ft2/s)*

Water, standard salt

Water having 3.5 per cent salinity and a temperature of 15°C (59°F) with:
density ρ = 1,02587 Kg/m3 (1.9905 lb s2/ft4)
Kinematic viscosity ν = 1.18831*10-6 m2/s. (1.27908*10-5ft2/s)* See also relevant items in General Section under Liquid Properties and Physical Constants

Wavy surface

Wind, angle apparent

(βAW)[-]
The direction of the relative wind with respect to a ship’s heading. The resultant direction of the wind induced by the ship’s motion and the true wind, if any.

Wind, angle true

(βTW)[-]
The direction of the wind , if any, with respect to a ship’s heading.

Wind direction

(θW)[-]
The direction of any natural or atmospheric wind blowing over the ground or over the surface of the sea, measured from the true North.

Wind resistance

Wind velocity, relative

(VWR)[LT-1]
The velocity of the wind relative to the ship. It is the resultant of the wind induced by the ship’s motion and the true wind, if any.

Wind velocity, true

(VWT)[LT-1]
The velocity of a natural wind relative to the ground.

Yaw, angle

(χ)[-]
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).

structured_dictionary/performance.txt · Last modified: 2014/01/10 10:00 by ubuwiki
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