Dictionary of Ship Hydrodynamics
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Dictionary of Ship Hydrodynamics (Alphabetic) Version 2008 Page I INTRODUCTION rection of motion of a ship or body can be This Dictionary is intended for a broad changed or maintained by its control devices. readership including practising naval architects who wish to acquire and apply knowledge of (performance) hydrodynamics and also physicists and theo- is concerned essentially with performance in retical hydrodynamicists who wish to apply the context of power required to propel a ship at their particular knowledge to the solution of a given speed and various factors and matters ship problems. related thereto. The propelling device is gener- ally understood to be a screw propeller. Engineering, physical and nautical terms in common use have not been included when they (propulsor, propulsion) did not require special definition in the context is concerned with propeller performance and of ship hydrodynamics or when their meanings various factor related thereto together with pro- were self evident. The terms are sorted alpha- peller geometry. Except where stated, the en- betically and for each term the context of it’s tries refer generally to screw propellers. usage is given with the following signifiers: (seakeeping) (cavitation) this section covers, in general, the behaviour is defined as the process of formation of the va- and performance of a ship in a seaway includ- pour of liquid when it is subjected to reduced ing, in particular, ship motions and the sea pressure at constant ambient temperature. It is states which cause them. used in the engineering context of liquid flow around bodies generally and, in particular, (ship geometry) screw-propellers and hydrofoils. signifies ship and hull geometry generally. (general) Under this is listed a number of general terms The order of entry for each item is: title, frequently encountered in the field of naval ar- symbol, and usage, dimensions, followed by the chitecture and marine engineering. To ensure definition. In each section the titles re arranged that their general meanings are retained and that in alphabetical order. In this way, having found they are employed in the proper manner, their the item required, perusal of the section will in- definitions are given here. dicate other related items which may be of in- Also definitions or descriptions are given of a terest. For general reference, there is an overall number of liquid properties and physical con- alphabetical index of all titles and against each stants concerned of ship hydrodynamics. is given the section and page where the item is to be found. (hydrodynamics) is concerned with fundamental aspects of the The symbols given are in accordance with resistance of a ship, or body, to motion through those in the latest ITTC list which is comple- calm water without consideration on the effects mentary document. of the method of propulsion. In a number of instances, the list give alter- (manoeuvring) native symbols and these are generally included is used to define the quality which determines except where a definite preference is indicated. the ease with which the speed, attitude and di- Page 1 ITTC Dictionary, Version 2008 A Acceleration zone (cavitation) In the sequence of cavitation erosion, the zone Figure 7-1: Geometry of turning circle of the curve of weight loss versus time in which a rapid increase in weight loss occurs (the re- gion between the incubation zone and the de- celeration zone which see). Formerly called the Accumulation zone. Active rudder (propulsion, propulsor) See: Rudder, active Added mass (seakeeping) [M] The total hydrodynamic force, per unit accel- eration, exerted on a ship or other body in phase with and proportional to the acceleration. Advance angle (of propeller blade section) Added mass coefficient (seakeeping) (Aij) [-] (propulsion, propulsor) A non-dimensional coefficient expressing See: Angle, advance added mass (which see) in ith mode due to jth motion. Advance angle, effective (propulsion, propul- sor) Admiralty coefficient (performance) See: Angle, effective advance A quasi-dimensionless coefficient used for as- sessing or comparing the performa nce of ship. Advance coefficient (propulsion, propulsor) Admiralty coefficient /, where Δ is (J) [-] the displacement, V speed and P any corre- A parameter relating the speed of advance of sponding power. propeller, VA to the rate of rotation, n, given by JVnD= A , where D is the propeller diame- Advance (manoeuvring) ter. The advance coefficient may also be de- The distance by which the centre of gravity fined in term of ship speed, V, in which case it (CG) of a ship advances in the first quadrant of is given by: JVn= D. a turn. It is measured parallel to the approach V path, from the CG position at rudder execute to the CG position where the ship has changed Advance coefficient, Taylor’s (propulsion, heading by 90 degrees (See Figure 7-1). Maxi- propulsor) (δ) mum advance is the distance, measured parallel A parameter defined as: to the approach path from the CG position at ⁄ A 101.27⁄ rudder execute to the tangent to the path of the CG normal to the approach path. The first of these terms is that most commonly used. Page 2 ITTC Dictionary, Version 2008 where n is the rate of propeller rotation in revo- Advance, speed of (propulsion, propulsor, per- lution per minute, D is the propeller diameter in formance) feet, and VA is the speed of advance in knots. See: Speed of advance. Advance maximum (in stopping) (manoeu- Air content(cavitation) vring) The term used loosely to describe gas content The distance travelled by a ship, in the direction (which see) when gas content is composed of of the approach path, before coming to rest af- components of air in the liquid. ter having executed a crash-back manoeuvre from a steady, straight-line motion ahead; it is Air content ratio(cavitation) also called Headreach. (See Figure 7-2). See See: Gas content ratio. also: Transfer, maximum (in stopping). Air, still, resistance (performance) See: Resistance, wind. Figure 7-2: Crash stop manoeuvre Amidships (ship geometry) (sometimes con- tracted to midship) ( ) [-] Near the centre of ship length, specially, the section of the ship at mid length (See Figure 2-12) Amplitude (seakeeping) Extreme value of a sinusoidal quantity with re- spect to the mean value. Analysis pitch (propulsion, propulsor) See: Pitch, analysis. Angle, advance (of a propeller blade section) (propulsion, propulsor) (β) [-] The inflow angle to a propeller blade section determined by the rotative speed, ω r, the axial velocity of the fluid, VX, and the tangential ve- locity of the fluid Vθ, according to the equa tion: tan , / , r is the radius of the blade section, ω the angu- lar rate rotation and θ the angular position of Advance ratio (propulsion, propulsor) (λ) [-] the blade section. A non dimensional speed parameter relating the A simpler definition, also in use is: speed of advance, VA and the rotational tip speed, πnD, given by: tan A⁄ ⁄ ⁄ where R is the propeller radius and VA the ad- vance speed. where J is the advance coefficient, D is propel- The induced velocities are not included in the ler diameter and n its rate of rotation. determination of the advance angle (See Figure 4-3). Page 2 ITTC Dictionary, Version 2008 Angle of attack (propulsion, propulsor, ma- Angle, deadrise (ship geometry) (β) [rad] noeuvring)) (α) [-] See: Deadrise angle. The angle measured in the plane containing the lift vector and the inflow velocity vector, be- Angle of diverging waves (hydrodynamics) tween the velocity vector representing the rela- See: Wave, angle of diverging tive motion between a body and a fluid and a characteristic line or plane of the body such as Angle, downwash or sidewash (manoeuvring) the chord line of an airfoil or hydrofoil, positive See: Downwash or Sidewash angle. in the positive sense of rotation about the y- axis. (See: Axes, co-ordinate in General Sec- Angle of drift or sideslip (manoeuvring, tion). Synonymous with angle of incidence. seakeeping) See: Drift or sideslip, angle of Angle of attack, effective (propulsion, propul- Angle, effective advance (propulsion, propul- sor) (αE) [-] ∗ The angle of attack relative to the chord line in- sor) (β ) [-] cluding the induced velocities. See Figure 4-3. A propeller inflow angle defined by the equa- tion: Figure 4-3: Typical velocity diagram for a propeller tan A⁄0.7 blade section at radius r where VA is the speed of advance, n is the rate of rotation, and R is the propeller diameter. UA (r) UT (r) Angle of entrance (ship geometry) αE (r) Chord line extended See: waterline VX (r,θ) φ(r) Angle of heel or list(manoeuvring, seakeeping) αG(r,θ) β(r,θ) See: Heel or list, angle of. βI(r,θ) Angle of heel or roll, projected (manoeuvring) (ωr-Vθ ))(r, θ (or angle of attack in roll) (γ) [-] The angular displacement about the x0 axis of the principal plane of symmetry from the verti- cal, positive in the positive sense of rotation Angle of attack, geometric (propulsion, pro- about the x0 axis. (See: Axes, co-ordinate). pulsor) (αG) [-] The angle of attack relative to the chord line of Angle, hydrodynamic flow (propulsion, pro- a section neglecting the induced velocities. See pulsor) (βI) [-] Figure 4-3. The inflow angle to a propeller blade section including the axial and tangential induced ve- Angle of attack, ideal (propulsion, propulsor) locities g iven by the equation: (α ) [-] I , Angle of attack for thin airfoil or hydrofoil for A I tan which the streamlines are tangent to the mean , T line at the leading edge. This condition is usu- UA and UT are induced axial and tangential ve- ally referred to as a “shock free” entry or locities respectively (which see).