The distance by which the centre of gravity (CG) of a ship advances in the first quadrant of a turn. It is measured parallel to the ap-proach path, from the CG position at rudder execute to the CG position where the ship has changed heading by 90 degrees
Maximum advance is the distance, meas-ured parallel to the approach path from the CG position at 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.
The distance travelled by a ship, in the direc-tion of the approach path, before coming to rest after having executed a crash-back ma-noeuvre from a steady, straight-line motion ahead; it is also called Headreach.
(See also: transfer, maximum in stop-ping)
The angle to the longitudinal body axis from the projection into the principal plane of sym-metry of the velocity of the origin of the body axes relative to the fluid, positive in the posi-tive sense of rotation about the y-axis. (See: Axes, co-ordinate)
See: Control surface angle.
See: Drift or sideslip.
See: Heel or list, angle.
(or angle of attack in roll) (γ) [-] The angular displacement about the x0 axis of the principal plane of symmetry from the ver-tical, positive in the positive sense of rotation about the x0 axis. (See: Axes, co-ordinate in General Section).
See: Neutral angle.
See: Pitch angle.
See: Roll angle.
The angle of a rudder, offset from the centre-plane, when in its zero lift or neutral position, it does not lie parallel to that plane. The rud-der “toes in” when its forward portion points inward toward the centreplane. To avoid am-biguity the terms “trailing edge out” or “trail-ing edge in” are often used.
See: Trim, angle of.
The vertical angle between the underwater path of the centre of gravity of a submerged body or submarine in motion and horizontal plane through that centre. The path angle is a combination of the trim angle and the angle of attack.
See: Yaw angle
See: Speed, approach
See: Control surface area.
(AHL, formerly AL) [L2] The area of the profile of the underwater hull of a ship when projected normally upon the vertical, longitudinal centreline, including the area of skegs, deadwood, ect. Usually areas which lie abreast of one another, such as those of multiple skegs, are included once only. Lateral area can refer not only to the whole body, but also to forebody, afterbody, entrance, run, ect. Thus AHLF, AHLA, AHLE, AHLR, ect.
See: ratio, aspect
The centre of the lateral area of the immersed portion of a ship or body, taken generally in the plane of symmetry.
The point in the plane of symmetry through which the resultant force would act to produce an effect equal to that of the total lateral hy-drodynamic force on a vessel. Coefficient of lateral area (CAL, formerly CLA)[-] The ratio of the lateral area of the bare hull of a ship to the area of a rectangle having the ship length L and a constant depth equal to draft TX at the station of maximum area.
Control devices comprise all the various de-vices that are used to control a body or ship, such as control surfaces, thruster, jets, ect.
Control surfaces are the rudders, hydroplanes and other hinged or movable devices used for controlling the motion of a body or ship.
(δFB, δR ect)[-] The angular displacement of any control sur-face about its hinge or stock, such as that of a bow fin δFB, or rudder δR. Positive hen turning in the positive sense of rotation of the ship, re-gardless of the effect this angle may have on the ship. See also: Rudder angle.
( AFB, AFS, AR, ect) [L2] The plan form area of any active or movable control surface, such as that of bow fins AFB, stern fins AFB or rudder AR, measured on the reference plane (generally the plane of sym-metry). See also: Rudder area.
A ship manoeuvre in which, while going ahead at normal or some other speed, the pro-pulsion devices are reversed in the shortest possible time.
(CC )[-] The ratio of the cross force C on a ship or body to the force corresponding to the dy-namic pressure times a specified area. It is customary to expressed it as CC=C/qA.
The hydrodynamic forces and moments which enter into the equations of motion are usually classified into three categories: static, rotary, and acceleration. The static deriva-tives are due to the components of linear ve-locity of the body relative to the fluid. Rotary derivatives are derived from angular velocity of the body and acceleration derivatives are from either linear or angular acceleration of the body.
The diameter of the circular arc described by the centre of gravity of a ship when it has achieved a steady-turning state.
The distance travelled by the centre of grav-ity of the ship normal to its original approach path in turning through 180 degrees. Tactical diameter is equal to the transfer at 180 de-grees change of heading.
(αIND, for-merly ε)[-] The angle of downwash meas-ured in a plane through the nose-tail line of the hydrofoil and perpendicular to the hydro-foil axis.
(CD)[-] A relationship between the drag D of a ship or body and the dynamic pressure times a specified area. It is customary to express it as CD=D/qA. (See also: drag and drag coefficient in Resistance Section)
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 the like. See also: 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 axis.
See: stability dynamic
The upstream and downstream edges, respec-tively, of a hydrofoil, propeller blade, rudder or similar device.
A fixed or movable hydrofoil, attached to a ship, generally in a longitudinal direction, to improve the dynamic stability or the manoeuvrability, or to provide a lift force to wind-ward, as in the fin keel of a sailing yacht.
(X,Y, Z) [LMT-2]
The components of the total hydrodynamic force on a body or ship as resolved along its x-, y- and z-axes respectively. Related to the flow over the body, the components are the drag component, D or R, in the direction of the relative flow; the lift component, L, in the principal plane of symmetry normal to the relative flow; the cross force, C, on the body normal to lift and drag.
A force exerted on a body, a hydrofoil, or a ship, with or without an angle of attack, at right angles to both the direction of lift and the direction of drag. Note: This is to be carefully distinguished from the lateral force; (See Force, sway).
The component of the total hydrodynamic force exerted by liquid on a body, acting perpendicular to the plane of symmetry. Specifically, the force developed on a ship, acting normal to the plane of symmetry, when the ship is caused to move sidewise in a horizontal plane, as in drifting, skidding or crabbing. 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.
A steady inclination of a ship about a longitudinal axis; to be distinguished from rolling, which is an oscillatory motion.
The angle, measured about a longitudinal axis, between a static inclined position of a ship and its normal upright position.
A relationship between the lift force L developed by a ship or body and the dynamic pressure times a specified area. It is customary to express it as CL = L/qA.
Manoeuvrability is that quality which determinates the ease with which the speed, attitude and direction of motion of a body can be changed or maintained by its control devices.
The process of executing various voluntary evolutions with a ship, such as starting, stopping, backing, steering, turning, diving, ris-ing, circling, zigzagging, dodging and the like.
See: Seakeeping Section
See: Seakeeping Section
A moment applied to a ship to cause it to assume angular dynamic motion about a vertical axis through the centre of gravity.
(N) [L2MT-2] A hydrodynamic moment due to environ-mental conditions acting on a ship which will tend to produce yawing in the form of an an-gular dynamic motion about the vertical or z-axis through the centre of the ship.
The angle between any characteristic line or plane of a body or ship and any other intersecting line or plane taken as reference, when the forces, moments or other actions on or by the body or ship have a value of zero.
A state of motion of a body or liquid in which, following a disturbance of the equilibrium conditions, the body or liquid returns toward equilibrium and passes beyond it, because of kinetic energy stored up in the system as it passes through the equilibrium position
See also Zigzagging
The angle, measured about the transverse body axis, between the instantaneous position of the longitudinal axis of a ship when pitching (which see) and its position of rest. (Positive bow up)
The angular component of the oscillatory mo-tion 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 under some conditions. (See: porpoising)
The cyclic oscillation of a high-speed craft primarily in clam water in which heaving motion is combined with pitching motion. The motion is sustained by energy drawn from the thrust.
See: Stability, course
The outline of a ship when projected on the fore-aft vertical centreline plane; also the out- line of parts of the ship, such as the stem, stern, and rudder, when similarly projected. Note: This definition also covers the contour of any flat or curved surface which acts as a hydrofoil or as a control surface; examples are the profiles of diving planes on submarines, fitted generally in a horizontal plane, and the profile of the blades on a screw propeller.
The ratio between the span of a hydrofoil, measured at right angles to the liquid flow, to the chord c of the hydrofoil, in the direction of flow. When the chord varies in length across the span, the aspect ratio is the span b divided by the mean chord c obtained generally dividing the hydrofoil projected area AP into the square of the span b, i.e. b2/ AP
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).
A control surface, which by its action or movement, controls the steering or the turn ing of a ship in horizontal plane. Specifically,hinged or movable control-surface appendage in the form of a hydrofoil, placed either at the bow or at the stern of a ship, or at both ends, to apply a turning moment to the ship.
The angular displacement of the rudder about its stock relative to the neutral position and measured in a plane normal to the stock. Positive when turning in the positive sense of rotation of the ship, regardless of the effect this angle may have on the ship. See also: Control surface angle.
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.
(AR, ART) [L2]
The total lateral area of the rudder (including fixed and movable parts) measured in the reference plane (generally the plane of symmetry). See also: Control surface area.
(AX) [L2] The lateral area of the sole fixed part of the rudder. See also: Control surface area.
(ARmov) [L2] The lateral area of the sole movable part of the rudder. See also: Control surface area.
Right or starboard rudder signifies that the main portion of the rudder aft of the stock has moved to the right or to starboard of the centreline, to cause the ship to turn to the right or to starboard in forward motion. Similarly, left or port rudder signifies movement in the opposite direction.
A vertical or nearly vertical member of the ship’s structure upon which the steering rudder is hung or supported.
That portion of the rudder, concentric with the axis of rotation, which provides bearing support and also transmits the operating torque.
The ratio of the maximum thickness of any horizontal section of a rudder to the corresponding chord length.
The motion of a ship resulting from the propeller thrust, drag forces, hydrodynamic side forces on rudder and hull or centrifugal forces in a turn, may have a component at right an-gles to the vertical plane through the longitu-dinal axis of the ship. This is called the side-slip. See also: Drift.
See also: Drift or sideslip angle of
A projection from or a fixed appendage applied to the underwater hull of a ship, generally to increase the lateral area an give increased swing damping and dynamic stability to the hull. A skeg is usually of large lateral area compared to its transverse thickness, is usually fitted in a vertical plane, and is in the after part of the vessel.
Any device ancillary to a hydrofoil or control surface or stabiliser to disturb the flow, in order to diminish the lift.
A body is said to have course stability if, when slightly disturbed from steady motion on a straight path, it returns to its original path, without any corrective control being applied.
Course stability in the horizontal plane does not normally exist, but a submarine can have it in the vertical plane. This is also known as positional motion stability.
A body is said to be a directionally stable if, when slightly disturbed from steady motion on a straight path, it returns to it original di-rection, but not necessarily its original path, without any corrective control being applied.
Directional stability in the horizontal plane does not exist, but a submarine can have it in the vertical plane. Note: The term directional stability is also commonly used to describe the more general case of straight-line stability (which see).
A body is said to be dynamically stable on a straight course or on a turn constant curvature if, when slightly disturbed from a steady mo-tion, it resume that same motion, but not nec-essarily along its original path, without any corrective control being applied.
A body is said to have straight-line stability if it is dynamically stable on a straight course. That is, when slightly disturbed from steady motion on a straight course, it resumes steady motion on a straight course, but not necessarily in its original direction, without any corrective control being applied.
Note: Straight-line stability is a special case of dynamic stability; directional stability is a special case of straight-line stability; and course stability,
The directional or inherent stability of a body which is so restrained that its only freedom of motion is that of rotation about an axis perpendicular to the direction of relative liquid motion. The body tends to align itself with the direction of flow after being disturbed. Note: In some quarters, as in wind tunnel establishments, this is also known as “static stability”.
In its general sense, the guiding of vessel in a horizontal plane by a rudder on control device; specifically, keeping a vessel on, or as close as practicable to, a given or designated course, despite various disturbances. As distinguished from turning and manoeuvring, the term steer-ing means keeping a vessel travelling in a given direction in a straight line.
See: Stability, straight line.
A small auxiliary foil, movable or fixed, attached to a control surface such as a rudder or diving plane, generally at its after edge, to reduce the control force or moment by applying local differential pressure to the main control surface.
(QR, QFB, QFS, etc.) [L2MT-2]
The torque applied to the stock or actuating mechanism of a control surface by the hydrodynamic forces acting upon it. Also the torque applied to the control surface through the stock or actuating mechanism to change the position or attitude of that surface, e.g. rudder torque QR, bow fin torque QFB, stern fin torque QFS, etc.
The point at which the towing force is ap-plied on a ship which is towing or on a craft which is being towed.
As applied to a movable appendage or control surface, that condition in which the surface aligns itself with the surrounding flow, leading end foremost when all control force or moment is removed. An unbalanced rudder pivoted at its forward edge always trails when going ahead.
The lateral offset of the CG of a body or ship in the first quadrant of turn, measured laterally from the extended approach path to the CG position when the body or ship has changed course 90 degrees.
The steady state longitudinal angular position of a ship; to be distinguished from pitching, which is an oscillatory motion.
The angle, measured about a horizontal axis, between the position of the longitudinal axis of a ship at rest and the horizontal plane.
That phase of manoeuvring in which a body or ship while moving ahead or astern, changes course or direction. The beginning of a turn, starting with the initial deviation from the approach path, is known as the “entry” into the turn; the end of a turn terminating in a new straight course, is known as the “sor-tie”. Figure 7-1.
The angle, measured about the vertical body axis, between the instantaneous position of the longitudinal centreplane of a ship when yawing and its mean heading. (Positive bow to starboard).
A ship manoeuvre in which the course of a ship is deliberately changed at frequent intervals, as a deceptive or evasive manoeuvre, or as a trial manoeuvre, in accordance with a predetermined or specified plan, while the average course made good remains approximately the same as if the ship were not zigzagging.