1 GENERAL

SHIP GEOMETRY

RESISTANCE

PROPELLER

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This section is concerned with ship and hull geometry generally. Propeller geometry is given in the Propeller Section.

(sometimes contracted to midship)

Near the centre of ship length, specially, the section of the ship at mid length (see figure below):

See: waterline

See: waterline

(ß) [rad] See: Deadrise angle

An additional structure or fitting to the main underwater hull of a ship, which generally results in a discontinuity in the fair surface of the main hull. Examples of appendages are: rudders, bossing, struts, shafts, bilge, keels, stabilizing fins, etc. (See appropriate items) }

(*A*_{BL}) [L^{2}]

The area of the ram projected onto the centreplane forward of the fore perpendicular.

(*A*_{BT}) [L^{2}]

The cross sectional area (full section port and starboard) at the fore perpendicular. Where the water lines are rounded so as to terminate on the fore perpendicular A_{BT} is measured by continuing the area curve forward to the perpendicular, ignoring the final rounding.

(*A*_{X}) [L^{2}]

See: section

(*A*_{M}) [L^{2}]

See: section

(*A*_{P}) [L^{2}]

Horizontally projected planing bottom area (at rest), excluding area of external spray strips (see figures below):

(*A*_{V}) [L^{2}]

Area of the portion of ship above the waterline projected to the direction of relative wind.

See: aspect ratio (Manoeuvrability Section)

The intersection of the baseplane with the centre (see 10). **NOTE This definition deviates from the ISO Definition**.

** Baseline and Keel line **

(*B*) [L]

A dimension expressing breadth or width of a body or ship in a transverse horizontal direction.

delete all below ?

When not otherwise defined the beam is the breadth moulded of a ship, measured amidships at the design waterline. According to the position were the breadth is measured, it is named:

**Beam, extreme**: maximum beam wherever it occurs on the hull above or below water.**Beam, immersed**: maximum: maximum beam of underwater body**Beam, maximum section**(*B*_{X}): beam measured on the designed waterline at the maximum section area.**Beam, midlenght**(*B*_{M}): beam at the midsection of the designed waterline.**Beam of design water line**(*B*_{WL}): maximum moulded breadth at design water line

For a hard chine hull the beam refers to the breadth or width of the planing bottom. According to the position were the breadth is measured, it is named:

**Beam, mean over chines**(*B*_{PA}) [L] mean breadth over chines; defined as the ratio between planing bottom area and projected chine length

**Beam, transom**(*B*_{PT}) [L]: Breadth over chines at transom, excluding external spray strips

**Beam, maximum over chines**(*B*_{PX}) [L]: Maximum breadth over chines, excluding external spray strips

The submerged transversally curved portion of the ship between the side and bottom. This region is also called the turn of the bilge. The minimum radius of the bilge at the section of maximum area is called bilge radius.

See: Keel

(<m>C_{B}</m>, formely δ) [-]

The ratio of displacement volume ∇ to the volume of a rectangular block having length L, beam equal to the waterline beam <m>B_{X}</m> an draught <m>T_{X}</m>:

<m> C_{B} = ∇/{LB_{X}T_{X}} </m>

If it is referred to length, beam or draught other than those defined above, they should be clearly defined.

Any hull or form which may be immersed or floating in a fluid, if a ship, usually its underwater portion. Particular parts of the body of a ship are:

- Forebody: the part forward of the midsection
- Afterbody: the part aft of the midsection
- Parallel middle-body, length of, (
*L*_{P}): the midship portion having the same transverse section throughout. - Entrance, length of (
*L*_{E}): the portion extending from the maximum area section, or from the fore end of the parallel middle-body, to the forward extremity of the underwater body. - Run, length of (
*L*_{R}): that portion extending from the maximum area section, or from the after end of the parallel middle-body, to the after extremity of the underwater body. See figure below for illustrations of these items:

The transverse sections of the ship projected on to a vertical transverse plane. The sections are generally equally spaced.

The part of the underwater hull of a ship which is carried outward beyond the fair form to enclose the propeller shafts or other external items. Bossing are of two general forms:

- Short, intended only to house the aftermost hull bearing of a propeller shaft or to form a faring where the propeller shaft emerges from the hull
- Long, enclosing the entire propeller shaft, shaft bearings, and the supporting frame from the hull to the propeller.

A long bossing is called contra or deflection type when its end is shaped to direct the flow of water against the direction of rotation of propeller (See Figure below):

A plate at, or near, the tip of a hydrofoil, or of an element acting as a hydrofoil, to suppress or reduce the tip vortex.

Intersection of a plane parallel to the centre plane with the moulded form of the forebody of the ship, both above and below the waterline. Similar intersections in the afterbody are called buttocks.

A length dimension expressing beam or width. (See: beam)

<m>(B^{C})</m> [-]

The ratio of the maximum breadth to the cube root of the volume displacement of a ship:

<m> B^{C} = B_{X}/∇^{1/3} </m>

in a consistent system of units.

A protection erected on the weather deck, generally forward, normally V-shape in planform, to prevent water shipped over the bow from running aft.

An appreciable swelling of the ship form generally below the waterline, involving increase of section area; frequently at the forward end lying just above the keel (bulbous bow), sometimes with increase of length beyond the forward perpendicular (ram bulb), sometimes the after end near the keel or at the level of the propeller shaft (stern bulb). The ram bulb dimensions are characterised by the transverse cross section area at the fore perpendicular <m>(A_BT)</m>, and the ram area in the longitudinal plane <m>(A_BL)</m>, which is the area of ram ahead of the fore perpendicular projected on to the centerplane. In non dimensional form:

- Taylor sectional area coefficient for bulbous bow <m>(f_BT)</m>, [-]:

<m>f_BT= A_BT/A_X</m>

- Area coefficient for ram bow <m>(f_BL)</m>, [-]

<m>f_BL=A_BL/LT</m>

When the waterlines are rounded so as to terminate on the forward perpendicular, A_{BT} is measured by continuing the area curve forward to the perpendicular, ignoring the final rounding. In some instances, the stem contour recedes aft the fore perpendicular below the load waterline before projecting forward to define the outline of the ram or fore end of the bulb. In such instances this area should be calculated using as datum the aftermost vertical tangent to the contour instead of the fore perpendicular.

The intersection of a plane parallel to the centreplane with the moulded form of the ship, both below and above the waterplane. Specifically, all such intersections in the afterbody, as distinguished from similar intersections in the forebody, called bowlines.

Generally applied to decks, where it represents the curvature in an athwartship or transverse vertical plane; the height of the deck at the centreline above the height at side. (See figure below):

(*B*) [-]

The geometric centroid, B of the submerged volume of a body or ship through which the total buoyancy may be assumed to act. Its position, measured as the distance from midship or from the fore (<m>overline{FB}</m>) or after perpendicular (<m>overline{FB}</m>) is called the *Longitudinal centre of buoyancy* and from the base line or keel (<m>overline{KB}</m>) the *Vertical centre of buoyancy*. In non dimensional form these distances are often expressed as ratios of length of the ship <m>overline{FB}</m>/<m>L</m> or <m>overline{AB}</m>/<m>L</m>, and of the draught <m>overline{KB}</m>/<m>T</m> respectively.

(*F*) [-]

The geometric centroid of the area of waterplane of any waterline. Its position measured as the distance from midships or from the fore or after perpendicular, is called *Longitudinal centre of flotation*, and is generally expressed as a ratio of the waterline length.

(*G*)[-]

The centre through which all the weights constituting the ship and its contents may be assumed to act. The distance measured from midships, from the fore perpendicular (<m>overline{FG}</m>), or from the after perpendicular (<m>overline{AG}</m>), and from the baseline or keel (<m>overline{KG}</m>), are called *Longitudinal and Vertical centre of gravity* respectively. They are generally expressed as ratios of the ship length <m>overline{FG}</m>/<m>L</m> or <m>overline{AG}</m>/<m>L</m> and of the ship depth <m>overline{KG}</m>/<m>D</m> respectively.

A more or less sharp corner or knuckle in the hull form, continuous over a significant length of the ship, as in the junction of side and bottom in planing craft. The chine is known as “soft” when the corner is rounded, and “hard” otherwise.

The angle at the junction between the two parts of a section, on either side of a chine or the angle between the tangents to these two parts, measured in a transverse plane.

The actual (in a “hard” chine), or imaginary (in a “soft” chine), locus of the intersections of the two parts of the hull form at the chine.

(*c*) [L]

The length of the chord line which is the straight line connecting the extremities of the mean line of a hydrofoil section. It passes through, or nearly through, the fore and aft extremities of the section. Synonymous with nose-tail line.

The clearances as indicated between the sweep line of a propeller and the hull or aperture in which is placed. As shown, the fore and aft clearances are generally measured at 0.70 of the propeller radius above and below the shaft centreline.

See: block coefficient.

See: Prismatic coefficient.

The overhanging portion of stern of a ship which lies between the designed waterplane and deck and which project abaft the waterline termination. See also *Stern, Counter or Fantail* and Figure below.

A narrow sharp portion of the stem of a ship at the waterline, or an appendage added to the stem to reduce the spray.

(β) [rad]
Angle between a straight line approximating the bottom part of a body section and the intersection between basis plane and section plane.

(See Figure)

According to the position were the deadrise angle is measured, it is named:

See: Skeg

*(D)* [L]
The moulded depth of a ship, defined as the height above the baseplane of the lowest point of a deck where it joins the side of ship.

The trace on the outside of a body marking the intersection of a plane passing through it at an angle other than 90° to the baseplane. Specifically for a ship of normal form, the diagonal plane is generally parallel to the baseline.

(-) [-] The complement of the acute angle between the plane of symmetry of a craft or body and the axis of a hydrofoil attached to it projected on to a transverse plane.

*(T)* [L]
The vertical distance, from the water surface to the bottom, of the underwater body of a ship. Specifically, the draught moulded, at midships to the designed waterplane. When different, the draught at the transverse section having maximum area is indicated as <m>T_X</m>

See body

This term is used to define the condition in which the ship has its keel parallel to the water surface. For vessels in which the keel is not straight or normally parallel to the water surface its use is not recommended: “zero trim” or “level trim” are preferred.

A fixed or moveable hydrofoil, attached to a ship generally in a longitudinal direction, to improve the dynamic stability or manoeuvrability, or to provide a lift force to windward, as in the fin keel of a sailing yacht.

The slant upward and outward from the vertical of a transverse section of a hull above the design waterline. Flare is opposite of tumblehome; its slope measured with respect to the horizontal, generally in the entrance and generally less than 90°, is called Angle of flare.

(-) [L]

The vertical distance above the baseline of the intersection point of the prolongation of the flat of the bottom at the maximum section area with a vertical straight line at half-beam from the centreplane.

The intersection of the hull form with a vertical transverse plane, at the position of a transverse frame of the ship.

(*f*) [-]

The vertical distance between the surfaces of the undisturbed water, in which a ship is floating, and the edge of a reference deck (Freeboard deck) or other reference point. In certain governmental load line rules, a minimum freeboard is specified at midship.

(-) [L]

The distance around the perimeter of any transverse station, section, or frame, between two selected points. For wetted surface calculations, these two points are generally the waterplane intersections.

(-) [L]

The half breadth, at any section, of the portion of the bottom, in the vicinity of the keel that is perpendicular to the centerplane, i.e. parallel to the baseline.

The body of a ship, including the above water and the underwater portions. It is used to express either its form or its structure.

The condition of a ship or model in which the fair form and the surface are represented without appendages or additions of any kind; it is also called bare hull.

(*b*) [L]

The length of a hydrofoil from tip to tip, from root to tip if cantilevered, or from end support to end support, measured normal to the direction of relative liquid motion.

The term is used, alone or characterised with an appropriate adjective, to indicate:

- The intersection of the plane of symmetry with the moulded hull surface at the bottom which is called the “keel line”. It may be parallel to the designed waterline or may be raked or sloped in the fore and aft direction.

- The keel as the central longitudinal girder. This may be of the flat type (Flat keel) or a heavy bar extending beyond the fair form of the bottom (Bar keel – See figure below).
- Appendages to improve the directional stability or reduce rolling: Bilge keel, an appendage, generally in the form of one or more long narrow fins, fitted along the side of a ship at the turn of the bilge to reduce rolling (See figure below):

A deep, relatively thin, generally fixed plate or hydrofoil, attached to the underside of a ship (generally a sailing ship), to reduce the leeway and improve the directional stability. This fin keel can be on, or parallel to, the longitudinal centreplane.

See chine

(*L*) [L]

The principal longitudinal dimension of a ship or body; specifically for a ship it can be defined in a number of ways as follows:

- Length overall (
*L*_{OA}) [L] - Length overall submerged (
*L*_{OS}) [L] - Length between perpendiculars (
*L*_{PP}) [L] - Length on waterline (
*L*_{WL}) [L]

When not defined, the length between perpendiculars is generally assumed. See also Amidships for and Perpendiculars for AP and FP. For a planning hull the following definitions of length are used: (See figure below):

- Length, chine wetted under way of planing craft (
*L*_{C}) [L]: the length of the wetted part of the chine - Length, keel wetted under way of planing craft (
*L*_{K}) [L]: the length of the wetted part of the keel - Length, mean wetted, of planing craft (
*L*_{M}) [L]: the mean length of the portion of the bottom of a planing craft actually wetted when under way. - Length, projected chine (
*L*_{PR}): Length of chine projected in a plane containing the keel and normal to longitudinal centre plane (See figures below):

or length – displacement ratio (*M*^{C}) [-]

The ratio of the ship length to the cube root of the volume of displacement:

*M*^{c}=<m>L/∇</m>_{1/3}

in a consistent system of units.

A drawing, depicting the form of a ship to the moulded shape and dimensions, showing the stations (transverse section or frames) waterlines, bowlines, buttocks and profile. (This includes a Body Plan which see.)

A commercial name applied to a certain type of hull form with pronounced V sections at the fore end.

(*M*) and longitudinal (*M*_{L}) [-]

The intersection of the vertical through the centre of buoyancy of an inclined body or ship with the upright vertical when the angle of inclination approaches to zero as limit, for transverse or longitudinal inclinations respectively.

**Metacentre, transverse and longitudinal; height above the baseline,**(<m>overline{KM}</m>)**and**(<m>overline{KM_L}</m>)**respectively**[L]. The height, measured vertically, of the transverse or longitudinal metacentre above the baseplane of a ship in the upright position.**Metacentre height, transverse**(<m>overline{GM}</m>)**and longitudinal**(<m>overline{GM_L}</m>) [L]. The distance between the centre of gravity and the transverse or longitudinal metacentre, measured vertically in the equilibrium position. It is positive when*M*is above*G*when the ship is said to have metacentric stability; that is, on inclination to a small angle a restoring moment arises which acts to return the ship to the vertical.**Metacentric radius, transverse**(<m>overline{BM}</m>)**and longitudinal**(<m>overline{BM_L}</m>) [L]. The height, measured vertically, of the transverse or longitudinal metacentre above the centre of buoyancy of a ship in the upright position. Geometrically, <m>overline{BM}</m> is the radius of curvature of the locus of the centre of buoyancy related to transverse inclinations, and <m>overline{BM_L}</m> the radius of curvature of the locus of the centre of buoyancy related to longitudinal inclinations. They are given by:

Where:
*I*_{T} = transverse second moment of area (or moment of inertia ) of the waterplane [L^{4}] (see: Moment of area, second);
*I*_{L} = longitudinal second moment of area (or moment of inertia) of the waterplane [L^{4}] (see: Moment of area, second);
*∇* = volume of displacement [L^{3}]
(See figure below for illustration of the transverse parameters.):

(or moment of inertia)[L^{4}]

The summation of the products of the elements of an area or surface squares and the squares of their distances from a given axis, generally in the surface. Especially for a ship:

**Second moment of the waterplane area (or moment of inertia) longitudinal**(*I*_{L}) about the transverse axis through the centre of flotation.**Second moment of the waterplane area (or moment of inertia), transverse**(*I*_{T}) about the longitudinal axis through the centre of flotation, generally the intersection of the intersection of the waterplane and the centerplane.**Second moment of free-water surface (or moment of inertia)**generally within a ship, calculated about an axis passing through the centre of area of that surface, parallel to the expected heeling or rolling axis.

An adjective used to indicate the generally fair form and dimensions of the hull as determined by the lines to the inside of the shell plating. For wooden ship it is taken to the outside of the planking.

One of a series of distances, measured from reference planes (normally from the centerplane), used for defining the size and the shape of a body or ship.

Any portion of the abovewater hull of a ship which when projected downward on to the designed waterplane, lies outside that designed waterline; it may be at the bow or stern or anywhere along the side.

Straight lines perpendicular to the designed load waterline of a ship through a fixed point as stated by classification rules: specially:

**Aft or after perpendicular**(AP). Through a fixed point at the stern; generally the aft side of the stern post, or centerline of the rudder stock in ship without a stern post.**Fore or forward perpendicular**(FP). Through a fixed point at the bow; generally the intersection of the fore side of the stem with the load waterline.

The co-ordinate planes, formed by an orthogonal co-ordinate system of axes x, y, z fixed in the ship to define the hull shape (see Axes, co-ordinate in General Section):

- Baseplane or x-y plane. The horizontal plane, parallel to the designed waterline and generally through the lowest point of the midsection.
- Centerplane or x-z plane. The vertical longitudinal plane, which coincides with the plane of symmetry.
- Plane, midstation, or y-z plane. The vertical plane at midstation, perpendicular to the baseplane and the centreplane or plane of symmetry.

The contour of a ship, a hydrofoil, or appendage projected orthogonally on to a plane parallel to the baseplane.

(*C*_{P}, formerly <m>phi</m>)[-]

The ratio of the volume of displacement to the volume of the cylinder having the length L and cross section of the maximum section of the ship. This sometimes called the longitudinal prismatic coefficient and is given by:

*C*_{P}=<m>∇</m>/(*L/A*_{X})

The prismatic coefficient can also be referred to the different parts of ship, such as afterbody, forebody, entrance and run. In any case the assumed length, as well as the cross section area if different from the above, is to be clearly indicated.

(*C*_{VP}, formerly <m>phi</m>* _{V}*)[-]

The ratio of the volume of displacement to the volume of a vertical cylinder having as horizontal section the waterline and as height the draught at midships. It given by:

When different, the draught of the transverse section having maximum area is used (TX).

The outline of a ship when projected on to the centerplane; also the outline of parts of the ship, such as the stem, stern, and rudder, when similarity projected. For different shapes and types of stem and stern profile, see Stem and Stern. (See also Manoeuvrability Section)

The ratio of the length L to the maximum diameter D of a body of revolution, or to the maximum breadth in other bodies.

(*M*^{C})[-]
See: length coefficient of Froude

See: body

An opening in the surface of the underwater body of a ship, which may or may not be fitted with a projection extending beyond that surface, designed for catching and taking water into a ship.

The intersection of a plane with a body or ship which it passes through in any position or direction; specifically for a ship, any transverse section perpendicular to the designed waterplane such as:

- Area, maximum section (
*A*_{X}) [L^{2}] - Area, midship section, midlength section, midsection or midstation section (
*A*_{M}) [L^{2}]

Any shape of transverse section considered typical in the development of ship forms. Some of this are:

- Blister (See Figure a)), in which an excrescence is added, near the waterline, to a more or less standard type of section.
- Bulb (See Figure b)), in which there is a local swelling below the waterplane generally at bow or stern. (For details and variations see special entry Bulb)
- Peg – top or battered (See Figure c)), in which there is a marked slope of the ship side outward and upward, generally but not necessarily above the designed waterline.
- U-shaped (See Figure d)), rounded at the bottom and with sensibly straigth, nearly vertical sides.
- V-shaped (See Figure e)), relatively sharp at the bottom and with sensibly straight but flaring sides.

(*C*_{X}), (*C*_{M}, formerly <m>beta</m>)[-]
The maximum transverse section coefficient, *C*_{X}, is given by

*C*_{X}=*A*_{X} / *B*_{x}*T*_{x}

where *A*_{X} is the area of a maximum transverse section; *B*_{X} and *T*_{X} are the beam and draught at this section respectively.
The *midship section coefficient* *C*_{M} is given by

*C*_{M}=*A*_{M} / *B*_{M}*T*

where *A*_{M} is midship section area; *B*_{M} and *T*_{M} are the beam and draught at midship respectively.

A diagram of transverse section areas up to the designed waterline plotted on a base of length *L*, representing the distribution of underwater volume along the length of a ship; this diagram may be made dimensionless by plotting each ordinate as the ratio of area *A* of any section to the area *A*_{X} of the maximum section and by plotting the position of that section as a fraction of a ship length *L* along the base from selected reference points (generally forward and after perpendicular or midships). The intercept of the tangent to the sectional area curve at the bow on the midship ordinate expressed as a ratio of a midship ordinate is called the Taylor tangent to the area curve or midperpendicular intercept ratio or terminal value of Taylor “t”. If the sectional area at the end ordinate is not zero (e.g. when there is a bulbous bow) both intercept should be diminished by that area in evaluating t. The midperpendicular intercept ratio was originally related to the tangent at the forward perpendicular only, but it can also be referred to the after perpendicular; therefore, the terms *t*_{E} and *t*_{R} may be used to indicate respectively the midperpendicular intercept ratio for entrance and the midperpendicular intercept ratio for run respectively.

The projection on to the plane of symmetry of the intersection of deck with the side, or the intersection of a deck with the plane of symmetry of a ship when the deck has no camber. The amount of rise of a sheer line above its lowest point is called the *Sheer*, forward or aft.

The portion of a ship, at the junction of the middle body with the entrance or the run, where the waterlines approach or reach their maximum width.

The thin portion of the hull at the stern of a vessel immediately forward of or in the vicinity of the rudder. A skeg is usually of large lateral area compared to its transverse thickness, is provided for the support of a propeller shaft, for structural strength, for docking support, for protection when grounding or to increase the lateral area and give increased roll damping and course keeping ability to the hull or for other reasons. It is placed generally at the aft end, but not necessarily on the centreline.

A relatively narrow strip, of small cross-section, attached to the hull of a ship for the purpose of controlling or diverting spray.

(-)[L]

The transverse horizontal distance between the centreplanes or the other designed plane or line of the two hulls of a catamaran or other multi-hulled craft.

An imaginary transverse plane, passing through a ship, perpendicular to the baseline, to define the shape and the position of the various parts. Generally the length between perpendiculars is divided by intermediate stations into 10 or 20 equal intervals. Specifically:

- Maximum area station, the station at which the transverse section has the maximum area;
- Midstation, the station at midlength.

The extreme forward end of a ship from the keel line to the top of the hull. Different names are given to various types and shapes and profile, such as:

- Clipper, in which the stem profile forms a concave curve which projects forward above the designed waterline, which a relatively large overhang. (See Figure a))
- Icebreaker, in which the stem profile below the designed waterline slops angle of much less than 45° which the baseplane. (See Figure b))
- Raked, a straight profile inclined forward. (See Figure c))
- Ram, in which the underwater stem profile extends beyond the forward perpendicular.(See Figure d), Figure e) and also Bulb)
- Vertical (plumb) , a straight profile coinciding with, or almost coinciding with, the forward perpendicular. (See Figure f))

The abrupt discontinuity in the profile of the bottom of a planing craft, designed to diminish resistance, to lessen the suction effects and to improve control of the longitudinal attitude. (See figure below):

Angle projected upon the designed waterline, between the lower corner of a step or a planing craft and the centreline. (See figure below):

The extreme after end of a ship from the keel line to the top of the hull. Different names are given to various types and shapes of stern profile, such as:

- Counter or fantail, in which the deck extends abaft the rudder post forming an elongated extension with appreciable overhang. With this type of stern the deck line is generally broad and full, but the waterlines are generally fine. (See Figure a))
- Cruiser, in which the stern profile as a convex shape, as indicated in Figure b)
- Transom, in which the buttocks and the waterlines, above and below the designed waterline, terminate abruptly in a transverse flat or convex surface or transom. The transom may be vertical or slightly raking aft. (See Figure c))

A curved non-symmetrical form of stern, or skeg ending just a head of a screw propeller, designed to impart a rotation to the propeller inflow against the direction of rotation of a propeller.

A strong, rigid member forming the after end of the structure of some ships, and supporting the rudder.

A paddle-wheel mounted at the stern of a vessel which is called a stern-wheeler, as distinguished from a side wheeler.

The shaft or spindle upon which a rudder, diving plane, or equivalent control surface is mounted. The rudder or plane is generally, but not necessarily, turned by the stock.

A bracket supporting the outboard end of a propeller shaft in twin or multiple–screw vessels having propeller shaft fitted off the centreplane. This is sometimes referred to as an “A” bracket. It usually consists of a barrel fitted with a bearing for the shaft, connected to the shell by one or two streamlined arms (Strut arms)

The angle between the axis of any strut arm and the baseplane of a ship when projected on to a transverse plane.

The angle between the meanline of a strut arm section normal to its axis at any selected point along the arm and a line lying in the plane of that section parallel to the centreplane or baseplane.

The angle between the axes of the two arms of a V-shaped strut, when projected on to a transverse plane.

(*S*) [L^{2}]

The surface area of the underwater body of a ship. This generally includes the area of the appendages which give an appreciable contribution to the frictional drag, such as bilge keel, propeller bossing, and rudder. It is usually expressed in non dimensional form viz:

- Wetted surface coefficient (
*C*_{S}) [-] where:*S*= wetted surface area,*L*= ship length, and*∇*= volume of displacement or - Froude’s wetted surface coefficient (
*S*^{C}) [-]

*S*^{C} = *S* / *∇* ^{2/3}

An inclination of ship or its parts from the vertical or upright position, generally in a transverse or athwartship plane.

See: stern

The difference between the draught forward *T*_{F} and the draught aft *T*_{A} for a ship with a designed level keel:

Trim = *T*_{F} - *T*_{A}

In non dimensional form the trim is expressed as a fraction of the ship length, i.e. (*T*_{F} - *T*_{A})*/L* and is called the trim ratio. It is referred to as trim by the bow or head if the forward draught is the greater, level trim if both are the same and trim by the stern if the draught aft is the greater. If the ship has a designed initial trim (raked keel or drag) the trim is generally measured with respect to this initial longitudinal inclination.

The slant inward from the vertical of a transverse section of a hull above the designed waterline. It is the opposite of flare.
Turtleback or turtleback deck
A form of weather deck with large camber which is rounded over at the sides in order to shed the water rapidly in heavy weather; also called *turtle deck*.

This term is used to indicate:

- The intersection line of the free water surface with the moulded surface of a ship, either in still water or when it is surrounded by waves of its own making.
- The intersection line of any selected plane, parallel to the baseplane, with the moulded surface of a ship. (See figure below)

The angle of the waterline at the bow in the horizontal plane neglecting local shape at stern is the *Angle of entrance*. This is generally designated as the *Half angle of entrance* (*i*_{E}) [-] i.e. with respect to the centreplane - See Figure below.

The angle of the waterline at the stern in the horizontal plane neglecting local shape of stern frame is the *Angle of the run*. This is generally designated as *Half angle of run* (*i*_{R}) [-] i.e. with respect to the centreplane – See figure below.

Any selected plane through a ship from and a parallel to the baseplane, specifically:

- Designed Waterplane, corresponding to the designed waterline.
- Maximum waterplane, corresponding to the waterline of a ship at the draught at which the waterplane area is maximum.

(*C*_{WP}, formerly <m>alpha</m>) [-]

*C*_{WP} = *A*_{W} / (*L* *B*_{WL})

where:

*L* = *L*_{WL} = Length on the waterline
*B*_{WL} = maximum breadth of the waterline.

- Longitudinal
*C*_{IL}=12*I*_{L}*/BL*^{3} - Transverse
*C*_{IT}= 12*I*_{T}*/B*^{3}L

where:

- I
_{L}= longitudinal second moment of area (or moment of inertia) of the waterplane. - I
_{T}=transverse second moment of area (or moment of inertia) of the waterplane.