## 2D, Two Dimensional

This is a term which refers to the general case of flat geometry, meaning that the geometry in question will lie on a plane. Strictly 2D geometry means that only two of the three coordinates [x,y,z] are saved and used. 2D, however, can also be used to refer to a portion of three-dimensional geometry which lies on a plane.

## 3D, Three Dimensional

This is a term which refers to geometry that is described using all three coordinate values [x,y,z]. All geometric models of objects use 3D geometry to define their shapes.

## amidships

This is a somewhat arbitrary location defining the mid-point of the boat. It usually is defined as one-half the distance between the forward end of the waterline to the aft end of the waterline. Since the forward end of the waterline is usually placed at X=0 in the coordinate system, then the amidships position is located at X = LWL/2. This amidships point is important since it is the location where the draft is measured.

## AP, Aft Perpendicular

This location is usually at the aft end of the design waterline (DWL) or located at the rudderpost for large ships.

## attribute

Attribute is a term used to describe the identifying features of an entity (point, curve, or surface). Attributes of an entity include things like color, visibility, and point type.

## baseline (BL)

The baseline is the location where Z=0 in the coordinate system. Normally, this is where the bottom of the boat or the keel is located. During the design process, however, the position of the keel may change, but the baseline is always at the Z=0 position.

## beam

This is the maximum width of the boat at any location along its length.

## body plan

In naval architecture, this term is used to describe a specific format for the section view of the boat. This section view involves drawing the forward stations of a boat on the right side of a vertical centerline, and the aft stations flipped over and drawn on the left side of the centerline. The amidships location is usually used to determine when to begin flipping the sections over to the left side. This is done so that the forward sections will not cover up the aft sections.

## bonded edge

This defines a common edge joining two NURB surfaces. Both surfaces must meet at that edge with exactly the same number of defining points and the connection must extend the full length of each surface edge. This is different from a merged edge in that a single relationship entry is created for this edge. Certain commands, like Add Row/Col will continue across a bonded edge, but not a merged edge.

## breakpoint

This is another term for a knuckle or knuckle point, which is a hard or non-smooth change point along a curve. On a chine-type hull cross-section, for example, the chine point may be called a knuckle point or a breakpoint.

## B-spline

This is class of piecewise polynomials that is used for curve and surface definition. The B stands for basis. B-splines are mathematically defined using vertex control points, which do not lie on the curve or surface.

## buttock

This is the curve created by intersecting a plane at a specific half-breadth (Y-value) with the hull geometry. The curve can be seen in the profile view of the boat.

## BWL, Beam WaterLine

This is the maximum width of the boat measured across a waterplane cut of the boat. Depending on the context, this value may or may not refer to the beam at the design waterline.

## centerline (CL)

The centerline is the Y=0 location on the coordinate system. It defines the global symmetry plane for the boat.

## centroid

The centroid (sometimes called a center of gravity) of a bounded surface is the center point or middle point of the area. Traditionally, this point is found by cutting the area out of cardboard and balancing it on a pin. The balance point location is the centroid.

## chine, chine line

This is a hard (not smooth) division line between two surfaces located below the sheer line. For example, most standard planing power boats contain at least one chine for spray separation.

## click

This term refers to the process of pressing and releasing a mouse button. This is usually the left mouse button, unless otherwise specified. A click is different than a pick, because with a click operation, you are not necessarily selecting an object or entity. The Add Point command, for example, requires to to position the cursor where you want the point and click the left mouse button to set the point.

## column, surface column

In discussing B-splines or NURBS, a column is one of the iso-parametric curves which define these surfaces. "Rows" lie in a direction perpendicular to columns. These surfaces are made up of a rectangular-like mesh of rows and columns.

## control point

This term is often used interchangeably with the term vertex point, because it is the point officially used by a B-spline to define its shape. In this program, however, you can use either the vertex point or a point on the curve or surface to control its shape. Therefore, "control point" may be used to refer to either a point on the curve or surface.

## current command

Many of the commands in this program will be used over and over again in succession, like the Move Point command. This program does not require you to reselect the Move Point command each time you want to move a point, because after the first menu selection, it becomes the current command and its label is shown on the right side of the Status Line. For the Move Point command, you simply pick and move the next point. The current command remains in effect until another command is selected.

## current plate

The current plate is the hull surface selected by the user for development. This is done using the Select Plate command. For development, the plate must have either two rows or two columns. Once a surface (plate) is current, you can display its Layout View or its Layout Numbers.

## curvature

This is a mathematically-defined term which refers to the amount of roundness located at a point on a curve or surface. If a curve is flat, then its curvature is zero. As the curve becomes more rounded, the radius of curvature goes down and the curvature goes up. Curvature 'k' = 1 / rho, where rho is defined as the radius of curvature. These two values ('k' and 'rho') are inversely related.

## defining point

This is the term used to identify those 3D points that are used to define an entity. For a point entity, there is just one defining point. For a curve, there are a series or string of defining points, ordered from one end of the curve to the other. For a surface, there are a grid or matrix of defining points, located at the intersections of all defining rows and columns.

## dependency

This term refers to a relationship between two entities. Some relationships, like Fix Pnt Rel Pnt, involve an independent and dependent entity. The point that is fixed relative to another is said to be dependent (form a dependency) on the other point. If the independent point is moved, then the dependent (relative) point moves along with it automatically.

## depth

This is a vague distance usually defined as the distance from the bottom of the hull (without appendages) to the highest point on the sheer line. If the program uses this number, it is only for sizing purposes and need not be perfectly accurate.

## developable

This term is used to describe hull surfaces which can be easily constructed out of flat material, like aluminum and plywood. Developable surfaces are combinations of flat, cylindrical, and conical shapes. Mathematically, a developbable surface is one where all points on the surface have a Gaussian curvature of zero. In addition, one may use the concept of ruling lines to define a developable surface.

## diagonal

This is the curve defined by intersecting a plane diagonally through the bilge area of the hull geometry. The curve can be seen in the profile or plan views. The diagonal plane is defined by two values: its intersection height (Z-value) on centerline and the heel angle of the plane.

## digitizing

This term refers to the process of tracing lines into the compuer using a tablet or digitizer pad. For boat design, this usually refers to taping a body plan to the tablet and tracing each station into the computer, one by one. These stations can then be fit or "skinned" with B-splines surfaces. (See the Digitizer program.)

## document

For this program, a document is the virtual representation of a particular 3D view of the model. A document view is created by scaling the 3D model view to the document coordinates using the user-defined scale factor (e.g. 1 inch = 1 foot) and adding the user-defined margins. If the document is to large to fit on the output device, it is automatically broken into pages covering the whole virtual document.

## double click

This term refers to the process of pressing and releasing a mouse button (usually the left) two times in a row, in rapid succession. This is a common technique used by many Windows programs for various purposes. It normally means that you want to edit an object or to look at its attributes.

## draft

This is the distance from the baseline (Z=0) to the waterline at amidships. Normally, draft is used to mean the distance to the design waterline(DWL), but it often depends on its context of use. In addition, the bottom of the boat is usually located at the baseline so that draft refers to the distance from the bottom of the boat to the waterline. However, if the bottom of the boat is NOT located at the baseline, you still must calculate draft from the baseline.

## drag

In reference to computer-aided design, drag is process of moving points or objects in a computer program. The process involves using the left button of the mouse to select (pick) an object or entity and move (drag) it to a new position. To begin the drag, use the left mouse button to select an object (such as a point). While holding down the left button, move the mouse to reposition the object. When it is in a desired location, fix it there by releasing the left mouse button.

## DWL - Design WaterLine

This is the length from the intersection of the design waterplane at the bow, to the intersection of the design waterplane at the stern. Often, people use just LWL, but DWL is more specific.

## DXF

DXF stands for Data eXchange File. It is a file that contains simple geometry entities (like lines, polylines, and text) that was created by AutoDesk, makers of AutoCAD. It has now become a standard means for transferring geometry data between CAD programs. These files are identified by their ".dxf" extensions.

## entity

This is a term used to refer to specific geometric objects. Most all CAD programs use entities as their basic unit of definition. For this program, the basic entities are: point, curve (includes polylines and combination curve and lines), and surfaces.

## fairing

This is the process of manipulating the definition of a hull shape to create a smooth surface for construction. Traditionally, this has been done on a drafting board using splines, ducks, and ships curves applied to stations, waterlines, buttocks, and diagonals. By computer, this is done using curvature analysis applied to the rows and columns of B-spline surfaces.

## fairness

This is a subjective, human term, which refers to the smoothness of a hull surface. There is no mathematical definition of fairness, although some define it as continuity of the second derivative. This may be a necessary condition, but it is not sufficient. A cubic B-spline guarantees second derivative continuity, but can easily be unfair. A better definition relates to the continuity of curvature of the curve or surface.

## FP, Forward Perpendicular

This is the position on the boat where X = 0. Usually this is set to the forward end of the design waterline.

## frame

This is the curve created by intersecting a plane at a specific transverse cut location (X-value) with the hull geometry. Although this term specifically refers to a structural member located at a particular transverse cross-section, it is often used interchangably with term station. The curve can be seen in the section view of the boat.

## freeboards

This term refers to the distance from the waterline to the sheer line. Often, freeboards at very specific locations are needed, in which case you have to be very careful in determining exactly where and how the freeboards are measured. Sometimes the sheer point is not well defined.

## Gaussian curvature

This is a mathematical term used to describe the double curvature of a point on a surface. If a surface is developable (flat in one direction), then the Gaussian curvature is zero. As the surface becomes more curved in two directions (twisted), the Gaussian curvature increases (positively or negatively). Mathematically, the Gaussian curvature is the product of the two principal curvatures.

## global

For B-splines, this refers to a technique which changes the shape of the curve or surface over the entire range of the curve or surface. The change might be very small, but there is no guarantee of no change. (see also local)

## grid line

This term is used to describe the straight lines drawn at the defined hull cut locations. These locations are defined in the HullCuts pull-down menu section. Their dispaly can be turned on and off using the Display Options dialog box, located under Options.

## heel angle

The heel angle is the angle in degrees that the waterline assumes with respect to the baseline of the boat. Upright, the boat has a heel angle of zero. For the calculations, the boat remains upright in the coordinate system and all calculations are done with a heeled waterline. That way, all of the numbers relate to the constant, upright coordinate system.

## hull cut

This is a generic term used to define a planar cut of the B-spline hull surfaces. Since the rows and columns that define the B-spline hull surfaces are not necessarily any of the traditional lines (stations, waterlines, buttocks, diagonals, and "other cuts"), then the program must calculate their shapes by mathematically cutting the B-spline surfaces with a plane. That is why the program often refers to the traditional lines as hull cuts.

## IGES

IGES stands for Initial Graphics Exchange Specification. It is an ANSI standard file definition for transferring 3D geometric data between CAD programs and allows for a large variety of entities, especially NURB curves and surfaces.

## interpolation

This term refers to the process of defining a curve (or surface) that passes through a set of points. Interpolation does not imply any one curve type or technique. It can be applied to almost any type of curve or surface. For B-splines, interpolation is a process whereby the program calculates the control vertices for a curve or surface which pass through the input defining points.

## K_Curve

This term refers to a curve defined using the curvature values of a curve (or row or column), the normal vector to the curve (or surface), and a scaling factor. The K_curve is drawn by connecting the K_Curve points, which are defined as:

K_Curve Point = Point on curve + curvature*scale factor*normal vector

This curve magnifies the fairness of a curve and is used for fairing curves and surfaces.

## KG, Vertical Center of Gravity

This term is used interchangeably with the term VCG. The 'K' refers to the keel of the vessel, which for many designs, is located at the baseline (Z=0). The 'G' refers to the center of gravity point. Therefore, KG is the vertical distance between 'K' and 'G', which is the same as VCG.

## knots, knot spacing

Refering to NURBS, a knot is a number which is part of the definition of a B-spline curve or surface and which is used to control its shape. The collection of all knots used by a B-spline is called a knot vector. The distance between knot values is called its knot spacing. Uniform B-splines set the knot spacing to 1 (usually), and NURBS (non-uniform) allow for uneven spacing of the knots.

## knuckle

This term refers to an abrupt or hard edged change in the shape of a curve or a surface. The point at which the hard change is located is called a knuckle point or a breakpoint on the curve. For surfaces, a knuckle line or curve usually forms the hard edge between two surfaces. (Most people call this a chine!)

## layout

This is a term use to describe the process of un-wrapping surfaces into their equivalent 2D shapes. It only applies to surfaces that are developable.

## LBP, Length Between Perpendiculars

This is the distance between the forward perpendicular (FP) and the aft perpendicular (AP). It is usually called LPP for large ships. For small vessels, this value is usually equivalent to LWL..

## LCB, Longitudinal Center of Buoyancy

This is the longitudinal X-position of the center of the volume of the underwater portion of the hull. It is calculated by the Hydro Calc command. It is referenced from the forward perpendicular (FP at X=0). Be careful, because some texts reference this value from amidships and some define it as a percentage distance (of LWL) aft of the forward end of the waterline.

## LOA - Length OverAll

This is the length of the boat from the tip of the bow to the furthest point aft on the stern.

## local

For B-splines, this refers to a technique which changes the shape of the curve or surface only over a limited range of the curve or surface. Outside of this range, the program guarantees that the shape won't change at all. (see also global)

## longitudinals

These are structural members which are attached (welded, perhaps) to the hull and which run along the length of the boat. They are often called stringers.

## LPP, Length Between Perpendiculars

This is the distance from the forward perpendicular (FP) to the aft perpendicular (AP). This is the same as LBP, but this term is more common in the big ship world.

## LWL - Length on the WaterLine

This is the distance from the intersection of the waterplane at the bow, to the intersection of the waterplane with the stern. Often, this term is used to mean the design or target waterline length. See also DWL.

## merged edge

This defines a common edge between two NURB surfaces, where all of the edge points of one surface have been Merge Pnt To Pnt to all of the edge points of a second surface. This is different from a bonded edge in that one merge relationship is stored for each connecting edge point and no commands will cross a merged edge boundary. For example, if you add a row into one surface that runs into a merged edge, the add row command will not continue into the merged surface.

## model

Model can be used as a noun or a verb. A geometric "model" is a 3D definition of an object such that a program can use it to perform analyses (volumetric, structural, etc.) and to create construction information (templates, NC cutting instructions, etc.). To "model" an object, refers to the process of defining the 3D shape of an object.

## non-uniform

For NURB surfaces, this term refers to the allowable uneven spacing of the defining knot values (knot vector). Knots are used in the definition of B-splines and can be used to control their shapes.

## numerical control (NC)

This term refers to using data extracted from a 3D geometric model and sending it to a cutting machine (lathe, mill, router, etc.) for automatic cutting of a part. For example, you can take the 2D frame shapes and the 2D developed plate shapes and transfer that information to an NC plasma cutter to cut out the pieces from large aluminum sheets.

## NURBS

Non-Uniform Rational B-Spline. This is the most general form of a B-spline commonly being used by CAD programs. Non-Uniform allows uneven spacing of the B-spline knots for more control over the shape of the curve or surface. Rational allows for exact representation of conic shapes and simpler perspective projections.

## offsets

This term normally refers to [x,y,z] points which define the intersection of waterlines and buttocks with each station of the hull. These points can then be organized into an "offsets table". X is the station location, Y is the half-breadth, and Z is the height of the offset.

## ooch, ooch point

This is a unique term which refers to the process of moving a point by a very small amount (an ooch distance). With the Move Point command it is impossible to change the shape by less than one pixel on the screen. Most programs require you to zoom in until you have the resolution you need. This is inconvenient for many tasks, so the ooch point command was invented to allow you to move a point by a fixed distance, which may be less than one pixel This command is very useful for fairing.

## other cut

This is the curve created by intersecting a plane with the hull geometry. This includes all hull cuts not covered by stations, waterlines, buttocks, and diagonals. Since three points in space define a unique plane, that is what is used to define an "other cut". This curve can be seen in any view.

## parameter

For B-splines, this refers to the independent value or parameter that is used to define the curve or surface. For curves, the parameter is called 'u' and its value changes from 0.0 at one end of the curve to 1.0 at the other end. Rectangular-like B-spline surfaces have two parameters: constant 'u' for the row direction and constant 'v' for the column direction. Their values also range from 0.0 to 1.0, so that every combination of 'u' and 'v' values between 0.0 and 1.0 define the entire surface.

## patch

This term refers to the area on a B-spline surface located between consecutive rows and columns of the surface. Therefore, all of the rows and columns subdivide the surface into a grid of patches. The program sometimes allows you to deal with the surface on a patch-by-patch basis.

## pick

This term is used to describe the process of selecting an object on the screen. First you position the cursor near the object or entity and then you press and release (click) the left mouse button. Many commands require you to select an object by "picking" it.

## Pixel

The pixel is the smallest unit of resolution for an output device. It is usually used to describe the resolution of the display screen. Although laser printer output is made up of dots (or pixels), one usually refers to its output in terms of dpi or "dots per inch".

## plate

This term usually refers to a surface of a boat which can be created out of flat material, like aluminum or plywood. This means that the surface is developable. This term is also used to refer to the sheet of steel or aluminum that will be used to construct the boat, whether or not the surface is developable.

## polyline

This is a series of 3D points in space connected by straight lines. For this program, polyline is considered to be a variation of the curve entity, which can define NURB curves, polylines, and combinations of the two.

## polynomial

This term refers to a particular type of equation that is used as the basis for all B-spline curves and surfaces. A polynomial has the general form: F(x) = a0 + a1*x + a2*x^2 + a3*x^3 + ... The highest exponent value that is used (3, in this example), is called the degree of the polynomial (degree=3 is also called a cubic polynomial). The number of coefficients ('a' values) is the order of the polynomial. Order is always one more than degree.

## relationship

This defines a geometrical connection between two entities that is enforced by the program. For example, if you fix a point to a curve, then you can only move or drag the point along that curve. Likewise, if you move the curve, the point will remain attached to the curve and remain in the same relative position.

## row, surface row

In discussing B-splines or NURBS, a row is one of the iso-parametric curves which define these surfaces. "Columns" lie in a direction perpendicular to rows. These surfaces are made up of a rectangular-like mesh of rows and columns.

## ruling lines

These are straight lines which lie entirely on a surface, traveling from one edge of the surface to another. If a surface can be constructed entirely of non-overlapping ruling lines, it is said to be developable and can be constructed out of flat material. Note that all flat, conical, and cylindrical surfaces contain ruling lines and are developable.

## sheer, sheer line

This is the well-defined curve which forms the junction between the deck and the hull.

## snap

This term refers to a technique whereby a point is automatically shifted to a near-by grid point. This is useful if you have to define or move points to positions which fall exactly on the intersections of a pre-defined grid. See the Set Grid, Grid ON, and Snap ON commands.

## station

This is the curve defined by intersecting a transverse cut plane (X-value) through the hull geometry. Although station specifically defines one of the cross-sections used to define the boat, it may be used in a more general sense for any cross-section, and may be used interchangably with the term "frame". This curve can be seen in the section view.

## status line

The status line is the line at the bottom of the application window which displays user prompts, warning messages, and error message. You should always look there if you are not sure what is happening.

## stringers

These are structural members which are attached (welded, perhaps) along the length of the boat. They are sometimes called longitudinals.

## TCB, Transverse Center of Buoyancy

This is the transverse distance from the centerline to the center of the volume of the underwater portion of the boat. It is calculated by the Hydro Calc command. This value is zero for symmetrical boats in the upright condition. It will not be zero if you specify an heel angle.

## toggle

This term refers to the process of turning on and off a switch type of command or option (like a light switch). The first time you use a command, you turn on the option, and the next time you select the command, you turn it off.

## uniform

For B-Splines, this term refers to the constant or uniform spacing of the defining set of knot values (knot vector). The knots are part of a B-spline definition and can be used to control the shape of the curve or surface.

## VCB, Vertical Center of Buoyancy

This is the distance from the baseline (Z=0) to the center of the underwater volume of the boat.

## VCG, Vertical Center of Gravity

This is the distance from the baseline (Z=0) to the vertical position of the center of gravity of all of the hull weights. This value is obtained from a weight study (see the Weights program) of the boat which identifies all of the weight items (hull, joinery, machinery, etc.) that make up the boat.. This is a very important number which affects the motions and stability of the boat.

## vertex point

This is a term used to describe the official defining points of a B-spline curve or surface. These points do not necessarily lie on the curve or surface, but they do influence its shape. For every vertex point, however, there is an associated point on the curve or surface. Since you can mathematically convert one point to the other, then sometimes the points on the surface are referred to as defining points. This program allows you to control shapes using either point.

## waterline

This is the curve defined by intersecting a horizontal plane (Z-value) through the hull geometry. All waterlines are parallel to the main design waterline or waterplane. The curve can be seen in the plan view of the boat.

## zoom

This is a generic term used by many programs to refer to the process of enlarging (and sometimes reducing) the view that appears on the screen. This can be done many ways: as a percentage magnification factor (zoom in), as a scale down factor (zoom out), or an enlarge defined by a user-specified rectangle in the view (zoom window).