Glossary

Illustrated Glossary
 

 

Definitions of nautical terms encountered and specific to the Boat Design Competition

Illustrated Glossary

Glossary

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Abeam

  At right angles to the fore-aft centerline away.  See Ship Directions.
 

Aft

  Toward, at, or near the stern.  See Ship Directions.
 

Aft Perpendicular (AP)

  For the purposes of the Boat Design Competition, a vertical line at the aftermost portion of the buoyant hull.  In actuality, the AP is normally at the rudder stock or at the intersection of the design waterline and the hull.  See Ship Dimensions.
 

Amidships

  At or near the midship section (middle portion) of the boat.  See Ship Directions.
 

Appendages

  Items or features outside the outline of the hull (e.g. rudder).
 

Athwartships

  See Abeam.
 

Awash

  Level with the surface of the water so as to be covered due to wave action.
 

Ballast

  Any weight(s) used to keep the boat from becoming top heavy or to change its draft or trim.
 

Baseline (BL)

  A horizontal line through the lower point of a boat's hull from which all vertical measurements are taken.  See Ship Dimensions.
 

Beam or Breadth

  Maximum width of the boat's hull.  See Ship Dimensions.
 

Bending

  Curvature is not a requirement.  Different types of hull forms can be accomplished without using curved plate, which has many benefits for designers and naval architects.  The greatest benefit of using flat plates is the ease of locating the center of gravity of each plate.  However, using multiple flat plates to accomplish a geometric form that can be accomplished by bending plates is superfluous.  This not only increases welding, but greatly increases the difficulty in lofting and construction.  The more complicated the design, the more difficult the calculations, construction, and other aspects of shipbuilding become. Curvature can be accomplished by rolling the plate to a dimensioned radius and angle.
 

Body Plan

  See Views.
 

Bow

  Forward end of the boat.  The length of the bow is arbitrary.  See Ship Directions.
 

Breaking

  When a ninety degree bend is required, a square break should be considered.  A typical location for a square break would be the turn-of-the-bilge (the transition between the bottom plating and the side plating).  Square breaks require extra dimensions on the drawings to accurately fabricate (depending on whether inside or outside breaks are used).
 

Bulkhead

  Partition or wall.
 

Buoyancy

  Ability to float.  For a boat to float, the weight of the boat and the buoyant force being imposed on the boat from the water must be equal.  The amount of buoyant force on the boat is determined by the amount of water the boat displaces, which is equal to the immersed volume of the hull (volumetric displacement).
 

Buttock

  Vertical longitudinal section planes.  The intersections of the these planes with the hull create buttock lines.  In combination with waterlines and stations, buttocks define the curvature of the boat's hull.
 

Capsize

  To overturn; turtle.
 

Catamaran

  A boat with two parallel hulls or floats joined by a deck or other structure.
 

Center of Buoyancy (CB)

  The point about which the boat floats; determined as the geometric center of gravity of the immersed volume of the displaced water.  Determined solely by the shape of the immersed body of the boat.  The center of buoyancy changes depending on the boat's motions (except pure surge and sway) and conditions (e.g. trim, list, etc.).  Even in these conditions a transverse component (TCB) is not normally defined because a boat's hull is usually symmetric.  The distance KM is the same as the vertical center of buoyancy, VCB.
 

Center of Flotation (CF)

  The geometric center of the waterplane at the design waterline.  When a boat trims, it inclines about the CF.  Weight added directly above or below the CF will increase the mean draft, but will not change trim.  The CF is usually referred to as the Longitudinal Center of Flotation (LCF) since the vertical component is zero (since the waterplane is an area at the design draft) and the transverse component is almost always nonexistent due to the waterplane being symmetrical.  The LCF is located midship when the boat has longitudinal symmetry (e.g. rectangular waterplane).
 

Center of Gravity (CG)

  The point at which the combined weight of all the individual items may be considered as concentrated.  Center of gravity is a geometric property of an object and is independent of the material used (unless the material is of varying density, nonhomogeneous).  The center of gravity can change due to shifting weights (e.g. passengers moving about, liquids in tanks, sliding cargo, etc.).  The distance KG is the same as the vertical center of gravity, VCG.  Unlike center of buoyancy, center of gravity does have a transverse component, TCG, since a boat is not always symmetric in regards to weight/cargo distribution.
 
 Center of Gravity Example
 

The following tables assume that the reference point is located at the front of Plate 1 and the bottom of Plate 5.  Plate weight density is 5.1 lbs/ft2.

Item

Dimensions

Area

Weight

LCG

LMOM

VCG

VMOM

TCG

TMOM

(in)

(in2)

(lb)

(in)

(in·lb)

(in)

(in·lb)

(in)

(in·lb)

Bow Plate (1)

22” x 18”

396

14.025

0.5

7.0125

9

126.225

0

0

Port Plate (2)

72” x 18”

1296

45.9

36

1652.4

9

413.1

-11.5

-527.85

Starboard Plate (3)

72” x 18”

1296

45.9

36

1652.4

9

413.1

11.5

527.85

Stern Plate (4)

22” x 18”

396

14.025

71.5

1002.79

9

126.225

0

0

Bottom Plate (5)

70” x 22”

1540

54.54

36

1963.44

0.5

27.27

0

0

Total

 

174.4

 

6278

 

1106

 

0

 

Item

Dimensions

Area

Weight

LCG

LMOM

VCG

VMOM

TCG

TMOM

(in)

(in2)

(lb)

(in)

(in·lb)

(in)

(in·lb)

(in)

(in·lb)

Bow Plate (1)

24” x 17”

408

14.45

0.5

7.225

9.5

137.28

0

0

Port Plate (2)

70” x 17”

1190

42.14

36

1517.04

9.5

400.39

-11.5

-484.61

Starboard Plate (3)

70” x 17”

1190

42.14

36

1517.04

9.5

400.39

11.5

484.61

Stern Plate (4)

24” x 17”

408

14.45

71.5

1033.18

9.5

137.28

0

0

Bottom Plate (5)

72” x 24”

1728

61.2

36

2203.2

0.5

30.6

0

0

Total

 

174.4

 

6278

 

1106

 

0

 

Notice that even though the plates of the two examples are different sizes (and therefore the center of gravity dimensions differ), the weight and the moments are the same.  If the reference point is moved, the moments would change, though the composite center of gravity would remain at the same point in the barge.

 The composite center of gravity is:

  • LCG = 35.5 inches aft of reference point (6278/174.4)
  • VCG = 5.84 inches above reference point (1106/174.4)
  • TCG = 0 inches aside reference point (0/174.4)
 
 Center of Gravity Techniques
 

Some plates do not have centroids that are easily defined, as in the center of gravity example.  This makes it difficult to find the LCG/VCG/TCG to the plate’s centroid/center of gravity.  If the plate is of a regular two-dimensional shape, the centroid should be relatively easy to find, even if the plate is rotated in space.  Use the internet to find formulas for finding the centroid of a two-dimensional shape.

 

The technique below is for finding the centroid of a bent rectangular plate.  Since the plate is a rectangle, the centroid is located halfway between its length and height, as shown in the xz-plane diagram.  However, as the plate is bent, the centroid moves outward (in the y-direction), as shown in the xy-plane diagram.  Therefore, bent plates have composite center of gravities that are not actually on the plate.  A horizontal line (in the -direction) drawn at the 2/3 dimension would illustrate that the same amount of material is above the line as below the line.  Therefore, that is how far the center of gravity moves away from the plate to compensate for the bend.

If the bent plate is triangular, as opposed to rectangular of the previous technique, then the center of gravity is found in a different manner.  Whereas a rectangle has its centroid midway along its length and height, a triangle’s centroid is located at 1/3 of its length and height.  This length moves the centroid over to the bulkier side of the plate, as shown in the xy-plane diagram.  Otherwise, the movement due to the bend is the same.

 

Centerline (CL)

  A line created by taking a vertical plane down the center of the boat, extending from bow to stern.  All transverse dimensions are taken from the centerline.
 

Coefficients

 

Coefficients help determine the efficiency of the hull.

 

Block Coefficient (CB) - the ratio of the volumetric displacement of the hull to a prism equal to the waterline length, beam, and draft of the hull (i.e. how much of a prism is occupied by the hull)

Midship Coefficient (CM) - the ratio of the midship area of the hull to a rectangle equal to the beam and draft of the hull (i.e. how much of a rectangle is occupied by the midship area)

Prismatic Coefficient (CP) - the ratio of the volumetric displacement of the hull to a body equal to the waterline length and midship area of the hull (i.e. how much of a body is occupied by the hull)

Waterplane Coefficient (CW) - the ratio of the waterplane area of the hull to a rectangle equal to the waterline length and beam of the hull (i.e. how much of a rectangle is occupied by the waterplane area)

 

Compartment

  A confined space (a room in a boat).
 

Curvature

 

Complex Curvature - bending a plate in two planes

Simple Curvature - bending a plate in one plane

 

Density

 

Mass per unit volume (Greek letter rho) or a weight per unit volume (weight density, specific weight, Greek letter gamma).  For the purposes of the Boat Design Competition, material density is in units of lb/ft2 for several reasons:

  • Steel plate and plywood areas, not volumes, are calculated
  • Steel plate in the shipyard is referred to by unit weight, which correlates to thickness
  • In conjunction with the above, the thickness dimension is removed from the denominator so that the units reduce to lb/ft2
  • All teams utilize the same thickness of steel plate and plywood
 

Depth (D)

  Vertical distance from the top of the hull to the baseline.  See Ship Dimensions.
 

Displacements

 

The amount of water that a boat displaces; measured by weight or by volume:

    Displacement (Δ) - total weight of the boat when afloat including everything on board.

      Volumetric Displacement () - displacement in terms of volume.

         

        Drafts

         

        Draft - vertical distance from the waterline to the baseline

        Mean Draft - draft measured at the center of flotation.  It is the average of the draft at the bow and the draft at the stern.  For purposes of the Boat Design Competition, the mean draft is taken as the design draft since they are approximately the same.

        Draft at Bow - draft measured at the forward perpendicular

        Draft at Stern - draft measured at the after perpendicular

        Design Draft - draft that the boat is expected to operate at

         

        Elevation View

          See Views.
         

        Forward or Fore

          Near, at, or toward the bow of the boat.  See Ship Directions.
         

        Forward Perpendicular (FP)

          For the purposes of the Boat Design Competition, a vertical line at the forward most portion of the buoyant hull.  In actuality, the FP is at the intersection of the design waterline and the stem.  See Ship Dimensions.
         

        Freeboard

          Depth minus the draft and therefore a measurement of the boat's hull above the waterline.  See Ship Dimensions.
         

        Half-Breadth (HB)

          A distance from the centerline to the hull.  For half-breadth plan, see Views.
         

        Heave

          See Ship Motions.
         

        Heel

          A semi-permanent condition where a boat leans to port or starboard due to effects on the hull.  Not to be confused with rolling, the continuous side-to-side motion.
         

        Hull

          For the purposes of the Boat Design Competition, the main structurally buoyant portion of a boat, including the decks, bulkheads, etc., but excluding the superstructure or appendages.  See Superstructure.
         

        Inboard

          Toward the centerline.  See Ship Directions.
         
        Joints
        When two plates meet at an angle, an option exists to either meet them at an exterior edge, which can result in an overlap, or meet them at an interior edge, which can result in a gap. To make a joint of the overlapping plates, one plate’s end needs to be modified across the thickness to make a flush joint. To make a joint of the gapped plates, weld metal will fill the gap. For the purposes of the Boat Design Competition, the cuts across the thickness/gaps between plates does not need to be specified in the construction drawings. Experienced shipyard workers will handle these small nuances in the construction. Instead, teams should focus on accurately dimensioning and weighing the plates’ major dimensions.
        Joint_Overlap.tif
        Joint_Gap.tif
        A butt joint is a connection between two pieces by joining their ends together. Butt joints should be used in the Boat Design Competition.
        Butt_Joint.tif
        A lap joint is a connection between two pieces by overlapping their ends. Lap joints should not be used for joining hull plates together (i.e. exterior of the boat should be flush).
        Lap_Joint.tif

        Keel

          The principal fore and aft member of a boat's hull (the boat's backbone) that runs along the bottom centerline.
         

        Lengths

         

        Length Between Perpendiculars (LBP) - distance between the FP and the AP

        Length Overall (LOA) - distance from structural tip to structural tip

        Length at Waterline (LWL) - distance between the forward draft and the aft draft.  In some instances, there may be two LWLs; one that defines only the buoyant portion of the boat and one that defines an actual length.

        For some boats of the Boat Design Competition, the LBP, the LOA, and the LWL may all be the same value (e.g. vertical-sided), may all be different values (e.g. a monohulls with cantilevered appendages), or may be a combination of the aforementioned (e.g. majority of boats).

        See Ship Dimensions.

         

        List

          A permanent condition, until corrected, of a boat to lean to port or starboard.  Not to be confused with rolling, the continuous side-to-side motion.  In contrast to trim, which is the permanent condition forward and aft.
         

         Load Waterline (Design Waterline) (DWL)

          Same as Design Draft.
         

        Longitudinal

          Parallel with the boat’s centerline.  See Ship Directions.
         

        Longitudinal Center of Buoyancy (LCB)

          See Center of Buoyancy.
         

        Longitudinal Center of Flotation (LCF)

          See Center of Flotation.
         

        Longitudinal Center of Gravity (LCG)

          See Center of Gravity.
         

        Longitudinal Metacentric Height (GML)

          See Longitudinal Stability.
         

        Longitudinal Metacentric Radius (BML)

          See Longitudinal Stability.
         

        Longitudinal Stability

          The tendency of a boat to resist a change in trim.  Longitudinal stability shares the same KB and KG as transverse stability and therefore only the measurements to the metacenter (KM, BM, GM) change.
         

        Main Deck

          For the purposes of the Boat Design Competition, the plane forming the top boundary of the hull (since a deck would enclose the hull and is not allowed per competition rules).
         

        Mean Draft (TM)

          See Drafts.
         

        Metacenter

          Fixed point in space above a boat about which it rotates.  See Transverse and Longitudinal Stability.
         

        Metacentric Height (GM)

          A vertical distance between the center of gravity and the metacenter.  A measure of stability: if M is above G, the metacentric height is positive and the boat is stable.  If M is below G, the metacentric height is negative and the boat is unstable.  If GM is large, the boat resists rolling/pitching and is said to be stiff.  If GM is small, the boat rolls/pitches slowly and is said to be tender.  Therefore, a large GM is desirable for resistance to the flooding effects of damage, but a smaller GM is desirable for passenger comfort, accurate gunfire, etc.  See Longitudinal and Transverse Stability.
         

        Metacentric Radius (BM)

          A vertical distance between the center of buoyancy and the metacenter.  So termed because for small angles of heel, the center of buoyancy can be traced by a circle.  See Longitudinal and Transverse Stability.
         

        Moment

          Force times a distance.
         

        Moment to Trim 1"

          Measure of a boat's ability to resist a one inch change in trim.  Units do not simplify; instead they remain a moment (force x distance) over a 1" trim (distance) [i.e. (in·lb)/in].
         

        Monohull

          A boat with only one hull, as opposed to a catamaran or trimaran.
         

        Origin

          See Reference Point.
         

        Outboard

          Away from the centerline.
         

        Nesting

          Layout of parts to be cut from a sheet of metal.
         

        Payload

          The total weight of cargo that a boat is designed to carry.
         

        Pitch

          See Ship Motions.
         

        Plan View

          See Views.
         

        Port

          The left side of the boat when looking forward.  See Ship Directions.
         

        Pounds Per Inch (PPI)

          The number of pounds of additional weight required to immerse a boat one additional inch of draft.  In the industry, this is referred to as Tons Per Inch (TPI) since pounds are relatively small for ships.  Like MT1", PPI is somewhat misleading since it is calculated from the design waterplane area, which theoretically changes size as the boat's draft changes.  However, since a boat's waterplane area does not change drastically in such a small increment, PPI/TPI is only used to determine small changes in draft from loading/discharging small weights.
         

        Reference Point

          For consistency, the reference point will be located at the same place for all teams.  This point is located at the intersection of the forward perpendicular and baseline.  This holds true for all types of hulls (monohulls/catamarans/etc.).  See Ship Dimensions.  The proper sign convention must be used in conjunction with the reference point.
         

        Roll

          See Ship Motions.
         

        Rudder

          Blade-shaped device used to steer the boat.
         

        Rudder Stock

          Vertical shaft that connects the rudder to the steering mechanism.
         
        Seams
        See Joints.

        Section View

          See Views.
         

        Sheer Plan

          See Views.
         

        Ship Dimensions

         

         

        Ship Directions

         

        Ship Motions

         

        Sign Convention

         

        Simpson's Rule

         

        Simpson’s Rule is a technique used in naval architecture to closely approximate areas and volumes through numerical integration (if distances are known, then areas are calculated; if areas are known, volumes are calculated).  For Simpson’s Rule to work, an even number of intervals is used.

         

        Specifically, Simpson’s First Rule will be used.  It assumes that the shape of the curve follows a second-order polynomial.  The basic formula for Simpson’s Rule for calculating a volume (Ñ) is:

         

        = (h/3)*(1*A0 + 4*A1 + 2*A2 + 4*A3 + 1*A4)

          

         

        where h is the spacing between the intervals and A is the area.  This formula would work for four intervals since there are five areas.  This is the basic formula that will be used in conjunction with station and waterplane areas in the next section.  The result will find the volumetric displacement and verify the draft.

         

        For increased precision, Simpson’s Rule may be modified.  If the hull is more uniform, less areas are required.  If the hull changes shape significantly, more areas are recommended.  For seven areas, the formula would look like this:

         

        = (h/3)*(1*A0 + 4*A1 + 2*A2 + 4*A3 + 2*A4 + 4*A5 + 1*A6)

          

         

        The values in front of the areas (1, 4, and 2) are called Simpson’s Multipliers.  Notice that in the middle of the equation for 7 areas, Simpson’s Multipliers go back and forth between 4 and 2 (as opposed to 1 and 4 for five areas).  The pattern for the multipliers is:

         

        for 3 areas                               1…4…1

        for 5 areas                               1…4…2…4…1

        for 7 areas                               1…4…2…4…2…4…1

        for 9 areas                               1…4…2…4…2…4…2…4…1

        for 11 areas                             1…4…2…4…2…4…2…4…2…4…1

         

        For calculations, the areas of the stations and the areas of the waterplanes are allowed to come from the CAD program.  If preferred (or unable to pull the areas from the CAD program), then the formula can be modified to find the areas by hand:

         

        A = (h/3)*(1*D0 + 4*D1 + 2*D2 + 4*D3 + 1*D4)

          

         

        where D is the half-breadth distance (do not forget that this only calculates the area of one side).  Obviously, some station and waterplane areas may be more easily found by geometric formulas than by Simpson’s Rule.

         

        Starboard

          The right side of the boat when looking forward.  See Ship Directions.
         

        Station

          Vertical transverse section planes.  The intersections of these planes with the hull create curved lines also called stations.  In combination with buttocks and waterlines, stations define the curvature of the boat's hull.
         

        Stem

          Edge composing the forward part of the bow.
         

        Stern

          Aft end of the boat.  The length of the stern is arbitrary.  See Ship Directions.
         

        Superstructure

          Structure that sits on top of the hull (e.g. deckhouses of cargo ships or the accommodation decks of cruise ships).
         

        Surge

          See Ship Motions.
         

        Sway

          See Ship Motions.
         

        Transverse

          At right angles to the boat’s centerline.  See Ship Directions.
         

        Transverse Center of Gravity (TCG) 

          See Center of Gravity.
         

        Transverse Metacentric Height (GMT) 

          See Transverse Stability.
         

        Transverse Metacentric Radius (BMT) 

          See Transverse Stability.
         

        Transverse Stability

          The tendency of a boat to resist a change in list.  Transverse stability shares the same KB and KG as longitudinal stability and therefore only the measurements to the metacenter (KM, BM, GM) change.
         

        Trim

          A difference between the forward and aft drafts.  A permanent condition, until corrected.  Not to be confused with pitching, a continuous rocking motion.  In contrast to list, which is the permanent inclination to port or starboard.  See Drafts.
         

        Trimaran

          A triple-hulled boat, the center hull usually being larger than the others.
         

        Vertical Center of Buoyancy (VCB)

          See Center of Buoyancy.
         

        Vertical Center of Gravity (VCG) 

          See Center of Gravity.
         

        Views

         

        Body - a view vertically and transversely forward of the boat with port to the right showing beam and depth and the front view of the boat.  In actuality, curves would be shown at designated stations to show the curvature of the hull.  Due to the symmetry of the hull, stations forward of midships would be shown on the right side of the view and the stations aft of midships would be shown on the left side.  For the purposes of the Boat Design Competition, a traditional view is all that is required.

        Isometric - a three-dimensional view represented two-dimensionally by projection

        Plan - a view horizontally and longitudinally above the boat with the bow to the right showing length and beam and the top view of the boat.  In actuality, curves would be shown at designated waterplanes to show the curvature of the hull.  Due to the symmetry of the hull, only one side would be shown (called a half-breadth plan).  For the purposes of the Boat Design Competition, a traditional plan view is all that is required.

        Section - a view cut through the boat to show an area hidden in another view

        Sheer - a view vertically and longitudinally abeam of the boat with the bow to the right showing length and depth and the elevation of the boat.  In actuality, curves would be shown at designated buttock planes to show the curvature of the hull.  For the purposes of the Boat Design Competition, a traditional view is all that is required.

         

        Volumetric Displacement

          See Displacements.
         

        Waterline/Waterplane

          Horizontal section planes.  The intersections of these planes with the hull create waterlines.  In combination with buttocks and stations, waterlines define the curvature of the boat's hull.
         

        Wetted Surface

          Surface area of the immersed portion of the hull.
         

        Yaw

          See Ship Motions.