HULLFORM

 

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A hull is the body of a ship or boat. It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking.

 

 

Hull wire frame using CAD computer aided design

 

"Hull Form"

 

 

General features

 

Nearly all watercraft, from small boats to the largest ships, adhere to a general form that serve the needs of stability and efficient propulsion, featuring:

 

  • horizontal cross-sections that have narrow, usually pointed, fronts (at the bow),

  • smooth widening from the bow until roughly the middle of the length (midships), and often narrowing smoothly but usually significantly to the extreme end (the stern), whose width may range from a large to an insignificant fraction of the beam width.

 

Hull shape

 

Hulls come in many varieties but are grouped primarily as follows:

  • Moulded, round bilged or soft-chined

defined as smooth curves

  • Chined or Hard-chined

have at least one pronounced knuckle throughout all or most of their length

After this they can be categorized as:

  • Displacement

the hull is supported exclusively or predominantly by the pressure of water displaced by the hull

  • Semi-displacement, or semi-planing

the hull form is capable of developing a moderate amount of dynamic lift, however, most of the vessel's weight is still supported through displacement

  • Planing

the Planing Hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed.

The inverted bell shape of the hull, with smaller payload the waterline cross-section is less, hence the resisitance is less and the speed is higher. With higher payload the outward bend provides smoother performance in waves.

Many hulls have composite shape, e.g., a fine entry forward and inverted bell shape aft.

 

 

Appendages

  • A protrusion below the waterline forward is called a bulbous bow and is fitted on some hulls to reduce the wave making resistance drag and thus increase fuel efficiency. Bulbs fitted at the stern are less common but accomplish a similar task.

  • A keel may be fitted on a hull to increase the transverse stability (if filled with a heavy weight), directional stability or to create lift as in a sail boat .

  • Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.

 

Terms

 

Bow is the frontmost part of the hull

Stern is the rear-most part of the hull

Portside is the left side of the boat when facing the Bow

Starboard is the right side of the boat when facing the Bow

Waterline is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.

Midships is the midpoint of the LWL (see below). It is half-way from the forwardmost point on the waterline to the rear-most point on the waterline.

Baseline an imaginary reference line used to measure vertical distances from. It is usually located at the bottom of the hull.

 

 

Hull LOA and LWL length over all and length water line

 

"LWL & LOA"

 

 

Metrics

 

Hull forms are defined as follows:

  • Block Measures that define the principal dimensions. They are:

  • Length Overall (LOA) is the extreme length from one end to the other

  • Length on the Waterline (LWL) is the length from the forwardmost point of the waterline measured in profile to the stern-most point of the waterline.

  • Length Between Perpendiculars (LBP or LPP) is the length of the summer load waterline from the stern post to the point where it crosses the stem.

  • Beam or breadth (B) is the width of the hull. (ex: BWL is the maximum beam at the waterline)

  • Depth (D) is the vertical distance from the bottom of the hull to the uppermost edge at the side.

  • Draft (d) or (T) is the vertical distance from the bottom of the hull to the waterline.

  • Freeboard (FB) is the difference between Depth and draft.

 

Hull design bean and draft, or water float depth

"Beam, draft & Depth

 

  • Form Derivatives that are calculated from the shape and the Block Measures. They are:

  • Volume (V or ) is the volume of water displaced by the hull.

  • Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull.

  • Longitudinal Centre of Buoyancy (LCB) is the longitudinal distance from a point of reference (often Midships) to the centre of the displaced volume of water when the hull is not moving. Note that the Longitudinal Centre of Gravity or centre of the weight of the vessel must align with the LCB when the hull is in equilibrium.

  • Vertical Centre of Buoyancy (VCB) is the vertical distance from a point of reference (often the Baseline) to the centre of the displaced volume of water when the hull is not moving.

  • Longitudinal Centre of Floatation (LCF) is the longitudinal distance from a point of reference (often Midships) to the centre of the area of waterplane when the hull is not moving. This can be visualized as being the area defined by the water's surface and the hull.

  • Coefficients help compare hull forms as well:

1) Block Coefficient (Cb) is the volume (V) divided by the LWL x BWL x T. If you draw a box around the submerged part of the ship, it is the ratio of the box volume occupied by the ship. It gives a sense of how much of the block defined by the Lwl, Bwl & draft (T) is filled by the hull. Full forms such as oil tankers will have a high Cb where fine shapes such as sailboats will have a low Cb.

 

 

V

Cb  = 
Lwl - Bwl - T

 

2) Midship Coefficient (Cm or Cx) is the Bwl x draft divided by the cross-sectional area (Ax) of the slice at Midships (or at the largest section for Cx). It displays the ratio of the largest underwater section of the hull to a rectangle of the same overall width and depth as the underwater section of the hull. This defines the fullness of the underbody. A low Cm indicates a cut-away mid-section and a high Cm indicates a boxy section shape. Sailboats have a cut-away mid-section with low Cx whereas cargo vessels have a boxy section with high Cx to help increase the Cb.

 

Am

Cm  = 
Bwl - T

 

3) Prismatic Coefficient (Cp) is the volume (V) divided by Lwl x Ax. It displays the ratio of the underwater volume of the hull to a rectangular block of the same overall length as the underbody and with cross-sectional area equal to the largest underwater section of the hull. This is used to evaluate the distribution of the volume of the underbody. A low Cp indicates a full mid-section and fine ends, a high Cp indicates a boat with fuller ends. Planing hulls and other highspeed hulls tend towards a higher Cp. Efficient displacement hulls travelling at a low Froude number will tend to have a low Cp.

 

 

V

Cp  = 
Lwl - Am

 

4) Waterplane Coefficient (Cw) is the waterplane area divided by Lwl x Bwl. The waterplane coefficient expresses the fullness of the waterplane, or the ratio of the waterplane area to a rectangle of the same length and width. A low Cw figure indicates fine ends and a high Cw figure indicates fuller ends. High Cw improves stability as well as handling behavior in rough conditions.

 

Aw

Cw  = 
Lwl - Bwl

 

 

 

Note:     Cb = Cp - Cm

 

History

 

Rafts have a hull of sorts, however, hulls of the earliest design are thought to have each consisted of a hollowed out tree bole: in effect the first canoes. Hull form then proceeded to the Coracle shape and on to more sophisticated forms as the science of Naval architecture advanced.

 

 

See also

 

LINKS and REFERENCE

 

 

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