Hull (watercraft)

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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 buouyancy that keeps the vessel from sinking.

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 (the beam), 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 by the press of water displaced by the hull

* 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. (see: Planing (sailing)).

Specific features

* 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. (see also : Naval architecture)

* 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

Waterline is a 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 verticle distances from. It is usually located at the bottom of the hull.

Metrics

Hull forms are defined as follows:

* Block Measures that define the principle 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 pont of the waterline.

:* 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.

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

:* Displacement (&delta) 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 (C_b) 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 C_b where fine shapes such as sailboats will have a low C_b.

::2) Midship Coefficient (C_m or C_x) is the BWL x draft divided by the cross-sectional area (A_x) of the slice at Midships (or at the largest section for C_x). 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 and high-speed displacement hulls have a cut-away mid-section with low C_x whereas cargo vessels have a boxy section with high C_x to help increase the C_b.

::3) Prismatic Coefficient (C_p) is the volume (V) divided by A_x. 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 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 C_p. Efficient displacement hulls travelling at a low Froude number will have a low C_p.

::4) Waterplane Coefficient (C_w) 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 secondary stability as well as handling behavior in rough conditions, but decreases hull speed.

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 construction then proceeded to the Coracle shape and on to more sophisticated forms as the science of Naval architecture advanced.

Hulls of the earliest design are thought to have each consisted of a hollowed out tree bole: in effect the first canoes. Hull construction then proceeded to keeled hulls, use of ballast, and on to modern double steel hulls with waterproof sections.

See also

• double hull
• Submarine hull
• ship
• naval architecture
• submarine
• Shipbuilding
• Froude number

External links

• [Nautical terms for the parts of sailing vessels]
• [Description of hydrostatics and coefficients]

Types of Sail Spars
Bowsprit | Fore (mast) | Gaff | Jigger (mast) | Main (mast) | Mast | Mizzen (mast) | Spar | Spinnaker Pole | Yard
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