Hull (watercraft) - Citizendia

A hull is the body of a ship or boat. A ship /ʃɪp/ is a large vessel that floats on water Ships are generally distinguished from Boats based on size A boat is a Watercraft of modest size designed to float or plane on water and provide transport over it It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking. In Physics, buoyancy ( BrE IPA: /ˈbɔɪənsi/ is the upward Force on an object produced by the surrounding liquid or gas in which it is

"Hull Form"

1 General features
2 Hull shape
3 Appendages
4 Terms
5 Metrics
6 History
7 Notes
8 References
9 See also
10 External links

General features

Nearly all watercraft, from small boats to the largest ships, adhere to a general form that serves 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. A watercraft is a Vehicle, vessel or craft designed to move across (or through Water, including saltwater and freshwater for pleasure recreation physical Ship stability is an area of Naval Architecture and ship design that deals with how a ship behaves at sea both in still water and in waves

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. A chine in boating refers to a relatively sharp angle in the hull, as compared to the rounded bottoms of most traditional boat hulls (see: Planing (sailing)). planing Boat 's hull skims across the surface of the Water rather than pushing through the water in the way a traditional Displacement hull

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. The inverted bell is a Metaphorical name for Geometric shape that resembles a bell upside down Waterline refers to an imaginary line marking the level at which Ship or Boat floats in the Water. 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. The bulbous bow, a standard feature of most large modern Ships with displacement hulls, is a protruding bulb at the bow (or front below the Waterline Wave making resistance is a form of drag that affects surface Watercraft, such as Boats and Ships and reflects the energy required to push In Fluid dynamics, drag (sometimes called fluid resistance) is the force that resists the movement of a Solid object through a Fluid (a 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, directional stability or to create lift. Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles In boats and ships keel can refer to either of two parts a structural element or a hydrodynamic element

Control devices such as a rudder, trim tabs or stabilizing fins may be fitted. A rudder is a device used to steer a Ship, Boat, Submarine, Hovercraft, or other conveyance that move through a fluid (generally air or

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. The bow (pronounced &mdashrhymes with how) is a nautical term that refers to the forward part of the hull of a Ship or Boat, The stern is the rear or aft part of a Ship or Boat, technically defined as the area built up over the Sternpost, extending upwards from the Counter Port is the nautical term (used on Boats and Ships) that refers to the left side of a ship as perceived by a person on board the ship and Starboard is the nautical term that refers to the right side of a vessel as perceived by a person on board a vessel and facing the bow (front Waterline refers to an imaginary line marking the level at which Ship or Boat floats in the Water.

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.

Metrics

"LWL & LOA"

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 overall, often abbreviated as ( LOA, o/a, oa or oa) refers to the maximum length of a vessel from the two points on the hull most distant
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 at the waterline, often abbreviated as lwl, w/l, wl or wl is term used to describe the length of a Ship.
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. Length between perpendiculars, often abbreviated as p/p, pp, pp or LBP is a term describing the length of a Ship. The stern is the rear or aft part of a Ship or Boat, technically defined as the area built up over the Sternpost, extending upwards from the Counter (see also p/p)
Beam or breadth (B) is the width of the hull. Length between perpendiculars, often abbreviated as p/p, pp, pp or LBP is a term describing the length of a Ship. The beam of a ship is its width at the widest point or at the mid-point of its length (ex: BWL is the maximum beam at the waterline)
Depth or moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side. ^[1]
Draft (d) or (T) is the vertical distance from the bottom of the hull to the waterline. The draft (or draught) of a ship's hull is the vertical distance between the Waterline and the bottom of the hull ( Keel) with the thickness Waterline refers to an imaginary line marking the level at which Ship or Boat floats in the Water.
Freeboard (FB) is the difference between Depth and draft. Freeboard or FREEBOARD may refer to Sporting Goods. The six-wheeled skateboard which acts like a snowboard (on pavement

"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. In Fluid mechanics, displacement occurs when an object is immersed in a Fluid, pushing it out of the way and taking its
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 ^[2] 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 L_wl, B_wl & 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.

$C_b = \frac {V}{L_{wl} \cdot B_{wl} \cdot T}$

2) Midship Coefficient (C_m or C_x) is the B_wl 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 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.

$C_m = \frac {A_m}{B_{wl} \cdot T}$

3) Prismatic Coefficient (C_p) is the volume (V) divided by L_wl x 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 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 C_p. Efficient displacement hulls travelling at a low Froude number will tend to have a low C_p. The Froude number is a Dimensionless number comparing inertial and gravitational forces

$C_p = \frac {V}{L_{wl} \cdot A_m}$

4) Waterplane Coefficient (C_w) is the waterplane area divided by L_wl x B_wl. 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.

$C_w = \frac {A_w}{L_{wl} \cdot B_{wl}}$

Note:

$C_b = {C_{p} \cdot C_{m} }$

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. The Stone Age is a broad prehistoric time period during which Humans widely used stone for toolmaking A tree is a perennial Woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or A canoe is a small narrow Boat, typically human-powered though it may also be powered by sails or small electric or gas motors Hull form then proceeded to the Coracle shape and on to more sophisticated forms as the science of Naval architecture advanced. A coracle (cwrwgl is a small lightweight Boat used mainly in Wales but also in parts of Western and South Western England, Ireland, and Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles

Notes

^ International Convention on Tonnage Measurement of Ships, 1969. International Conventions. Admiralty and Maritime Law Guide (1969-6-23). Retrieved on 2007-10-27. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 312 - Constantine the Great is said to have received his famous Vision of the Cross. , Annex 1, Regulations for determining gross and net tonnages of ships, Reg. 2(2)(a). In ships with rounded gunwales, the upper measurement point is take to the point at which the planes of the deck and side plating intersect. The gunwale (ˈɡʌnəl "gunnel" to rhyme with "tunnel" is a nautical term describing the top edge of the side of a Boat. Id. , Reg. 2(2)(b). Ships with stepped decks are measured to a line parallel with the upper part. Id. , Reg. 2(2)(c).
^ Rawson, E. C. & Tupper (1976), Basic Ship Theory Vol 1 (2nd ed. ), Longman, pp. 12-14, ISBN 058244523X

References

Hayler, William B. ; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0-87033-549-9.
Turpin, Edward A. ; McEwen, William A. (1980). Merchant Marine Officers' Handbook, 4th, Centreville, MD: Cornell Maritime Press. ISBN 0-87038-056-X.

External links

Description of hydrostatics and coefficients

A boat is a Watercraft of modest size designed to float or plane on water and provide transport over it A double hull is a Ship hull design and construction method where the bottom and sides of the ship have two complete layers of watertight hull surface one outer layer The draft (or draught) of a ship's hull is the vertical distance between the Waterline and the bottom of the hull ( Keel) with the thickness The Froude number is a Dimensionless number comparing inertial and gravitational forces Hull speed, sometimes referred to as displacement speed, is a Rule of thumb used to provide an approximate maximum efficient speed for a hull A monohull is a type of boat having only one hull, unlike Multihulled boats which can have two or more individual hulls connected to one another A multihull is a Ship, vessel craft or boat with more than one hull. Naval architecture is an engineering discipline dealing with the design construction and repair of marine vehicles A ship /ʃɪp/ is a large vessel that floats on water Ships are generally distinguished from Boats based on size See also Shipbuilding (song. Shipbuilding is the construction of Ships It normally takes place in a specialized facility known as a A submarine is a Watercraft that can operate independently below water as distinct from a Submersible that has only limited underwater capability The term light hull is used to describe the outer hull of a submarine which houses the pressure hull providing hydrodynamically efficient shape but not holding pressure difference

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