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A hull is the watertight body of a
ship A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...
,
boat A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats. Small boats are typically found on i ...
, or
flying boat A flying boat is a type of fixed-winged seaplane with a hull, allowing it to land on water. It differs from a floatplane in that a flying boat's fuselage is purpose-designed for floatation and contains a hull, while floatplanes rely on fuselag ...
. The hull may open at the top (such as a
dinghy A dinghy is a type of small boat, often carried or towed by a larger vessel for use as a tender. Utility dinghies are usually rowboats or have an outboard motor. Some are rigged for sailing but they differ from sailing dinghies, whic ...
), or it may be fully or partially covered with a deck. Atop the deck may be a
deckhouse A cabin or berthing is an enclosed space generally on a ship or an aircraft. A cabin which protrudes above the level of a ship's deck may be referred to as a deckhouse. Sailing ships In sailing ships, the officers and paying passengers wo ...
and other
superstructure A superstructure is an upward extension of an existing structure above a baseline. This term is applied to various kinds of physical structures such as buildings, bridges, or ships. Aboard ships and large boats On water craft, the superstruct ...
s, such as a funnel, derrick, or
mast Mast, MAST or MASt may refer to: Engineering * Mast (sailing), a vertical spar on a sailing ship * Flagmast, a pole for flying a flag * Guyed mast, a structure supported by guy-wires * Mooring mast, a structure for docking an airship * Radio mas ...
. The line where the hull meets the water surface is called the
waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...
.


General features

There is a wide variety of hull types that are chosen for suitability for different usages, the hull shape being dependent upon the needs of the design. Shapes range from a nearly perfect box in the case of scow barges to a needle-sharp surface of revolution in the case of a racing multihull sailboat. The shape is chosen to strike a balance between cost, hydrostatic considerations (accommodation, load carrying, and stability), hydrodynamics (speed, power requirements, and motion and behavior in a seaway) and special considerations for the ship's role, such as the rounded bow of an
icebreaker An icebreaker is a special-purpose ship or boat designed to move and navigate through ice-covered waters, and provide safe waterways for other boats and ships. Although the term usually refers to ice-breaking ships, it may also refer to smaller ...
or the flat bottom of a
landing craft Landing craft are small and medium seagoing watercraft, such as boats and barges, used to convey a landing force (infantry and vehicles) from the sea to the shore during an amphibious assault. The term excludes landing ships, which are larger. ...
. In a typical modern steel ship, the hull will have watertight decks, and major transverse members called bulkheads. There may also be intermediate members such as
girders A girder () is a support beam used in construction. It is the main horizontal support of a structure which supports smaller beams. Girders often have an I-beam cross section composed of two load-bearing ''flanges'' separated by a stabilizin ...
, stringers and webs, and minor members called ordinary transverse frames, frames, or longitudinals, depending on the structural arrangement. The uppermost continuous deck may be called the "upper deck", "weather deck", "spar deck", "
main deck The main deck of a ship is the uppermost complete deck extending from bow to stern. A steel ship's hull may be considered a structural beam with the main deck forming the upper flange of a box girder and the keel The keel is the bottom-mos ...
", or simply "deck". The particular name given depends on the context—the type of ship or boat, the arrangement, or even where it sails. In a typical wooden sailboat, the hull is constructed of wooden planking, supported by transverse frames (often referred to as ribs) and bulkheads, which are further tied together by longitudinal stringers or ceiling. Often but not always there is a centerline longitudinal member called a
keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...
. In
fiberglass Fiberglass ( American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cl ...
or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a
monocoque Monocoque ( ), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word ''monocoque'' is a French term for "single shell". First used for boats, ...
arrangement. In many cases, composite hulls are built by sandwiching thin fiber-reinforced skins over a lightweight but reasonably rigid core of foam, balsa wood, impregnated paper honeycomb, or other material. Perhaps the earliest proper hulls were built by the Ancient Egyptians, who by 3000 BC knew how to assemble wooden planks into a hull.Ward, Cheryl. "World's Oldest Planked Boats," in ''
Archaeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landsc ...
'' (Volume 54, Number 3, May/June 2001).
Archaeological Institute of America The Archaeological Institute of America (AIA) is North America's oldest society and largest organization devoted to the world of archaeology. AIA professionals have carried out archaeological fieldwork around the world and AIA has established re ...

Archaeology.org
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Hull shapes

Hulls come in many varieties and can have composite shape, (e.g., a fine entry forward and inverted bell shape aft), but are grouped primarily as follows: * Chined and hard-chined. Examples are the flat-bottom (chined), v-bottom, and multi-chine hull (several gentler hard chines, still not smooth). These types have at least one pronounced knuckle throughout all or most of their length. * Moulded, round bilged or soft- chined. These hull shapes all have smooth curves. Examples are the round bilge, semi-round bilge, and s-bottom hull.


Planing and displacement hulls

* Displacement hull: here the hull is supported exclusively or predominantly by
buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
. Vessels that have this type of hull travel through the water at a limited rate that is defined by the waterline length except for especially narrow hulls such as sailing multihulls that are less limited this way. * Planing hull: here, the planing hull form is configured to develop positive
dynamic pressure In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5 :q = \frac\rho\, u^2 where (in SI units): * is the dynamic pressure in pascals ( ...
so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and more rarely, round-bilged. The most common form is to have at least one chine, which makes for more efficient planing and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. An effective planing hull must be as light as possible with flat surfaces that are consistent with good sea keeping. Sailboats that plane must also sail efficiently in displacement mode in light winds. * Semi-displacement, or semi-planing: here the hull form is capable of developing a moderate amount of dynamic lift; however, most of the vessel's weight is still supported through buoyancy.


Hull forms

At present, the most widely used form is the round bilge hull. With a small payload, such a craft has less of its hull below the
waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...
, giving less resistance and more speed. With a greater payload, resistance is greater and speed lower, but the hull's outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.


Chined and hard-chined hulls

A chined hull does not have a smooth rounded transition between bottom and sides. Instead, its contours are interrupted by sharp angles where predominantly longitudinal panels of the hull meet. The sharper the intersection (the more acute the angle), the "harder" the chine. More than one chine per side is possible. The Cajun "pirogue" is an example of a craft with hard chines. Benefits of this type of hull include potentially lower production cost and a (usually) fairly flat bottom, making the boat faster at planing. A hard chined hull resists rolling (in smooth water) more than does a hull with rounded bilges (the chine creates turbulence and drag resisting the rolling motion, as it moves through the water, the rounded-bilge provides less flow resistance around the turn). In rough seas, this can make the boat roll more, as the motion drags first down, then up, on a chine: round-bilge boats are more seakindly in waves, as a result. Chined hulls may have one of three shapes: * Flat-bottom chined hulls * Multi-chined hulls * V-bottom chined hulls. Sometimes called hard chine. Each of these chine hulls has its own unique characteristics and use. The flat-bottom hull has high initial stability but high drag. To counter the high drag, hull forms are narrow and sometimes severely tapered at bow and stern. This leads to poor stability when heeled in a sailboat. This is often countered by using heavy interior ballast on sailing versions. They are best suited to sheltered inshore waters. Early racing power boats were fine forward and flat aft. This produced maximum lift and a smooth, fast ride in flat water, but this hull form is easily unsettled in waves. The multi-chine hull approximates a curved hull form. It has less drag than a flat-bottom boat. Multi chines are more complex to build but produce a more seaworthy hull form. They are usually displacement hulls. V or arc-bottom chine boats have a Vshape between 6°and 23°. This is called the angle. The flatter shape of a 6-degree hull will plane with less wind or a lower-horsepower engine but will pound more in waves. The deep Vform (between 18and 23degrees) is only suited to high-powered planing boats. They require more powerful engines to lift the boat onto the plane but give a faster, smoother ride in waves. Displacement chined hulls have more wetted surface area, hence more drag, than an equivalent round-hull form, for any given displacement.


Smooth curve hulls

Smooth curve hulls are hulls that use, just like the curved hulls, a centreboard, or an attached keel. Semi round bilge hulls are somewhat less round. The advantage of the semi-round is that it is a nice middle between the S-bottom and chined hull. Typical examples of a semi-round bilge hull can be found in the
Centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
and
Laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...
sailing
dinghies A dinghy is a type of small boat, often carried or towed by a larger vessel for use as a tender. Utility dinghies are usually rowboats or have an outboard motor. Some are rigged for sailing but they differ from sailing dinghies, which ...
. S-bottom hulls are sailing boat hulls with a midships transverse half-section shaped like an ''s''. In the s-bottom, the hull has round bilges and merges smoothly with the keel, and there are no sharp corners on the hull sides between the keel centreline and the sheer line. Boats with this hull form may have a long fixed deep keel, or a long shallow fixed keel with a centreboard swing keel inside. Ballast may be internal, external, or a combination. This hull form was most popular in the late 19th and early to mid 20th centuries. Examples of small sailboats that use this s-shape are the
Yngling The Ynglings were a dynasty of kings, first in Sweden and later in Norway, primarily attested through the poem '' Ynglingatal''. The dynasty also appears as Scylfings ( Old Norse ''Skilfingar'') in ''Beowulf''. When ''Beowulf'' and ''Ynglingatal' ...
and Randmeer.


Appendages

* Control devices such as a
rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally air or water). On an aircraft the rudder is used primarily to counter adve ...
,
trim tabs Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a pa ...
or stabilizing fins may be fitted. * A
keel The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in Br ...
may be fitted on a hull to increase the transverse stability, directional stability or to create lift. * Retractable appendages include centreboards and daggerboards. , Section E.1.2 Hull Appendage Types * A forward protrusion below the waterline is called a
bulbous bow A bulbous bow is a protruding bulb at the bow (or front) of a ship just below the waterline. The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability. Large ships w ...
. These are fitted on some hulls to reduce the wave making resistance drag and thereby increase
fuel efficiency Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device ...
. Bulbs fitted at the stern are less common but accomplish a similar task.


Terms

* Baseline is a level reference line from which vertical distances are measured. * Bow is the front part of the hull. * is the middle portion of the vessel in the fore and aft direction. *
Port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...
is the left side of the vessel when facing the bow from on board. *
Starboard Port and starboard are nautical terms for watercraft and aircraft, referring respectively to the left and right sides of the vessel, when aboard and facing the bow (front). Vessels with bilateral symmetry have left and right halves which ar ...
is the right side of the vessel when facing the bow from on board. *
Stern The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. Or ...
is the rear part of the hull. *
Waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...
is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.


Metrics

Hull forms are defined as follows: Block measures that define the principal dimensions. They are: * Beam or breadth (B) is the width of the hull. (ex: BWL is the maximum beam at the waterline) *
Draft Draft, The Draft, or Draught may refer to: Watercraft dimensions * Draft (hull), the distance from waterline to keel of a vessel * Draft (sail), degree of curvature in a sail * Air draft, distance from waterline to the highest point on a vesse ...
(d) or (T) is the vertical distance from the bottom of the keel to the
waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...
. *
Freeboard In sailing and boating, a vessel's freeboard is the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship. In commercial vessels, the latter criterion measured relativ ...
(FB) is depth plus the height of the keel structure minus draft. *
Length at the waterline A vessel's length at the waterline (abbreviated to L.W.L)Note: originally Load Waterline Length is the length of a ship or boat at the level where it sits in the water (the ''waterline''). The LWL will be shorter than the length of the boat over ...
(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 Length between perpendiculars (often abbreviated as p/p, p.p., pp, LPP, LBP or Length BPP) is the length of a ship along the summer load line from the forward surface of the stem, or main bow perpendicular member, to the after surface of the ster ...
(LBP or LPP) is the length of the summer load waterline from the
stern The stern is the back or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail. The stern lies opposite the bow, the foremost part of a ship. Or ...
post to the point where it crosses the
stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...
. (see also p/p) *
Length overall __NOTOC__ Length overall (LOA, o/a, o.a. or oa) is the maximum length of a vessel's hull measured parallel to the waterline. This length is important while docking the ship. It is the most commonly used way of expressing the size of a ship, and ...
(LOA) is the extreme length from one end to the other. * Moulded depth (D) is the vertical distance measured from the top of the keel to the underside of the upper deck at side., Annex 1, ''Regulations for determining gross and net tonnages of ships'', Reg. 2(2)(a). In ships with rounded
gunwale The gunwale () is the top edge of the hull of a ship or boat. Originally the structure was the "gun wale" on a sailing warship, a horizontal reinforcing band added at and above the level of a gun deck to offset the stresses created by firi ...
s, the upper measurement point is take to the point at which the planes of the deck and side plating intersect. ''Id.'', Reg. 2(2)(b). Ships with stepped decks are measured to a line parallel with the upper part. ''Id.'', Reg. 2(2)(c).
Form derivatives that are calculated from the shape and the block measures. They are: * Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull. * Longitudinal centre of buoyancy (LCB) is the longitudinal position of the centroid of the displaced volume, often given as the distance from a point of reference (often midships) to the centroid of the static displaced volume. 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. * Longitudinal centre of flotation (LCF) is the longitudinal position of the centroid of the waterplane area, usually expressed as longitudinal distance from a point of reference (often midships) to the centre of the area of the static waterplane. This can be visualized as being the area defined by the water's surface and the hull. * Vertical centre of buoyancy (VCB) is the vertical position of the centroid of displaced volume, generally given as a distance from a point of reference (such as the baseline) to the centre of the static displaced volume. * Volume (V or ∇) is the volume of water displaced by the hull. Coefficients help compare hull forms as well: # (Cb) is the volume (V) divided by the LWL × BWL × TWL. 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, beam (B) & 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. C_b = \frac # Midship coefficient (Cm or Cx) is the cross-sectional area (Ax) of the slice at midships (or at the largest section for Cx) divided by beam x draft. 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. C_m = \frac # Prismatic coefficient (Cp) is the volume (V) divided by LWLx Ax. It displays the ratio of the immersed volume of the hull to a volume of a prism with equal length to the ship and cross-sectional area equal to the largest underwater section of the hull (midship section). This is used to evaluate the distribution of the volume of the underbody. A low or fine Cp indicates a full mid-section and fine ends, a high or full 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 In continuum mechanics, the Froude number (, after William Froude, ) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). The Froude number is based on ...
will tend to have a low Cp. C_p = \frac # 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. C_w = \frac Note: C_b = C_ \cdot C_


Computer-aided design

Use of
computer-aided design Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve co ...
has superseded paper-based methods of ship design that relied on manual calculations and lines drawing. Since the early 1990s, a variety of commercial and freeware software packages specialized for naval architecture have been developed that provide 3D drafting capabilities combined with calculation modules for hydrostatics and hydrodynamics. These may be referred to as geometric modeling systems for naval architecture.


See also

* * * * * * * * * *
Hull classification symbol The United States Navy, United States Coast Guard, and United States National Oceanic and Atmospheric Administration (NOAA) use a hull classification symbol (sometimes called hull code or hull number) to identify their ships by type and by ...
* * * * * * * * * *


Notes


References

* * {{Authority control Shipbuilding Watercraft components Naval architecture Structural system Structural engineering Ship measurements Egyptian inventions