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Half-hull of the 46-gun ship of the line Tigre, build from 1724 in Toulon after plans by Blaise Coulomb

A hull is the watertight body of a ship or boat. Above the hull comes the maindeck (except in an open small boat), and various superstructure. The line where the hull meets the water surface is called the waterline.

The structure of the hull varies depending on the vessel type. In a typical modern steel ship, the structure consists of major transverse and longitudinal members called watertight (and also sometimes non-tight) bulkheads, intermediate members such as girders, stringers and webs, and minor members called ordinary transverse frames, frames, or longitudinals, depending on the structural arrangement.

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. In fiberglass or composite hulls, the structure may resemble wooden or steel vessels to some extent, or be of a monocoque 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.

"Hull Form"

Contents

General features

The shape of the hull is entirely 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 (load carrying and stability) and hydrodynamics (speed, powering, and dynamic motion behavior).

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:

  • Moulded, round bilged or soft-chined. Examples are the round bilge, semi-round bilge and s-bottom hull.
defined as smooth curves
  • Chined and Hard-chined. Examples are the flat-bottom (chined), v-bottom and multi-bottom hull (hard chined).
have at least one pronounced knuckle throughout all or most of their length

Categorisation

After this they can be categorized as:

  • Displacement
the hull is supported exclusively or predominantly by buoyancy. They travel through the water at a limited rate which is defined by the waterline length. They are often heavier than planing types, though not always.
  • 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 buoyancy
  • Planing
Royal Navy World War II MTB planing at speed on calm water showing its Hard chine hull - note how most of the forepart of the boat is out of the water
the planing hull form is configured to develop positive dynamic pressure 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 sometimes 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. (see: Planing (sailing), Hull speed).

Most used hull forms

At present, the most widely used form is the round bilge hull.[1]

The inverted bell shape of the hull, with smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With higher payload the outward bend provides smoother performance in waves. As such, the inverted bell shape is a popular form used with planing hulls.

Hull forms

Smooth curve hulls

Smooth curve hulls are hulls which use, just like the curved hulls, a sword 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 and Laser cruising dinghies.

S-bottom hull (A), compared to a hard (B) and soft (C) chine hull

S-bottom hulls are hulls shaped like an s. In the s-bottom, the hull runs smooth to the keel. As there are no sharp corners in the fuselage. Boats with this hull have a fixed keel, or a kielmidzwaard. This is a short keel which still sticks a sword. Examples of cruising dinghies that use this s-shape are the yngling and Randmeer.

Chined and hard-chined hulls

A chined hull consists of straight plates, which are set at an angle to each other. The chined hull is the most simple hull shape because it worked only with straight planks. These boards are often bent lengthwise. Most home-made constructed boats are chined hull boats. Benefits of this type of boating activity is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Chined hulls can also make use of a sword or attached keel.

Chined hulls can be divised up into 3 shapes:

  • V-bottom chined hulls
  • flat-bott chined hulls
  • and multi-chined hulls.

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. (see also: Naval architecture)
  • A keel may be fitted on a hull to increase the transverse stability, directional stability or to create lift.
  • 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

Port 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.

Metrics

Principal hull measurements
"LWL & LOA"
"Beam, draft & Depth"

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 at 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. (see also p/p)
  • Beam or breadth (B) is the width of the hull. (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.[2]
  • 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.
  • 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[3] 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 Lpp, 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 {V}{L_{pp} \cdot B \cdot T}
2) 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 {A_m}{B \cdot T}
3) Prismatic Coefficient (Cp) is the volume (V) divided by Lpp 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.
 C_p = \frac {V}{L_{pp} \cdot A_m}
4) Waterplane Coefficient (Cw) is the waterplane area divided by Lpp x B. 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_{pp} \cdot B}
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. Hull form then proceeded to the Coracle shape and on to more sophisticated forms as the science of Naval architecture advanced.

Notes

  1. ^ [Zeilen:Van beginner tot gevorderde by Karel Heijnen]
  2. ^ "International Convention on Tonnage Measurement of Ships, 1969". International Conventions. Admiralty and Maritime Law Guide. 1969-6-23. http://www.admiraltylawguide.com/conven/tonnage1969.html. Retrieved 2007-10-27.  , 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. Id., Reg. 2(2)(b). Ships with stepped decks are measured to a line parallel with the upper part. Id., Reg. 2(2)(c).
  3. ^ 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 ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87038-056-X.  

See also

External links


Simple English


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.

Contents

General features

Nearly all watercraft, from small boats to the largest ships, have a general form that is necessary for stability and efficient propulsion, which includes:

  • 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).
  • 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.

How does it float?

Archimedes, a Greek doctor, inventor, and mathematician, made the science of "hydrostatics", the study of liquids and pressure.[1] The Archimedes principle says that the force holding up an object that is inside a liquid in part or totally. So, a ship actually does sink, until it moves an amount of water that is the same weight of the weight of the ship. The weight must be over a wide area to work. This explains why ships and boats (including submarines) float. An object's weight pulls down in the direction of gravity, but the water's buoyant (floating) force pushes it up. Because these forces counteract each other, the object seems to lose weight. NASA uses this principle to prepare their astronauts for the weightlessness of space, so they had the astronauts work on parts of the space ship inside water tanks 25 feet (7.6 m) deep.[1]

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.

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 (see also o/a)
  • 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. (see also w/l)
  • 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.
  • Form Derivatives that are calculated from the shape and the Block Measures. They are:
  • Volume (V) 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.

Other pages

References

  1. 1.0 1.1 Sherwin, Frank (2004). The Ocean Book. P.O. Box 726, Green Forest, AR 72638: Master Books. ISBN 0-89051-401-1. 

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