Hull (watercraft)

Hull form lines, lengthwise and in cross-section

The hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. 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 watertight and non-tight decks, major transverse and watertight (and also sometimes non-tight or longitudinal) members called bulkheads, intermediate members such as girders, 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", or simply "deck". The particular name given depends on the context—the type of ship or boat, the arrangement, or even where it sails. Not all hulls are decked (for instance a dinghy).

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.

History

Cross-section of the Swedish warship, Vasa.

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.

By around 3000 BC, Ancient Egyptians knew how to assemble wooden planks into a hull.[1]

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 (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 or the flat bottom of a landing craft.

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:

Categorization

After this they can be categorized as:

The hull is supported exclusively or predominantly by buoyancy. Vessels that have this type of hull travel through the water at a limited rate that is defined by the waterline length. They are often, though not always, heavier than planing types.

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 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. Sail boats that plane must also sail efficiently in displacement mode in light winds.

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.

Most used hull forms

At present, the most widely used form is the round bilge hull.[2]

In the inverted bell shape of the hull, with a smaller payload the waterline cross-section is less, hence the resistance is less and the speed is higher. With a 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

Chined and hard-chined hulls

A chined hull consists of straight, smooth, tall, long, or short plates, timbers or sheets of ply, which are set at an angle to each other when viewed in transverse section. The traditional chined hull is a simple hull shape because it works with only straight planks bent into a curve. These boards are often bent lengthwise. Plywood chined boats made of 8' x 4' sheets have most bend along the long axis of the sheet. Only thin ply 3–6 mm can easily be shaped into a compound bend. Most home-made constructed boats are chined hull boats. Mass-produced chine powerboats are usually made of sprayed chop strand fibreglass over a wooden mold. The Cajun "pirogue" is an example of a craft with hard chines. Benefits of this type of hull is the low production cost and the (usually) fairly flat bottom, making the boat faster at planing. Sail boats with chined hull make use of a dagger board or keel.

Chined hulls can be divided up into three shapes:

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 sail boat. 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 V shape between 6 and 23 degrees. This is called the deadrise angle. The flatter shape of a 6 degrees hull will plane with less wind or a lower horse power engine but will pound more in waves. The deep V form (between 18 and 23 degrees) is only suited to high power 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 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.

(A) S-bottom hull
compared to a
(B) hard and
(C) soft 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 (literally "keel with sword"). This is a short fixed keel, with a swing keel inside. Examples of cruising dinghies that use this s-shape are the Yngling and Randmeer.

Appendages

  • Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.
  • A keel may be fitted on a hull to increase the transverse stability, directional stability or to create lift.
  • 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)

Terms

Metrics

Principal hull measurements
"LWL & LOA"

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 (d) or (T) is the vertical distance from the bottom of the keel to the waterline.
  • Freeboard (FB) is depth plus the height of the keel structure minus draft.
  • 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)
  • Length overall (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.[3]
  • 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 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.
  • 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.
  • 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.
  • Volume (V or ∇) is the volume of water displaced by the hull.
  • Coefficients[4] 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, 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.
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.
3) Prismatic coefficient (Cp) is the volume (V) divided by Lpp x 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 will tend to have a low Cp.
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.
Note:

See also

Notes

  1. ↑ Ward, Cheryl. "World's Oldest Planked Boats," in Archaeology (Volume 54, Number 3, May/June 2001). Archaeological Institute of America. Archaeology.org
  2. ↑ Zeilen: Van beginner tot gevorderde by Karel Heijnen
  3. ↑ "International Convention on Tonnage Measurement of Ships, 1969". International Conventions. Admiralty and Maritime Law Guide. 1969-06-23. 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).
  4. ↑ Rawson, E.C.; Tupper (1976). Basic Ship Theory. 1 (2nd ed.). Longman. pp. 12–14. ISBN 0-582-44523-X.

References

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