solid mechanics Solid mechanics (also known as mechanics of solids) is the branch of continuum mechanics that studies the behavior of solid materials, especially their motion and deformation (mechanics), deformation under the action of forces, temperature chang ...

structural engineering Structural engineering is a sub-discipline of civil engineering in which structural engineers are trained to design the 'bones and joints' that create the form and shape of human-made Structure#Load-bearing, structures. Structural engineers also ...

, and

shipbuilding Shipbuilding is the construction of ships and other Watercraft, floating vessels. In modern times, it normally takes place in a specialized facility known as a shipyard. Shipbuilders, also called shipwrights, follow a specialized occupation th ...

, hogging and sagging describe the shape that a

beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Radio beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially lo ...

or similar long object will deform into when loading is applied. ''Hogging'' describes a beam that curves upwards in the middle, and ''sagging'' describes a beam that curves downwards.

Ships

Dynamic stress

Hogging is the stress a ship's

hull Hull may refer to: Structures * The hull of an armored fighting vehicle, housing the chassis * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a sea-going craft * Submarine hull Ma ...

keel The keel is the bottom-most longitudinal structural element of a watercraft, important for stability. On some sailboats, it may have a fluid dynamics, hydrodynamic and counterbalancing purpose as well. The keel laying, laying of the keel is often ...

experiences that causes the center or the keel to bend upward. Sagging is the stress a ship's hull or keel is placed under when a wave is the same length as the ship and the ship is in the trough of two waves. This causes the middle of the ship to bend down slightly, and depending on the level of bend, may cause the hull to snap or crack. Sagging or dynamic hogging may have been what sank the ''

Prestige Prestige may refer to: Arts, entertainment and media Films *Prestige (film), ''Prestige'' (film), a 1932 American film directed by Tay Garnett: woman travels to French Indochina to meet up with husband *The Prestige (film), ''The Prestige'' (fi ...

'' off Spain on 19 November 2002. The 2013 loss of container ship '' MOL Comfort'' off the coast of

Yemen Yemen, officially the Republic of Yemen, is a country in West Asia. Located in South Arabia, southern Arabia, it borders Saudi Arabia to Saudi Arabia–Yemen border, the north, Oman to Oman–Yemen border, the northeast, the south-eastern part ...

was attributed to hogging. Subsequent lawsuits blamed the shipbuilder for design flaws.

Time-induced stress

Hogging, or "hog", also refers to the semi-permanent bend in the keel, especially in wooden-hulled ships, caused over time by the ship's center's being more buoyant than the bow or stern. At the beginning of her 1992 refit, had over 13 inches (33 cm) of hog. The keel blocks in the

drydock A dry dock (sometimes drydock or dry-dock) is a narrow basin or vessel that can be flooded to allow a load to be floated in, then drained to allow that load to come to rest on a dry platform. Dry docks are used for the construction, maintenance, ...

were set up especially to support this curve. During her three years in drydock, the center keel blocks were gradually shortened, allowing the hog to settle out. Additionally, the diagonal riders specified in her original design to resist hogging, which had been removed in an earlier refit, were restored. The similar-sized had 36 inches (91 cm) of hog before refitting in the mid-1990s.

Cargo loading

During loading and discharging cargo, ships bend (hog or sag) due to the distribution of the weights in the various holds and tanks on board. The maximum amount of cargo that a vessel can load often depends on whether her

Plimsoll mark The load line, also known as Plimsoll line, indicates the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves. The load line is ...

is submerged or not. Therefore, sagging can reduce her effective cargo capacity – especially if her loadline has already been reached prematurely due to the sag. This is taken into account when calculating

cargo In transportation, cargo refers to goods transported by land, water or air, while freight refers to its conveyance. In economics, freight refers to goods transported at a freight rate for commercial gain. The term cargo is also used in cas ...

, by applying what is called a "3/4 mean draft". This method is also called the "two-thirds mean correction", directly derived from Simpson's first rule.

Structural beams

In building construction, the sagging of beams is called "

deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...

". The amount of deflection varies in accordance with the beam's stiffness, the span between supports, and the load it carries. Sagging is a common problem in older houses.

References

External links

''Constellations 1994 restoration article
has several photos showing the hogging.

{{DEFAULTSORT:Hogging And Sagging Structural analysis Nautical terminology