Hull deformation

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Part of a series on vessels'

Structure

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Concepts

Hull Deformation

A ships hull is subjected to a multitude of ship loads from service, cargo and environmental conditions. The natural response of the hull is to deform to an extent which these forces are balanced. Larger ships (complete with larger loading moments) can risk considerable damage or failure of the hull and its constituent structure if these deflections are too great.

Contents

Longitudinal

Longitudinal deformations are typically caused by static imbalances between the vessels distribution of weight and distribution of buoyancy. These are perhaps most pronounced in bulk carriers: the hull is particularly full through the cargo hold areas. When empty, they provide excess buoyancy through amidships, a hogging situation. When loaded, particularly when care is not taken in distributing load through the length of the vessel, they can carry excess weight through midships, leading to sagging.

Waves can also induce transient hogging or sagging conditions. When wavelengths approach a vessels length, pronounced changes in the distribution of buoyancy can occur; for example, a crest at midships decreases buoyancy in the vessels ends and increases it at it's centre. The weight distribution remains the same, resulting in a hogging condition. In the opposite condition, a trough at midships decreases buoyancy through midships, causing the vessel to sag. If the induced stress is great enough, wooden ships may take on water from the separation and twisting of planking. Steel ships can suffer from fatigue or cracking.

For vessels particularly prone to longitudinal stresses, such as by load cases (as described above), or from normal seaways loads where the vessel is sufficiently long, a longitudinal framing system is generally adopted. This creates continuous longitudinal structures in the ordinary stiffeners (in addition to girders, stringers & deck, shell & bottom plate) to increase longitudinal strength and stiffness (see hull girder).

Measurement

A hull's longitudinal deflection can be determined from multiple draft readings, similar to a lightship survey (where applicable, lightship surveys should take deflection into account). Freeboard measurements are taken along the length of a vessel at multiple locations. When plotted against the vessels linesplan, the direction and magnitude of deflection can be visually observed by the resulting curvature in the waterline.

Hogging

Hogging is the bending upwards about amidships of a ships hull, about it's transverse axis. Early wooden vessel designs often adopted a natural hogging moment, some to the degree that a hogging truss were fitted.

Sagging

Sagging is the bending downwards about amidships of a ships hull, about it's transverse axis. Sagging conditions are typical of laden cargo vessels with holds placed either at amidships or slightly forward.

Transverse

Transverse deformations are typically the results of deck cargo loads, or tall & lightly built superstructures, in rolling seas. Transverse deformations are known as racking, and are opposed by continuous transverse structures - primarily, bulkheads.

Racking

Racking is deformation of the hull in the transverse direction, about the longitudinal axis.

Hydrodynamic

Hydrodynamic deformations are caused by the flow of water across the hull and the operation of the propulsion machinery.

Vibration

Vibration, knocking and shudders can be caused by the operation of propellers. This can be particularly felt at natural resonant frequencies of the propeller/hull system, during aggressive maneuvers with the propeller heavily loaded or cavitating (such as emergency stops), or when there is an insufficient propeller aperture. Azimuthing drives can generate rapidly changing pressure conditions in way of the propulsors, such as when the drives point in opposition. These conditions can cause local deformation of the shell plate in way of the propulsion units.

Panting

Panting is a in-and-out deformation of shell plate in the bow. Panting is caused by the cyclical loading of the plate by consecutive waves from a seaway. The effects of panting can be diminished by decreasing frame spacing, increasing bow plate thickness, or adding panting frames.

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