A vessels hull may be comparable to a simple beam ( in fact a box section) under even loading (weight of structure plus contents) and supported evenly (buoyancy in water). Simple beam thery may be applied in calculating the compound bending and shear stresses caused by loading and wave action. In addition torsional stresses may be significant especially during grounding.
Stresses are divided into three groups
- Primary (structural) Affecting the Hull Girder
- Secondary (Local) Affecting definable areas within the Hull e.g. tank, bulkhead etc
- Tertiary (Very localized), Affecting small areas of plating or stifferners
The total stress in the hull is a combination of all three
Causes of Primary Structural stresses
With the vessel at a constant draft forces, both bending and shear, may be altered by movement of loads within a vessel. Were weights are locted such as to cause deflection downwards of the ends this is termed Hogging.
Were the weights are more centrally located such that the midships section deflects downwards then this is termed Sagging
Uneven point loading caused by tank, cargo and stores distribution as well as hull design cause stresses within the hull structure
All weights contained within the vessel are bourne by bouyancy resulting from water pressure. The effects of this pressure is to distort the structure inwards an is resisted by the hull plating and associated stifferning arrangements
Uneven water pressure caused by wave action leads to distortion of the structure as shown and is resisted by the Shear Stresses in the structure including most significantly the Transverse ulkheads and framing. Racking stresses are highest at the corners of the box section and thus the corner brackets are specially inspected.
When a vessel is docked on keel blocks only there is a tendency to sag transversely. This is reduced by includin additinal rows of blocks outbd of this
This leads to special case loading on the hull dependent on how the vessel has landed. Uneven loading on the hull structure will occur when the vessel is landed at one end due to uneven bouyancy distribution. Where the landing is narrow transerse sagging can also occur
Causes of Secondary and Tertiary Structural stresses
Caused by oscillatory motion on the shell plating at the Bow and Stern caused by the uneven water pressure as the vessel passes through waves. It is resisted by a system of Panting Beams, Panting Stringers, Breast Hook and a deep floor
Pounding or slamming.
Caused by bow pitching clear of the water then coming dow heavily on the sea. This is resisted by a reduction in framing pitch and possibly increased plate thickness
Local Loads. These are due to such as deck Machinery, hull design etc
Vibration. Localised stresses resisted by adjacent stresses are caused by vibrations from main engine, propellers etc