Static stresses and Dynamic stresses of a ship

Static stresses

Hogging:

• Longitudinal bending stresses could be caused while loading the ship in still water.
• Hogging is a stress that causes the ship's hull or keel to bend upwards at amidships.
• Look at the effect when the load concentration is more on the ends of the vessel.
• The ship structure bend downwards at the end.

Sagging: 

• When load concentration is more at the centre, the structure tends to bend downwards in the middle.
• This action is called sagging.
• The vessel is designed with the certain allowable maximum bending moment and shear forces which are tabulated in the approved ship's stability booklet.
• The loading and distribution of weights will be monitored by chief officer of the vessel according to the prescribed limits with the help of a load indicator and stability booklet.
• He is responsible for minimizing this 'SAG' and 'HOG' of the vessel.

Water pressure:

Act on the sides and bottom of the ship.

If the ship's sides are not strengthened, it will tend to bend inwards, as shown.

Drydock: 

• When the ship is in a dry dock, a ship's sides are no longer supported by water pressure.
• The only things holding the ship are keel blocks, bilge blocks and side shores.
• The result is that the sides of the ship tend to bulge outwards and the bottom tends to sag.
• To counteract drydocking stress, the entire bottom of a ship is strengthened.

Localised stress: 

• When a heavyweight is loaded over a small area or if there is a concentration of weight in a particular area, it causes localized stresses.
• Localized stress is counteracted by extra strengthening.

Dynamic stresses

Panting stress:

• Fluctuation in water pressure causes in and out movement of a ship's side plating at bow and stern.
• This movement results in panting stress.
• It is more pronounced at the bow, which pushes the bow ahead into the water or swell.
• This stress is compensated by fitting panting beams with a distance of two metres between every alternate frames.

• The panting beams are connected to the frames by beam knees and supported by a wash plate at the Centre.
• Side stringers, in line with panting stringers, are fitted throughout this deep framing region.
• Breast hooks fitted to support the radiused stem plate and to prevent in and out movement of the side shell.

Pounding stress:

• Pounding stress occurs at the bottom plating of a ship near the bow during excessive pitching.
• To compensate slamming down of a ship's bottom, outer bottom plating is thickened and connections to inner shell and inner bottom girder are strengthened.

• The thickness of plates near the pounding is increased by thirty percent and transverse frame spacing is reduced from nine hundred millimeters; to seven hundred millimetres.
• Solid floors fitted at every frame space are welded to the bottom shell and transverse floors are fitted at alternate frames.

Racking stress:

• When a ship rolls in a seaway, there is a tendency for the ship's side to get deformed.
• This deformation is caused when the deck moves laterally relative to the bottom structure.
• This in turn causes the side shells to move vertically relative to each other.
• Such deformation is called racking stress.

• This stress is more pronounced on the corners of the ship due to wave action on the hull.
• Racking stresses are reduced by fitting beam knees and tank side brackets.
• But the most effective way to resist racking is to fit transverse bulkheads.