Naval Arch(rolling And Stabilisation).pptx

Naval Architecture

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Rolling and stabilisation  When a ship is heeled by an external force, and the

force is suddenly removed, the vessel will roll to port and starboard with a rolling period which is almost constant.  This is known as the ship’s natural rolling period.  The amplitude of the roll will depend upon the applied heeling moment and the stability of the ship.  For angles of heel up to about 15° the rolling period does not vary with the angle of roll. 4/3/2019

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Rolling and stabilisation  The angle reduces slightly at the end of each swing and

will eventually dampen out completely .  This dampening is caused by the frictional resistance between the hull and the water, which causes a mass of entrained water to move with the ship.

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Rolling and stabilisation  The natural rolling period of a ship may be estimated

by the formula: Rolling period P = (2Πk)/(√g . GM) seconds where GM is the metacentric height, and k is the radius of gyration of the loaded ship about a longitudinal polar axis.  Thus a large metacentric height will produce a small period of roll, although the movement of the ship may be decidedly uncomfortable and possible dangerous. 4/3/2019

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Rolling and stabilisation  A small metacentric height will produce a long period

of roll and smooth movement of the ship.  The value of the radius of gyration will vary with the

disposition of the cargo.  In tankers and OBO vessels it is possible to change the

radius of gyration and not as easy to change the metacentric height. 4/3/2019

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Rolling and stabilisation  If for such vessels the cargo is concentrated in the

centre compartments, with the wing tanks empty, the value of the radius of gyration is small, producing a small period of roll.  If, however, the cargo is concentrated in the wing

compartments, the radius of gyration increases, producing a slow rolling period.

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Rolling and stabilisation  Problems may occur in a ship which travels in a beam

sea, if the period of encounter of the waves synchronises with the natural frequency of roll.  So even with a small wave force the amplitude of roll

may increase to alarming proportions.  In such cirumstances it may be necessary to change

the ships heading and alter the period of encounter of the waves. 4/3/2019

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Rolling and stabilisation  Reduction of Roll :  When ships were first built of iron instead of wood a bar

keel was fitted, one of its advantages being that it acted as an anti-rolling device.

 Then, with the fitting of the flat plate keel the anti-rolling

properties were lost. So an alternative method was supplied in the form of ‘Bilge Keels’ which are now used in the majority of the ships.

 These projections are arranged at the bilge to lie above the

line of the bottom shell and within the breadth of the ship, thus being partially protected against damage.

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Rolling and stabilisation  The depth of the bilge keels depends to some extent on

the size but there are two main factors to be considered : a) The web must be deep enough to penetrate the

boundary layer of water travelling with the ship. b) If the web is too deep the force of water when rolling may cause damage.

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Rolling and stabilisation  ‘Bilge keels’ 250mm to 400mm in depth are fitted to

ocean going ships.  The keels extend for about one half of the length of the ship amidships and are tapered gradually at the ends.  This ‘Bilge Keels’ are fitted in two parts, the connection to the shell plating being stronger than the connection between the two parts.  In this way it is more likely, in the event of damage, that the web will be torn from the connecting angle than the connecting angle from the shell. 4/3/2019

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Rolling and Stabilisation  The ‘Bilge Keels’ reduce the initial amplitude of roll

as well as subsequent movements.

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Rolling and Stabilisation

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