Simplified Stability Data

Stability Information may be supplied in a simplified form

• It has become evident that the masters’ task of ensuring that his ship complies with the minimum statutory standards of stability is in many instances not being adequately carried out.• This simplified presentation of stability information has

been adopted in a large number of small ships and is considered suitable for wider application in order to overcome the difficulties

• Considerable experience has now been gained and three methods of presentation are in common use. These are:• (a) The Maximum Deadweight Moment Diagram or Table.• (b) The Minimum Permissible GM Diagram or Table.• (c) The Maximum Permissible KG Diagram or Table.

• In any condition of loading the KG from line D of the Displacement Table or as you just calculated, must lie within the area under the curve.

• In Condition No. 5, with the actual amount of fresh water on board, the vessel’s KG is 3.28 metres. Is this KG too high for adequate stability? Now consult the Curve of Limiting KG Table shown to the left. This curve tells us that in any condition of load the KG of the vessel must be below the curve indicated. Now enter the curve with the displacement of the vessel (187.16 tonnes) along the bottom, and the KG (3.28 metres) on the vertical. The black dot representing these two values is indeed below the curve of limiting KG and the vessel has adequate stability.

• Curve of maximum KG or Minimum GM to ensure adequate stability in the event of partial loss of intact buoyancy are provided in passenger ship.

Free Surface Moment

• The free surface effect is a mechanism which can cause a craft to become unstable and capsize. It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior approximates that of liquids — to move in response to changes in the attitude of a craft's cargo holds, decks, or liquid tanks in reaction to operator-induced motions (or sea states caused by waves and wind acting upon the craft). When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".

• Free surface moments are to be added to the deadweight moment when using the diagram of maximum deadweight moment.

MAXIMUM PERMISSIBLE DEADWEIGHT–MOMENT CURVE

• This is simply a graph of displacements against maximum-deadweight moments. The deadweightmoment has been calculated about the keel. Note that this ship’s Lightweight is 370tonnes.

• It is very important to realise that the total deadweight–moment at any displacement must not under any circumstances exceed the maximum deadweight–moment at this displacement.

• For example when the displacement is 1000tonnes, then a deadweight–moment of 1200tm is acceptable. However, a deadweight-moment of 1480tm would mean this ship has deficient stability and is not acceptable. A deadweight–moment value of 1375tm would be just allowable.

• If the maximum KG for this 1375tm is required it is simply:

• Total deadweight moment or KG is less than the maximum permissible value, the ship will have adequate stability.

Deadweight moment

• Mass in tonnes times vertical height of the mass above the keel.

Maximum KG = Deadweight Moment Deadweight