Explain the use of KN curves.

Stability Cross Curves for a ship are constructed by plotting the righting levers for an assumed height of the centre of gravity above the keel. In some cases the curves are constructed for an assumed KG of zero. The curves are then referred to as KN curves, KN being the righting lever measured from the keel.

To obtain the righting levers for a particular displacement and KG the values of KN are first obtained from the curves by inspection at the displacement concerned. The correct righting levers are then obtained by subtracting from the KN values a correction equal to the product of the KG and sin heel.

List the precaution necessary before an inkling experiment is carried out.

In order to find the KG for the light condition that the Inclining Experiment is performed. The experiment is carried out by the builders when the ship is as near to completion as possible; that is, as near to the light condition as possible.

The following conditions are necessary to ensure that the KG obtained is as accurate as possible:

1. There should be little or no wind, as this may influence the inclination of the ship. If there is any wind the ship should be head on or stern on to it.

2. The ship should be floating freely. This means that nothing outside the ship should prevent her from listing freely. There should be no barges or lighters alongside; mooring ropes should be slacked right down, and there should be plenty of water under the ship to ensure that at no timeduring the experiment will she touch the bottom.

3. Any loose weights within the ship should be removed or secured in place.

4. There must be no free surfaces within the ship. Bilges should be dry. Boilers and tanks should be completely full or empty.

5. Any persons not directly concerned with the experiment should be sent ashore.

6. The ship must be upright at the commencement of the experiment.

7. A note of ‘weights on’ and ‘weights off’ to complete the ship each with a VCG and LCG.