Inclining Test

It is an experiment performed on a ship to determine its:

• Stability (GM)
• Vertical centre of gravity (KG)
• Light ship weight (Displacement)

The metacentric height (GM) of a ship plays a vital role in setting loading capacity & stability of ship. The initial metacentric height can be estimated from its design. But the actual value of GM is determined by an inclining experiment after the ship is completely built.

This test is applied to:

• Newly constructed ships greater than 24 m in length
• Ships which have undergone major structural changes that could affect stability

The test procedure is specified by IMO.

Requirements/Preparation before test:

1. The experiment is carried out in light ship condition: i.e. The ship should be empty or as near to empty as possible.
2. The ship must be in upright position with reasonable trim. (The trim should be such that the deviation from her design trim does not exceed 1% of total length of ship)
3. The ship should be sheltered in calm water. Ideally there should be no wind, current & tide. Water depth should be sufficient enough.
4. Mooring ropes should be slack enough to allow the ship to be inclined without restraint. Gangway should be lifted.
5. All tanks in the ship must be empty or pressed up tight to reduce free surface effect.
6. The contents & weights of all the ship’s compartments should be known.
7. Deck should be free of water & anything unnecessary or temporary should be removed. All derricks, boats etc. should be secured in their sea-going condition.
8. Only the people required to conduct the experiment should be on board.
9. Ship’s light condition displacement should be known from builder.
10. Ship’s forward, mid & aft draft and water density should be known.

Description of the instrument arrangement:

1. The ship, in its upright position & light condition, is fitted with a wire plumb line (Pendulum) from a high point on the transverse centre line.
2. The wire should be piano wire or of any other monofilament material. The top connection of the pendulum should afford unrestricted rotation of pivot point. Example: The pendulum wire is attached with a washer & suspended from a nail.
3. The pendulum should be long enough to give a deflection of at least 15 cm to each side of upright position. It is recommended that pendulum lengths should be used of 4 to 6 m in lengths.
4. The plumb bob of the pendulum is set into a horizontal trough, which is filled with light oil or any other viscous substance to dampen the oscillation of the bob, when the vessel is inclined. The trough should be deep enough to prevent the plumb bob from touching the bottom.
5. The edge of the trough should be fastened with a wooden batten, which is graduated with a mm scale.
6. The batten should be:
   • Smooth
   • Light colored wood
   • 1 to 2 cm thick
   • Securely fixed with a wooden saw horse

   So that an inadvertent contact will not cause them to shift when inclined. The batten should be aligned close to the pendulum wire but not in contact with it.

7. Four test weights are placed on board just prior to test. These weights are fitted with a wheeled platform usually, to assist movement on the deck during test period.
8. All test weights should be verified prior to experiment in the presence of a class surveyor using a recently calibrated scale.
9. All weights should be of equal mass. The total mass of the weights should be at least 1/500 of the light ship weight.

Experiment: (Procedure)

1. Four masses are placed on the ship’s deck, two on each side of the mid ship, placed away from the centre line.
2. In the next step, the masses are moved transversely only, one at a time until all four are on the same side, then, all four on the other side & lastly two on each side.
3. For all the movement of the mass; the deflection of the pendulum is recorded. An average of this deflection is used to determine KG of the ship.$$K_G = \frac{\omega \times d}{W}$$
4. IMO code for intact stability states that while conducting this experiment, heel produced for any shift of mass shall not be less than 2 degrees and not more than 4 degrees. For large ships, lower limit of heel can be up to 1 degree. The deflection of the pendulum bob should not be less than 15 cm.
5. By the following formula we can calculate GM: $$GM = \frac{\omega \times d}{w} \times \frac{AB}{BC}$$ Where:
   • ω = mass moved weight (Tonne)
   • d = distance moved by mass (meter)
   • ω x d = list moment (tonne-meter)
   • W = Ship’s weight in light condition (Tonne)
   • AB = length of plumb line when upright (meter)
   • BC = Average deflection of plumb line (meter)

So after successful and satisfactory completion of the test, class surveyor will endorse the intact stability booklet with inclining test certificate and this will eventually lead to successful achievement of safety construction certificate.

Note: Stabilograph consists of a heavy metal pendulum balanced on a knife edge and connected to a pointer to record the heel angle readings.

Read another note on inclining experiment: INCLINING EXPERIMENT by marineinbox