This experiment is carried out on new ships and those that have had large structural alterations made, to find out the new centre of gravity (kg) and metacentric height (gm) and further to know the stability of the ship.
- During the experiment one or more weights are moved through a known distance ‘l’, transversely across the deck.
- The resulting angle of heel ‘ǿ’ is measured by means of two pendulum, one at each end of the ship (or by using an instrument known as stabilograph).
- When pendulum is used, deflection of pendulum is measured against a wooden batten.
Note : pendulum should be as long as possible, to improve the accuracy of measurement, and a fine wire with heavy bob to ensure free movement. The bob may be immersed in oil or water to dampen it’s movement.
- Refer to figure,
as mass ‘W’ is moved across the deck through a distsnce ‘L’, the centre of gravity of the ship moves from ‘G’ to ‘G1’.
W x L = ∆ x GG1 ————————————(1)
where ∆ = displacement
with this ship will be inclined to some angle ‘ǿ’ and will be in stable equilibrium state. The centre of buoyancy will move from ‘B’ to ‘B1’, vertically beneath G1.The movement of ‘GG1’ is parallel to the movement of ‘W’ and therefore angle ‘G1GM’ is right angle.
GG1 = GM x tanǿ ——————————–(2)
CONDUCTING INCLINING EXPERIMENT
- The inclining experiment is carried out on the ship when it is as near complete as possible and in almost light weight condition. Because with light weight condition any adjustment on load can be made to establish the centre of gravity.
- Procedure :
- Two sets of weights are used, one on each side of the ship.
- The inclination for the movement of each weights both to port, then back again and then to starboard and back again is noted.
- Displacement of the ship is determined by noting the draught of ship & the specific gravity of water in which ship is floating.
PRECAUTIONS DURING EXPERIMENT
- A calm day with no wind should be chosen
- The ship should be free to move with all the moorings slack.
- Any loose items should be secured.
- All tanks should be empty or full to avoid free surface effect.
- As few personnel as possible should be on board and they should be on the centreline of ship, when readings are being taken.
- A large trim should be avoided.
- The Metacentric height of the ship plays an important role in determining the loading capacity and stability of the ship. The Initial metacentric height of the ship is determined by an inclining experiment after the ship is completely built in the yard