Effect of free surfaces of liquids in tanks are :
- For all loading conditions, the initial metacentric height and the righting lever curve should be corrected for the effect of free surfaces of liquids in tanks.
- Free surface effects should be considered whenever the filling level in a tank is less than 98% of full Free surface effects need not be considered where a tank is nominally full, i.e. filling level is 98% or above. Free surface effects for small tanks may be ignored .
- Tanks which are taken into consideration when determining the free surface correction may be in one of two categories:
- Tanks with filling levels fixed (e.g. liquid cargo, water ballast). The free surface correction should be defined for the actual filling level to be used in each tank.
- Tanks with filling levels variable (e.g. consumable liquids such as fuel oil, diesel oil and fresh water, and also liquid cargo and water ballast during liquid transfer operations). Except as permitted in paragraph 5 and 6, the free surface correction should be the maximum value attainable between the filling limits envisaged for each tank, consistent with any operating instructions.
- In calculating the free surface effects in tanks containing consumable liquids, it should be assumed that for each type of liquid at least one trans- verse pair or a single centre line tank has a free surface and the tank or combination of tanks taken into account should be those where the effect of free surfaces is the greatest.
- Where water ballast tanks, including anti-rolling tanks and anti heeling tanks, are to be filled or discharged during the course of a voyage, the free surface effects should be calculated to take account of the most onerous transitory stage relating to such operations.
- For ships engaged in liquid transfer operations, the free surface corrections at any stage of the liquid transfer operations may be determined in accordance with the filling level in each tank at that stage of the transfer operation.
- The corrections to the initial metacentric height and to the righting lever curve should be addressed separately as follows.
- In determining the correction to initial metacentric height, the transverse moments of inertia of the tanks should be calculated at 0 degree angle of heel according to the categories .
- The righting lever curve may be corrected by any of the following methods subject to the agreement of the Administration:
- Correction based on the actual moment of fluid transfer for each angle of heel calculated.
- Correction based on the moment of inertia, calculated at 0 degree angle of heel, modified at each angle of heel calculated.
- Correction based on the summation of M fsvalues for all tanks taken into consideration
The values of Mfs for each tank may be derived from the formula:
M f =vbρk(d)1/2
where:
Mfs is the free surface moment at any inclination, in m.tonnes
v is the tank total capacity, in m3
b is the tank maximum breadth, in m
ρ is the mass density of liquid in the tank, in tonnes/m3
d is equal to v/blh (the tank block coefficient)
h is the tank maximum height, in m
1 is the tank maximum length, in m
k is the dimensionless coefficient to be determined from table.
