3.1.1 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.
3.1.2 Free surface effects should be considered
whenever the filling level in a tank is less than 98% of full condition.
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 under condition specified in 3.1.12.footnote
But nominally full cargo tanks should be corrected for free
surface effects at 98% filling level. In doing so, the correction
to initial metacentric height should be based on the inertia moment
of liquid surface at 5° of heeling angle divided by displacement,
and the correction to righting lever is suggested to be on the basis
of real shifting moment of cargo liquids.
3.1.3 Tanks which are taken into consideration
when determining the free surface correction may be in one of two
categories:
-
.1 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; or
-
.2 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 3.1.5 and 3.1.6, the free surface correction should
be the maximum value attainable between the filling limits envisaged
for each tank, consistent with any operating instructions.
3.1.4 In calculating the free surface effects
in tanks containing consumable liquids, it should be assumed that
for each type of liquid at least one transverse pair or a single centreline
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.
3.1.5 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.
3.1.6 For ships engaged in liquid transfer operations,
the free surface corrections at any stagefootnote of
the liquid transfer operations may be determined in accordance with
the filling level in each tank at that stage of the transfer operation.
3.1.7 The corrections to the initial metacentric
height and to the righting lever curve should be addressed separately
as follows.
3.1.8 In determining the correction to initial
metacentric height, the transverse moments of inertia of the tanks
should be calculated at 0° angle of heel according to the categories
indicated in 3.1.3.
3.1.9 The righting lever curve may be corrected
by any of the following methods subject to the agreement of the Administration:
-
.1 correction based on the actual moment of fluid
transfer for each angle of heel calculated; or
-
.2 correction based on the moment of inertia,
calculated at 0° angle of heel, modified at each angle of heel
calculated.
3.1.10 Corrections may be calculated according
to the categories indicated in 3.1.2.
3.1.11 Whichever method is selected for correcting
the righting lever curve, only that method should be presented in
the ships stability booklet. However, where an alternative method
is described for use in manually calculated loading conditions, an
explanation of the differences which may be found in the results,
as well as an example correction for each alternative, should be included.
3.1.12 Small tanks which satisfy the following
condition corresponding to an angle of inclination of 30°, need
not be included in the correction:
where:
-
M fs
free surface moment
(mt)
-
Δmin
is the minimum
ship displacement calculated at dmin (t)
-
dmin
is the minimum mean
service draught of the ship without cargo, with 10% stores and minimum
water ballast, if required (m).
3.1.13 The usual remainder of liquids in empty
tanks need not be taken into account in calculating the corrections,
provided that the total of such residual liquids does not constitute
a significant free surface effect.