285-Vessel Stability / 748
Vessel Stabilit
y
BOS-PRD-07.900601
4
.1.
7
Stabilit
y
s
of
twa
r
e l
o
a
d
i
n
g
c
o
nd
iti
o
n
test.
The purpose is to ensure the operational accuracy of the stability software and test records are maintained.
During the loading test, one of the ship's
Se
n
i
o
r
O
ff
i
c
e
r
s
is to operate the loading instrument and calculate
the test conditions using the
BOS-INS-0701.1403 - Test Procedure for Loading Computer Systems
.The
results obtained from the loading software are to be identical to those stated in the stability booklet.
This test
shall
be perfomed at least annually.
4
.1.
8
F
r
ee s
ur
f
a
c
e e
ff
e
c
t
If a tank is completely filled with liquid, no movement is possible and stability is precisely the same as if the
tank contained solid material. When a quantity of liquid is withdrawn from the tank the situation changes and
the stability of the ship is adversely affected by what is known as the free surface effect.
The adverse effect on the stability is referred to as a loss in metacentric height (GM) or a virtual rise in
Vertical Centre of Gravity (VCG).
The free surface effects of all slack tanks
shall
be taken into account in any calculations.
When it is intended to introduce or discharge ballast water during a voyage the
Maste
r
shall
prior to
departure ensure that the required stability criteria is maintained at all times due to loss of stability by partially
filled tanks.
The free surface
shall
be kept to a minimum; if possible only one tank of each kind of liquid
shall
have a free
surface. The minimum GM
shall
be maintained as specified in the
Stability Booklet
.
When a tank is to be ballasted at sea, to counteract the loss of consumables it
shall
be noted that the tanks
have a complete free surface as soon as ballasting commences and the free surface
shall
adversely affect
the stability of the vessel before any benefit can be assumed from the weight of ballast water taken on board
Free surface values for different levels can be found in the tanks capacity tables.
4
.1.
9
F
r
ee s
ur
f
a
c
e i
n
te
r
a
c
ti
o
n
s
o
n
stabilit
y
The maximum free surface moment of each tank
shall
be used when they are not full (pressed-up), or are
not empty (stripped dry), throughout a given voyage. The influence of trim is usually
n
o
t
taken into account
when hand calculating the free surface moments. Manual calculations are conservative because maximum
free surface correction is used, whereas software calculates real free surface effect.
4
.1.1
0 Gu
a
rd
i
n
g
a
g
ai
n
st a
nd
p
r
e
v
e
n
ti
n
g
f
r
ee s
ur
f
a
c
e e
ff
e
c
t
To guard against and prevent free surface effect the number of partially filled tanks
shall
be kept to a
minimum. Tanks that are either completely full or completely empty do not have a free surface effect and
therefore do not reduce the vessel’s metacentric height (GM).
Anti-rolling tanks have a free surface effect, which decreases the vessel’s metacentric height (GM). They
should
, therefore, always be emptied when the metacentric height is small.
Free surface effects are not only caused by partially-filled tanks. They can, for example, also be caused by
accumulated water on deck. To enable the water to run off quickly, a vessel
shall
have adequate freeing
ports. Water
shall
flow easily to the freeing ports which
shall
always be clear.
When loading deck cargoes, care
shall
be taken to avoid any obstruction of the freeing ports or areas
necessary for the drainage of pipe stowage positions.
Version 04. Original date: 22/06/2012. Version date: 31/12/2014. Valid version only on QMS site. Uncontrolled document when printed.
Procedure prepared by: BOS Operations Department Project Coordinator. Approved by: BOS Head of Operations. Validated by: BOS Quality Representative.
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