3.24 TRIMMING THE SHIP
During the initial stages of loading, some trim and list is acceptable,
even unavoidable. However, the loading plan should be arranged so that
the ship is close to an even keel by the time the first set of tanks
is topped off and the list and trim should be minimised thereafter.
Excessive trim and/or list can negatively affect the topping off operation.
Trim and list must be limited to permit the ship to depart the berth
at any time during loading and to prevent grounding as the loading nears
3.24.1 Removing list
If the ship begins to list, the cargo watch officer must first determine
the cause of the list by checking the ullage readings in all off-centre
tanks. Some ships have a natural list, requiring more cargo to be put
into the opposite side to counteract it. List can be caused by asymmetrical
loading of wing tanks, mooring lines too tight, loading of fuel, excessive
free surface, or the vessel being aground. In most cases, uneven levels
in the wing cargo tanks is the cause. In this case, the cargo watch
officer must adjust the loading valves to reduce the flow of cargo to
the low side of the ship and/or increase the flow of cargo to the high
If the list is not due to differential levels in wing tanks, then a
diligent search must be conducted, stopping the loading of cargo if
necessary, until the cause of the list is definitely determined.
3.24.2 Adjusting trimChanging the final quantities to be loaded in each of the trim tanks.
As the loading operation nears completion, the ship should be close
to the desired final trim. When the second set of cargo tanks has been
topped off and the ship is loading the final set of 'trim tanks', the
chief officer should closely observe the forward and after draft of
the ship and recalculate the trim which will result from the final quantities
to be loaded. If this check indicates a final trim which is more or
less than desired, then an adjustment must be made.
Trim can be adjusted by:
Shifting cargo between tanks after loading is completed.
Shifting fuel between fuel tanks.
Adding water ballast.
The best method is to amend the loading plan, changing the trim tank
quantities to achieve the desired final trim.
The cargo watch officer should be instructed to confirm the chief officer's
trim calculations. In the absence of such instruction, he should do
so on his own initiative. Use of the trimming table is the most convenient
way to do this. The officer determines the hundreds of tonnes to be
loaded in each trim tank, then multiplies it by the trimming factor
for the applicable tanks, adding (subtracting) the resulting figures
to (from), the forward and after drafts.
A rrimming table provides a rapid means of determining
the draft effect of each 100 tonnes loaded as the vessel approaches
Shifting cargo between tanks after loading requires charterer's approval
(unless done at the loading port before gauging). If the cargo layout
and piping arrangement permit, cargo may be gravitated from full to
partially full or empty tanks after loading is completed to achieve
desired trim. Shifting cargo is not normally possible with multi-grade
Shifting of fuel for trim is only possible where the ship has forward
fuel tanks and will have sufficient fuel on board arriving at the discharge
port to maintain the desired trim.
Ballast can only be added to change trim when the ship will not be loaded
to its leadline mark with cargo alone.
During the final minutes of loading the chief officer will go onto the
dock and maintain a close watch on the trim and the loadline, indicating
to the cargo watch officer any adjustments of cargo flow to the trim
tanks to achieve the desired final trim. If the draft cannot be observed,
he should finish the loading by calculated volumes.
3.24.3 Salinity differential allowance
The tanker's loadline amidship must never be immersed below the applicable
level, regardless of the forward and aft drafts and the corresponding
'mean draft'. It is the amidship freeboard which is the controlling
factor whenever the ship is loaded 'to the marks'. The amidship marks
assume that the vessel is in pure salt water. That is rarely the case
in a loading berth, where the actual density of the water may be somewhat
less that the 1.025 nominal value for salt water. The portion of the
fresh water allowance that applies (salinity differential allowance,
or 'SDA'), is determined by first measuring the density (or salinity),
of the berth water. While the final cargo tanks are filling, a clean
bucket on a heaving line should be used to take a water sample from
the berth. The specific gravity of the sample must be measured using
a calibrated hydrometer.
For this Panamax tanker - an observed salinity
of 1.005 indicates that the summer loadline may be immersed 23 cm
The SDA to be applied to the limiting loadline is calculated by the
SDA = (1.025 - observed density) x fresh water allowance / 0.025
If the tanker has a fresh water allowance of 50 cm and the berth water
density measurement is 1.020, then the ship can load so that the applicable
loadline is immersed by
(1.025 - 1.020) x 50 / 0.025 = 50/5 = 10 cm
10 cm below its upper edge.