Although the stability criteria for individual vessels may vary
depending on their survey class, the proceedure and documentation
for checking that a vessel will pass the set criteria follows a
similar path for most vessel. As a guide the following steps will
be required for most stability assessments:
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1
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Hull shape
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Determining the exact shape of the hull is necessary
to assess the vessel's stability characteristics. Determining
the shape can be done by two principal methods, these being
(a) using the vessel's lines plan if available, or (b) by direct
measurement. Direct measurements can be taken in a variety of
ways from simply measuring with a tape measure, to using sophisticated
surveying equipmnet or more recently photogrametry. |
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2
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Hull Model
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After determining the hull shape a 3-dimensional
computer model will be made in order to calculate the stability
characteristics. |
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3
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Hydrostatic Particulars
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From the 3D computer model it is possible to calculate
the hydrostatic particulars for the hull shape at various waterlines.
This data is typically calculated for a range of drafts covering
at minimum the expected weight range of the vessel from its
lightest condition to its heaviest condition. The hydrostatic
details calculated for a given waterline usually include displacement,
centre of buoyancy, centre of flotation, waterplane area, wetted
surface area etc. These particulars are calculated for the vessel
in the upright position (ie. level trim and no heel).
Click here for typical hydrostatic
data abreviations and definitions. |
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4
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Cross Curves of stability
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Cross curves of stability are a measure of the
restoring force (tendecy for the vessel to return to the upright
position) from an angle of heel. These curves are a measure
of the vessel stability while heeling and are typically calculated
for a range of operating displacements, trim angles and heel
angles. |
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5
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Lightship
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Once the model has been made and the necessary
hydrostatic data and stability curves calculated it is necessary
to obatin an accurate weight for the vessel in each condition.
This is normally achieved by measuring the draft of the vessel
in a known condition (ie. known tank contents and consumables).
From this draft reading the displacement and longitudinal centre
of gravity can be calculated from the hydrostatic particulars. |
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6
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Vertical Centre of Gravity
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Calculating the vertical centre of gravity requires
a practical stability test commonly called an inclining experiment.
As the name implies the vessel is inclined to both port and
starboard by moving weights accross the deck. The angle of inclination
is measured, usually with a pendulum, for each weight movement.
Using this data and the hdrostatic particulars for the vessel
it is possible to calculate the vertical centre of gravity.
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7
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Stability Conditions
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Once the displacement and centre of gravity (vertical
and longitudinal) are known for the reference condition they
can then be calculated for all other expected conditions of
loading. Each expected condition is then simulated in a computer
program and the upright and inclined stability calculated. The
values obtained are checked against the criteria for the vessel's
operational class. |
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8
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Report
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The results of the above calculations will be collated into
a booklet called a Trim and Stability Report. This report
should include some or all of the following:
- Vessel Particulars
- Vessel profile
drawing
- Tank plan and tank capacity tables
- Hydrostatic data table
- Loading conditions (lightship, Departure, Typical working,
Crew only, Arrival etc.)
- Stability Criteria and associated calculations to ensure
compliance
- Damage stability conditions
- Details of the inclining experiment or lightship measurement
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