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APPENDIX III
MEMORANDUM TO ADMINISTRATIOS ON AN APPROXIMATE DETERMINATION
OF SHIP’S STABILITY BY MEANS OF THE ROLLING PERIOD TESTS
(for ships up to 70 m in length)
- 1. Recognizing the desirability of supplying to masters of small ships
instructions for a simplified determination of initial stability, attention
was given to the rolling period tests. Studies on this matter have now been
completed with the result that the rolling period test may be recommended as
a useful means of approximately determining the initial stability of small
ships when it is not practicable to give approved loading conditions or
other stability information, or as a supplement to such information.
2. Investigations comprising the evaluation of a number of inclining and
rolling tests according to various formulae showed that the following
formula gave the best results and it has the advantage of being the
simplest:
Where:
f = factor for the rolling period/rolling coefficient (different
for feet and metric system)
B = breadth of the ship in feet or metric units
Tr = time for a full rolling period in seconds (i.e.
for one oscillation "to and fro" port - starboard - port,
or vice versa).
3. The factor "f" is of the greatest importance and the data
from the above tests were used for assessing the influence of the
distribution of the various masses in the whole body of the loaded ship.
4. For coasters of normal size (excluding tankers), the following average
values were observed:
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|
metric system |
feet system |
|
|
a) |
Empty ship or ship carrying ballast |
f ~ 0.88 |
f ~ 0.49 |
|
|
b) |
Ship fully loaded and with liquids in tanks comprising the following
percentage of the total load on board (i.e. cargo, liquids, stores etc.). |
|
|
|
|
|
|
|
|
|
|
|
1 20 per cent of total load |
f ~ 0.78 |
f ~ 0.435 |
|
|
|
2 10 per cent of total load |
f ~ 0.75 |
f ~ 0.415 |
|
|
|
3 5 per cent of total load |
f ~ 0.73 |
f ~ 0.405 |
The stated values are mean values. Generally, observed f-values were
within ±0.05 of those given above.
5. These f-values were based upon a series of limited tests and,
therefore, Administrations should re-examine these in the light of any
different circumstances applying to their own ships.
6. It must be noted that the greater the distance of masses from the
rolling axis, the greater the rolling coefficient will be.
Therefore it can be expected that:
The rolling coefficient for an unloaded ship, i.e. for a hollow body,
will be higher than that for a loaded ship.
The rolling coefficient for a ship carrying a great amount of bunkers
and ballast - both groups are usually located in the double bottom, i.e.
far away from the rolling axis - will be higher than that of the same
ship having an empty double bottom.
7. The above recommended rolling coefficents were determined by tests
with vessels in port and with their consumable liquids at normal working
levels; thus, the influences exerted by the vicinity of the quay, the
limited depth of water and the free surfaces of liquids in service tanks are
covered.
8. Experiments have shown that the results of the rolling test method get
increasingly less reliable the nearer they approach GM-values of 0.20 m and
below.
9. For the following reasons, it is not generally recommended that
results be obtained from rolling oscillations taken in a seaway:
- Exact coefficients for test in open waters are not available.
- The rolling periods observed may not be free oscillations but
forced oscillations due to seaway.
- Frequently, oscillations are either irregular or only regular for
too short an interval of time to allow accurate measurements to be
observed.
- Specialized recording equipment is necessary.
10. However, sometimes it may be desirable to use the vessel’s period
of roll as a means of approximately judging the stability at sea. If this is
done, care should be taken to discard readings which depart appreciably from
the majority of other observations. Forced oscillatins corresponding to the
sea period and differing from the natural period at which the vessel seems
to move should be disregarded. In order to obtain satisfactory results, it
may be necessary to select intervals when the sea action is least violent,
and it may be necessary to discard a considerable number of observations.
11. In view of the foregoing circumstances, it needs to be recognized
that the determination of the stability by means of the rolling test in
disturbed waters should only be regarded as a very appropriate estimation.
12. The formula given in paragraph 2 above can be reduced to:
and the Administration should determine the F-value(s) for each vessel.
13. The determination of the stability can be simplified by giving the
master permissible rolling periods, in relation to the draughts, for the
appropriate value(s) of F considered necessary.
14. The initial stability may also be more easily determined graphically
by using one of the attached sample nomograms for feet and/or metric units
as described below:
- The values for B and f are marked in the relevant scales and
connected by a straight line (1). This straight line intersects the
vertical line (mm) in the point (M).
- A second straight line (2) which connects this point (M) and
the point on the Tr scale corresonding with the
determined rolling period, intersects the GM scale at the requested
value.
15. The Annex to Appendix III shows an example of a recommended form in
which these instructions might be presented by each Administration to the
masters. It is considered that each Administration should recommend the
F-value or values to be used.
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