3.1 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 showed 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.
3.2 Investigations comprising the
evaluations 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 the being the simplest:
where:
|
f |
= |
factor for the
rolling period (rolling coefficient) as given in 3.4;
|
|
B |
= |
breadth of the
ship in metres; |
|
Tr
|
= |
time
for a full rolling period in seconds (i.e. for one oscillation “to
and fro” port - starboard - port, or vice versa). |
3.3 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.
3.4 For coasters of normal size
(excluding tankers) and fishing vessels, the following average values
were observed;
|
|
f-values
|
| Empty ship or ship carrying ballast
|
f ~
0.88
|
| Ship fully loaded and with liquids in tanks
comprising the following percentage of the total load on board (i.e. cargo,
liquids, stores, etc.)
|
|
|
|
20% of total
load
|
f ~
0.78
|
|
|
10% of total
load
|
f ~
0.75
|
|
|
5% of total
load
|
f ~
0.73
|
| Double boom shrimp fishing boats
|
f ~
0.95
|
| Deep sea fishing boats
|
f ~
0.80
|
| Boats with a live fish well
|
f ~
0.60
|
3.5 The above 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.
3.6 It should 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:
-
.1 the rolling coefficient for an unloaded ship,
i.e. for a hollow body, will be higher than that for a loaded ship;
and
-
.2 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.
3.7 The above recommended rolling
coefficients were determined by tests with ships 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.
3.8 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.
3.9 For the following reasons,
it is not generally recommended that results be obtained from rolling
oscillations taken in a seaway:
-
.1 exact coefficients for tests in open waters
are not available;
-
.2 the rolling periods observed may not be free
oscillations but forced oscillations due to seaway;
-
.3 frequently, oscillations are either irregular
or only regular for too short an interval of time to allow accurate
measurements to be observed; and
-
.4 specialized recording equipment is necessary.
3.10 However, sometimes it may
be desirable to use the ship'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 oscillations corresponding to the sea period
and differing from the natural period at which the ship 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.
3.11 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 approximate estimation.
3.12 The formula given in paragraph 3.2 above can be reduced to:
and the Administration should determine the F-value(s)
for each ship.
3.13 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.
3.14 The initial stability may also be more easily
determined graphically by using the attached sample nomogram as described below:
-
.1 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) at the point (M).
-
.2 A second straight line (2) which connects this
point (M) and the point on the Tr scale corresponding with the
determined rolling period, intersects the GM scale at the requested value.
Figure 1
3.15
Section
3.16.1 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.