1.1.1 The stability should be sufficient to satisfy
the provisions of 2.3 and 2.4 of this Code.
1.1.2 Heeling moment due to turning
The heeling moment developed during manoeuvring of the craft
in the displacement mode may be derived from the following formula:
where:
|
MR
|
= |
moment
of heeling; |
|
Vo
|
= |
speed
of the craft in the turn (m/s) |
|
Δ |
= |
displacement
(t); |
|
L |
= |
length of the
craft on the waterline (m) |
|
KG |
= |
height of the
centre of gravity above keel (m). |
This formula is applicable when the ratio of the radius of the
turning circle to the length of the craft is 2 to 4.
1.1.3 Relationship between the capsizing moment
and heeling moment to satisfy the weather criterion
The stability of a hydrofoil boat in the displacement mode can
be checked for compliance with the weather criterion K as follows:
where:
|
Mc
|
= |
minimum
capsizing moment as determined when account is taken of rolling; |
|
Mv
|
= |
dynamically
applied heeling moment due to the wind pressure. |
1.1.4 Heeling moment due to wind pressure
The heeling moment Mv is a product of wind pressure
Pv, the windage area Av and the lever of windage
area Z.
Mv = 0.001 PvAvZ
(kNm)
The value of the heeling moment is taken as constant
during the whole period of heeling.
The windage area Av is
considered to include the projections of the lateral surfaces of the
hull, superstructure and various structures above the waterline. The
windage area lever Z is the vertical distance to the centre of windage
from the waterline and the position of the centre of windage may be
taken as the centre of the area.
The values of the wind
pressure in Pascal associated with Force 7 Beaufort Scale depending
on the position of the centre of windage area are given in table 1.
| Table 1
|
| Typical wind pressures for Beaufort scale 7,100 nautical miles from
land
|
| Z above waterline
(m)
|
1.0
|
1.5
|
2.0
|
2.5
|
3.0
|
3.5
|
4.0
|
4.5
|
5.0
|
| Pv (Pa)
|
46
|
46
|
50
|
53
|
56
|
58
|
60
|
62
|
64
|
| Note: These values may not be applicable in all areas.
|
1.1.5 Evaluation of the minimum capsizing moment
Mc in the displacement mode
The minimum capsizing moment is determined from the static
and dynamic stability curves taking rolling into account.
-
.1 When the static stability curve is used, Mcis
determined by equating the areas under the curves of the capsizing
and righting moments (or levers) taking rolling into account, as indicated
by figure 1, where θzis
the amplitude of roll and MK is a line drawn parallel to the abscissa
axis such that the shaded areas S l and S2 are
equal.
|
Mc
|
= |
OM,
if the scale of ordinates represents moments, |
|
Mc
|
= |
OM
x Displacement, if the scale or ordinates represents levers. |
-
.2 When the dynamic stability curve is used, first
an auxiliary point A should be determined. For this purpose the amplitude
of heeling is plotted to the right along the abscissa axis and a point
A' is found (see figure 2). A line
AA' is drawn parallel to the abscissa axis equal to the double amplitude
of heeling (AA'= 2θz) and the required auxiliary
point A is found. A tangent AC to the dynamic stability curve is drawn.
From the point A the line AB is drawn parallel to the abscissa axis
and equal to 1 radian (57.3°). From the point B a perpendicular
is drawn to intersect with the tangent in point E. The distance 
is equal to the capsizing moment if measured along the
ordinate axis of the dynamic stability curve. If, however, the dynamic
stability levers are plotted along this axis, 
is then the capsizing lever, and in this case the capsizing
moment Mc is determined by multiplication of ordinate 
(in metres) by the corresponding displacement in tonnes
-
.3 The amplitude of rolling θ zis
determined by means of model and full-scale tests in irregular seas
as a maximum amplitude of rolling of 50 oscillations of a craft travelling
at 90° to the wave direction in sea state for the worst design
condition. If such data are lacking the amplitude is assumed to be
equal to 15°.
-
.4 The effectiveness of the stability curves should
be limited to the angle of flooding.
Figure 1 Static Stability Curve
Figure 2 Dynamic Stability Curve