GHS is capable of evaluating
the stability of a vessel based on the Weather Criterion (46 CFR
170.170). This criterion assumes that the righting arm curve has
the form of a sine curve and the heeling arm curve has the form
of the cosine as shown in (1) and (2).
where
P = wind pressure
A = projected lateral area
H = the vertical distance from the center of the projected lateral
area to the center of the underwater lateral area.
W = displacement
phi = 14 degrees or the angle at which one-half the freeboard to
deck edge is immersed, whichever is less
The angle at which the heeling arm curve and righting arm curve
intersect is the angle of equilibrium. Therefore, at equilibrium
The Weather Criterion requires that (4) must be satisfied for each
condition of loading and operation. Three LIMIT commands are required
by GHS in order to apply this criterion.
The first LIMIT command assures that the angle of equilibrium occurs
before 14 degrees or the angle at which one-half the freeboard to
deck edge is immersed, whichever is less. It is defined as follows:
LIMIT(1) ANGLE from EQU to ABS 14 or HF > 0
The HF keyword (short for Half Freeboard)
refers to the angle at which one-half the freeboard to deck edge
is immersed. In this case, the angle from equilibrium to the lesser
of 14 or HF must be greater than zero.
Before you can use the HF keyword, you must first mark the deck
edge of the vessel. The deck edge is marked in Part Maker using
the MARGIN command. For this criterion, the amount of the margin
does not matter. You could use zero or any other value. For example,
modify hull\hull.c
margin 0
/
When you view the vessel using the DISPLAY command, you should see
a line marking the deck edge.
GHS does not make the assumption that the righting arm curve is
a sine curve -- actual righting arms are calculated for each angle
of heel. If only the angle of equilibrium is considered, it is possible
to have a righting arm curve which satisfies LIMIT(1) above but
has very little righting energy up to the angle or, in extreme cases,
a negative GM at zero heel. In order to insure that the true righting
arm curve provides as much energy as would a sine curve, a second
LIMIT command is required. It is derived from (1) and (2) as follows:
The following LIMIT command can therefore be used to force the righting
arm curve to be equal or better than the sine curve with regard
to area (energy).
LIMIT(2) ABS RATIO from ABS 0 to ABS 14 > 0.492
An additional limit is required to guarantee that stability exists
beyond the angle of equilibrium. This could be a measure of the
range of stability or the residual area. For example, if the range
of stability is to be at least 15 degrees beyond equilibrium the
limit command would be,
LIMIT(3) ANGLE from EQ to RA0 > 15
To evaluate the stability of a vessel based on the Weather Criterion,
you must also set the heeling moment. You can use the WIND and HMMT
commands as follows:
WIND(PRESSURE) P
HMMT WIND /CS
where P equals the wind pressure as calculated according to 46 CFR
170.170(a). The values of A and H will be determined by GHS from
the model and current waterplane. All superstructure that is to
be included in the lateral area calculations must be modeled. If
superstructure is to be included in the lateral area calculations
but not in the buoyant volume of the vessel, it must be modeled
as a SAIL part. These commands will then generate the heeling arm
curve as defined in (2).
Following is a simple run of commands which can be used to perform
a weather criterion analysis:
READ filename
REPORT
WEIGHT w, l, t, v
LIMIT(1) ANGLE from EQ to ABS 14 or HF > 0
LIMIT(2) ABS RATIO from ABS 0 to ABS 14 > 0.492
LIMIT(3) ANGLE from EQ to RA0 > 15
WIND(PRESSURE) P
HMMT WIND /CS
HEEL = 0
RA 0,2 ... 14 /LIM |