tank reduces the hydrostatic stability of the ship by virtue of
the shifting of the fluid within the tank. GHS and BHS account for
this "free surface" effect by actually calculating the
new C.G. of the fluid with each change of trim and heel.
Another method of representing the free surface effect is to elevate
the ship's center of gravity by a suitable amount. This simplifies
the calculation of righting moments as a function of heel since
the tank's contribution to the center of gravity is considered to
be fixed. The amount by which the C.G. is elevated may be chosen
such that the additional righting moment produced by a small change
of heel is the same as would be produced by the shifting of the
tank's contents. This elevation of the C.G., multiplied by the weight
of the ship, is called the free surface moment, or FSM.
The primary disadvantage of using the FSM is that it does not accurately
represent the tank's effect on stability beyond a small increment
of heel, since the FSM itself can be very different at different
Applications of the FSM
In the past, the primary advantage of using the FSM was its
relative simplicity, and although the availability of high-speed
computers has made that less of an issue today, there remain some
applications where the FSM is still a useful shortcut.
One such application uses maximum FSM values to represent the free
surface even in a case of loading where the true FSM is less than
maximum. The intent is to make a few sample cases representative,
in a worst-stability sense, of all the various loads which would
occur during normal operation.
Another application where the FSM is necessary is in the use of
precomputed maximum VCG data for a quick stability check. In order
to use maximum VCG data to assess the stability of a particular
load case, the effect of slack tanks is conveniently and quickly
represented by the FSM. The present VCG is increased by an FSM adjustment
before being compared with the maximum VCG.
GHS Facilities for using FSM
In order to accommodate these applications, GHS associates
two FSM functions with each tank. One function is used for load
factors between zero and just under 95%. The other is used with
loads from 95% to 100%.
Each FSM function is allowed to have one of three forms: 1) a constant
value; 2) a constant value except at zero and 100% loads where the
value becomes zero; and 3) a variable equal to the true FSM value
at each particular load.
GHS allows the value used by the first two forms of the FSM function
to be either specified directly by the user or taken from the FSM
present in a particular load at a particular heel and trim. A means
of finding the load which has the maximum FSM at a particular heel
and trim is available.
2 In addition, GHS provides for a minimum FSM -- a "floor"
value which becomes the effective FSM when it is greater than the
FSM sum from the tanks. In addition to the overall FSM floor, individual
FSM floors can be designated for each of the various descriptions
of tank contents.
Assigning FSM functions to a tank is done by means of the
command which has the form,