Better Ballasting
The scheme we chose in the previous analysis (Click here to review.) fills the compartments indiscrimently. Because of free surface effects the ballast plan in the previous example will probably be difficult to work with. In this example we will get a better ballast condition.

Most of our examples contain a common set of "beginning" commands as well as a common "ending" command. Click here to get documentation for these commands. They will not be discussed directly. The files which are discussed here are:

There are two sets of changes made to the cif file. The first set are basically to add some steps in the ballasting process. The commands are:

   &status compartm
   &select :outer -select @p @s
   &cmp_bal cbrg180 :outer
   &status
   &compartment  -percent @ 0 \
                          4p 100 1.0255 4s 100 1.0255 \
                          3p 100 1.0255 3s 100 1.0255 \
                          2p  15 1.0255 2s  15 1.0255 \
                          1p 100 1.0255 1s 100 1.0255
   &status
   &status compart
   &equi
   &STATUS B_W
The second set of changes to the cif file is to demonstrate the options available when computing righting arm and heeling arm. Towards the end of the file you will find the following set of commands.
   &subtitle default
   RARM 2.5 10 -WIND 100
      REPORT
   END
   &subtitle -cen_lateral 90 0 5
   RARM 2.5 10 -WIND 100 -cen_lat 90 0 5
      REPORT
   END
   &subtitle -u_current
   RARM 2.5 10 -WIND 100 -u_current
      REPORT
   END
   &subtitle -u_current initial
   RARM 2.5 10 -WIND 100 -u_current initial
      REPORT
   END
Normally one desires two things from a ballast condition: to use the outboard compartments and for them to be pressed full. Using outboard compartments increases the radii of gyration and having them full minimizes the reduction in metacentric height due to partially full compartments. Pressed compartments rarely result when the &CMP_BAL command is used.

Remember above that we used "@" to select the tanks for ballasting in the previous example. If we want to use only the outer tanks, we need to specify something else. Here, we change the &SELECT command to:

     &SELECT :OUTER -SELECT @P @S
     &CMP_BAL CBRG180 :OUTER

The first of these commands defines a selector called :outer. It selects only the compartments which end with the letter "P" or the letter "S". Please note that the selector name begins with a colon (:). In MOSES, all selector names will begin with a colon. Now, this selector is used in the second command so that when it computes ballast, it only uses the compartments selected instead of all of them. You can click here to look at the log file. You will notice that indeed the program only chose to use those compartments which end with the letter "P" or the letter "S". In addition, one should notice that 2P, 2S, 3P, 3S, 4P and 4S are not filled to capacity. This generally is not acceptable. One would like, at most, to have two slack tanks - here we have six.

We can fix these problems by using the computed results to intelligently reballast manually. In particular, we next use the command:

   &compartment  -percent @ 0 \
                          4p 100 1.0255 4s 100 1.0255 \
                          3p 100 1.0255 3s 100 1.0255 \
                          2p  15 1.0255 2s  15 1.0255 \
                          1p 100 1.0255 1s 100 1.0255
This command fills 4P and 4S 100 percent full, fills 3P and 3S 100 percent full, fills 2p and 2s to 15 percent full, and fills 1p and 1s to 100 percent full. The command &STATUS COMPART confirms our ballasting. Notice that all the compartments are emptied in the beginning of the command with "@ 0". This is why compartments 5P and 5S are changed from 100 percent to 0 percent. (For details, click here.)

The next step is to determine whether the vessel will float in an acceptable manner with the ballast arrangement just described. The command:

     &EQUI
instructs the program to find the equilibrium position for the current ballast arrangement. A summary of the state of the system is produced automatically whenever equilibrium is requested. (For details, click here.) Here, it shows that the vessel will float at a draft of 6.78 feet with a trim of 0.03 degrees. This generally is an acceptable floating position. This change in ballast arrangement will be seen in the righting arm calculations. The comparison between the righting arm in these output results and the previous ones is left to the user.

A word of caution is in order here. For many arrangements there are no ballast or equilibrium solutions found. MOSES will place a warning message only in log file, not in the output file. MOSES will proceed with the analysis after the warning message has been printed. It is therefore advised that the log file be checked for warning messages whenever the &cmp_bal or the &equi command is used.

We have now reached the second set of changes to the cif file. Here we demonstrate the use of the options -cen_lat and -u_current. What we are trying to show in this presentation is that the option -cen_lat requires three value follow it, while the option -u_current does not require a value. You can read more about these options here. The use of these options changes the heeling arm results. The comparison between the heeling arms in the output results is left to the user.

This concludes the tutorial section on stability and ballasting. The next section covers motions.