General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

SEA /SHW (extra)
(Requires GHS version 16.26 or later with SK)

A few weeks ago we did a rather uninteresting COW on SEA /SHW. Well, we decided it might be worthwhile to revisit the parameter with an interesting example. This isn't as much a "part two" as it is an extended director's cut.

The question arises: would it be possible to compute the required freeboard such that the frequency of deck submergence at a particular location is less than a certain threshold in a given time period? Lucky for us, SEA is a command, so we can definitely do this!

There are initially two different ways to approach this problem:

We may assume the underwater hull geometry is constant, i.e. the loading condition and all geometry below the waterline may not be changed. In this case, we assume the freeboard may be adjusted by modifying the topsides of the vessel, perhaps by increasing the foc'sle height, for example.


We may assume that the topsides cannot be modified, and therefore the only means by which we may adjust the freeboard is to change the draft. In this scenario, the underwater hull geometry does change, because we are varying the draft.

With some clever run file work, we can investigate the effects of both methods in a single run file, and then use the results to gain some insight into the influence of varying draft vs. varying the topsides.

To investigate varying the height of the topsides, we start by creating a vertical "line" of critical points. Each point has the same longitudinal and transverse coordinate with the location we are interested in. However, each point has a different vertical height, where the first is even with the existing deck.

 vari crt_list
 crt (1)  "bow DK+0.0" 97.573f,0,11.9
 crt (2)  "bow DK+0.5" 97.573f,0,12.4
 crt (3)  "bow DK+1.0" 97.573f,0,12.9
 crt (4)  "bow DK+1.5" 97.573f,0,13.4
 crt (5)  "bow DK+2.0" 97.573f,0,13.9
 crt (6)  "bow DK+2.5" 97.573f,0,14.4
 crt (7)  "bow DK+3.0" 97.573f,0,14.9
 crt (8)  "bow DK+3.5" 97.573f,0,15.4
 crt (9)  "bow DK+4.0" 97.573f,0,15.9
 crt (10) "bow DK+4.5" 97.573f,0,16.4
 set crt_list="1,2,3,4,5,6,7,8,9,10"
To investigate varying the draft, we create a loop as follows:

 vari drft=5
 vari inc=0.25
 macro drafts
  dr {drft}
  vcg 6
  so we lcg tcg
  di status profile, body@50f /sync
  \\ DRAFT = {depth} \ | \\
  sea /speed:9 /shw:crt /crt:{crt_list} /samp:10 /time:1
  set drft = {drft} plus {inc}
 wave (spe) pm 30
 report off
Notice that the macro 'drafts' is called 13 times, and after each call, the variable 'drft' is increased by 'inc'. This means drafts from 5 m. to 8.25 m. will be considered. Then for each draft, the shipping water statistics for the critical points are computed via /CRT and /SHW:CRT parameters. All of this information is then stored in a report file. Also notice that we are computing statistics for a 1 hour period using the /TIME parameter.

Upon completion of the run, we parse the report file with a couple simple macros (an example of report file parsing may also be found in COW073) and create a custom plot of the results. On the vertical axis we plot the shipping water frequency and on the horizontal axis we plot the draft. Each series represents a different critical point, i.e. a different vertical height above the deck in the location of interest. It is now possible to interpolate the required draft and/or topsides height to achieve the desired maximum shipping water frequency.

Like many of our COWs, this example highlights the incredible flexibility a command language approach offers users. Find a complete run file of this example here.

Questions, comments, or requests?
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