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

SEA /SPREAD:datafile
(Requires GHS version 16.62 or later with SK)

Some time ago we introduced the /SPREAD parameter on the SEAKEEPING command. While objectively not as exciting as the spread in last weekends Michigan-OSU game (and a fine spread that was), it is a Tuesday afternoon, and SEA /SPREAD will have to suffice.

As introduced in COW151, /SPREAD gives users the option to compute motions in a short-crested seaway. While the SK Manual has all of the play-by-play mathematical details (fine reading, regardless of the Michigan-OSU game), the main idea is that the direction waves are spread by a so-called spreading function. In other words, this spreading function makes it so that waves aren't just coming from one heading, but a range of headings.

The default spreading function in GHS is a COS^2 spreading function. Without getting too technical, the COS^2 function is symmetrical about the dominant wave heading, and it is a good option when little additional information is known about the wave propagation.

But what if we do know more about the wave environment? What if we have buoy data or some hind-cast model that gives us a more specific spreading function? Perhaps it is asymmetric, or perhaps it is narrower than the default COS^2 function. If this is the case, /SPREAD will also accept a spreading data file.

The spreading data file is a comma separated file that looks something like what is shown below: where the first line is a title line, the second line specifies the units of the file, and the data lines are pairs of sub-headings and spreading function values. The figure on the right of the data is a plot of what the spreading function in this file actually looks like.

The only trick to making this work is to note that the headings in the spreading data file are sub-headings, not primary headings. You will still need to specify your dominant heading with /HEAD on the SEAKEEPING command. The sub-headings in the file are added to, or subtracted from, the dominant heading to yield the specified spread at any dominant heading. A simple example run file while using a spreading file is given below.


we 250 0 0 5 /gyr:gf,gf,gf

wave (spe) p2 2.5

report /box:bw
 status weight products inertia
 sea /head:135 /spread:spread_data.csv
report off /preview

The files used in this COW can be found here.

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