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

(Requires GHS version 16.62 or later with SeaKeeping)

Waves don't always travel in the same direction. In fact, it is often more realistic to consider a seaway as a collection of not only waves of different frequencies and amplitudes, but of different directions as well. This type of seaway is often called short-crested as it is characterized by its generally short wave crests and "confused" nature. The following photograph is a good representation of a moderate short crested sea in Port Townsend bay:

So how do we model this in GHS? We use the /SPREAD parameter.

/SPREAD applies a COS^2 spreading function to sinusoidal and spectra type waves. The user has the option to specify the spreading half-angle as well as the number of headings to be used to compute the spread. For example, if you were using a JONSWAP spectrum with a modal period of 9.3 seconds, a significant wave height of 3.25 ft, and a known spread of approximately 90 degrees, you could generate the corresponding short-crested seaway using:

WAVE (SPE) J2 9.3,3.25

Note that when /SPREAD is present, /HEAD now refers to the dominant wave heading, and the spread half-angle is the angle of spread to either side of the dominant heading. In this case, the largest waves would be encountered at a 180 degree heading, with all waves encountered between headings of 135 degrees (180-45) and 225 degrees (180+45).

Now the Wave Components table will give the /SPREAD parameters and list the wave components for each heading within the spread, as shown below:

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