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General HydroStatics
Ship Stability Software |
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Command of the Week
(New or interesting aspects of GHS that you may not know about) |
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SEA /SPREAD
(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: |
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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 SEA /HEAD:180 /SPREAD:45 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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Questions, comments, or requests?
Contact Creative Systems, Inc. USA phone: 360-385-6212 Fax: 360-385-6213 Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday Mailing address: PO Box 1910 Port Townsend, WA 98368 USA Click here for an index to this and previous COWs |