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

The Time Domain
(Requires GHS version 16.56B or later with SK module...sort of)

A few weeks ago we published a COW that utilized the new SK library "SK.LIB" to create a time domain plot of relative motion in a sinusoidal wave. The COW was still hot off the press when we received a call from John Bonn. He told us that he didn't really care about relative motions at all but that he loved the time domain plot and wondered if there was a way to use similar methods to create an animation of a vessel in regular waves. He explained that it would look really cool. We tried to explain that it would have limited engineering value, but we could not deny that it would look cool. Really cool. So we made it with a run file (and SK.LIB):

[FV.GF with a forward speed of 8 knots in a rather exciting bow-quartering wave. The wave has a wavelength of 120 ft and a wave height of 5 ft. The files needed to create this animation will be found here.]

John was happy, we were entertained, and the animation was useless.

Although animations provide some level of intuition, in a practical sense what is really needed is the ability to generate six-degree-of-freedom time domain visualizations in irregular waves. These visualizations (and the data behind them) can be used to test phase sets (you can create them!), investigate phase relationships between responses, select design values, and perform all sorts of real engineering. At this point, John was no longer interested.

[Translational time domain responses of FV.GF with a forward speed of 8 knots in a Pierson-Moskowitz wave spectrum (Hs=3.5 ft, Heading=165 deg). The phase set used was created from pseudo-random phase angles. The files used in this example are available here.]

Using a time domain approach, it becomes possible, for example, to identify the phase relationship between angular displacements (roll and pitch angles) and accelerations (longitudinal, transverse, and vertical "g's") during extreme events. This insight can be used to calculate more realistic dynamic loads when designing seafastenings and equipment foundations, or to simulate cargo shifting.

One final note: Even if you do not have the SK module, you can still run the examples. Just use the included data files and comment out any SEA command lines. For the hydrodynamically curious, we've also included a zero-speed, long-period wave data file with the animation example to show convergence at low wave frequencies. Be sure to grab the latest SK.LIB here.

Questions, comments, or requests?
Contact Creative Systems, Inc.

USA phone: 360-385-6212 Fax: 360-385-6213
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Mailing address:
PO Box 1910
Port Townsend, WA 98368 USA

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