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

Long-Term Wave Statistics
(Requires GHS version 17.5 or later)

Imagine you are driving south on, say, US highway 101 (or any coastal road for that matter). You are dutifully (as any naval architect would) observing the waves in the ocean. You notice the stochastic nature of the waves: they change from one minute to the next. But on this particular day, in this particular season, the waves could all be described by a single random distribution. Perhaps looking something like this:

An example JONSWAP "J2" spectrum with a modal period 8 seconds and significant wave height of 3 feet as taken from a SEAKEEPING report.

But what happens if you make that same drive in mid-November, just as a Pacific storm is making landfall? If you can even see the waves through your rain-inundated windshield, wipers going to the max, you'll see the seaway looks a bit different. Sure, in this moment, the waves could be described by a distribution, but it's certainly not the same distribution as before, and yet these waves are observed in the exact same location.

This is the concept of a non-stationary or long-term seaway--given enough time, most marine environments will change, sometimes dramatically. This presents a problem: which wave spectrum do you use if you need to analyze this environment in a design scenario? The worst one? An average between the two? How do we account for the fact that some more severe waves occur less often?

Fortunately, since v17.5, long-term wave data may be easily entered and manipulated in GHS. The new Long-Term "LT" Wave Statistics Wizard makes it all possible.

Above is a view of the Wave Scatter input section of the wizard. It is a customizable dialog to allow input of all sorts of different wave data observations in the form of a scatter table. Scatter tables are simply a matrix of observed wave spectra. The number of observations for each spectrum is recorded in each cell, and (in this example) each spectrum's corresponding period and significant wave height is entered in the columns and rows. The user can customize their row and column data types using the corresponding drop-down menus to suit the needs of the available data.

As for the answers to the many questions mentioned earlier: you'll have to wait until a future COW. But if you want to get started using the new LT wizard now, you may download it here.

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

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