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

(Requires GHS version 16.70 or later with SeaKeeping)

A fundamental aspect of a stability analysis is that you can model virtually any loading condition. You can then compute the hydrostatic behavior of the vessel subject to that condition--or even better, subject to many conditions in a run file. But that all seems pretty basic, right?

Yet the same could not be said for a seakeeping analysis. At least, not until the SEA /COUPLE parameter was introduced.

This might surprise you, but virtually all commercial seakeeping codes invoke what is called the "symmetry assumption", which assumes:

    • The vessel is geometrically symmetric about the centerline;
    • The loading condition is symmetric about the centerline (that includes mass distribution and tank loads);
    • The vessel's condition is perfectly zero-heel and often zero-trim.

The trouble is that most real-world scenarios are not that simple. And when you're used to the open-ended nature of loading conditions in GHS, it doesn't seem so unreasonable to want the same flexiblity in your seakeeping analyses.

So why do most codes make the symmetry assumption? Because it reduces computation time and simplifies the theory. Given the computational power of modern computers, the first reason is almost irrelevant. And if you don't mind a few months of off-and-on whiteboard derivations to get the math right, the second reason doesn't seem so convincing either....

At a basic level, by invoking the symmetry assumption the vertical modes of motion (surge, heave, pitch) are uncoupled from the horizontal modes (sway, roll, yaw). This means that roll motions cannot affect heave motions, for example. But this also means half of the terms in the physical mass, added mass, damping, and restoring forces are assumed zero. SEA /COUPLE retains and computes all of those terms and solves a fully-coupled equation of motion, giving you the ability to run seakeeping on virtually any loading condition.

To put those few months of whiteboard derivations to the test, we recently got our hands on some third party experimental and computational research data for a naval frigate heeled to 15 and 20 degrees. Extreme, we know, but the interest was in the dynamic behavior of a damaged vessel, and since there's not a lot of published data when it comes to fully-coupled seakeeping studies, we were eager to give our parameter a real test:

[Comparison data source: Andrewartha, T., Thomas, G., & Cannon, S. (2008). Seakeeping Behavior of a Damaged Warship. RINA - International Maritime Conference, Pacific, (pp. 53-64).]

That's right: the heeled vessel was rolling in head seas! You'll also notice the strong coupling between roll and heave for both the 90 degree and 180 degree headings. GHS captured this unusual condition and motion with reasonable agreement given myriad experimental and computational uncertainties. Not bad for a couple of 15-line run files.

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