Question:

What are some considerations for predicting roll with Seakeeping?

Answer:

With any seakeeping code, roll predictions are only as good as your damping model. We believe GHS-SK has the best out-of-the-box damping model available, and we put a lot of effort into making it as good as possible.

In terms of methods, SK uses surface integration to compute the friction and eddy damping components. The theoretical details are in the SK Manual, but the main idea is that SK uses the actual geometry to compute the damping and not just a regression formula.

In addition, SK has options to input 1st, 2nd, and 3rd order damping coefficients from model test or CFD roll decay data. You can also use critical damping ratios by entering a % damping value (commonly people use 5 to 10%).

SK has a geometry-based bilgekeel parameter which uses components from in your geometry file. This can also be used for skegs.

If you really need to get crazy with it, you have complete access to the entire damping matrix, so you can modify any linear coefficient.

To get good roll predictions, you'll also need accurate roll gyradii, which will depend on weight estimates and tank loads. A detailed loading condition in GHS helps, as does lightship roll gyradii from a structural weight estimate.

Of course, accurate knowledge of the wave conditions is important.

The only stipulations for your geometry file is that your main hull is a single component (which it typically is) and any appendages you want to include are modeled as separate components (which they often are).

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