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

(Requires GHS version 16.46 or later with SK)

In many ways, John Bonn asked for roll damping before he even asked for seakeeping. We told him it doesn't work that way. Fundamentally, roll damping is a function of response. We pointed to a page in an old textbook. He ignored the pointing.

Regardless, John Bonn (mostly) knew what he was talking about. Roll damping is an important topic, as roll can be the most difficult response to "get right" using numerical predictions. But our developers were not happy about it. Why? Because there really is not a perfect solution to the problem. Roll damping models vary dramatically, methods lack standardization, and legacy methods that may be assumed "standard practice" are based on strong assumptions and ship-type requirements and copious amounts of hand-waving.

What John Bonn did not know, however, is that roll damping is actually part of a much larger topic: appendages. By default, SEA computes motions for the unappended hull. This is standard practice. However, appendages can have marked effects on motions. Bilge keels are an excellent example of this. So, it became clear that if we were going to add roll damping, we would also need to add support for certain appendages. The developers drank a second cup of coffee.

This narrative does reach a happy ending though. A damping model has finally been integrated into the SEA command. At the most basic level, the model will automatically include hull eddy, friction, and lift damping without additional user input as it is all computed directly from the geometry. The nonlinear damping is displayed separately in the HYDROS.DAT file for those interested. There is also a /NODAMP parameter to omit all non-wave-related hull damping terms if desired.

For appendage support, the new SEA /BILGEKEEL parameter makes it possible to specify the bilge keel component(s) in your geometry file. The effects of bilge keels, including added mass, damping, and stiffness terms, are then computed based on the actual appendage geometry, not just an approximate method. Once your bilge keels are modeled, it is very easy to see the effects:

set head_speed_samp="/head:90 /speed:16 /samp:60,0.1,1.0"

sea {head_speed_samp} /data:re /nodamp
sea {head_speed_samp} /data:re /bilgekeel:hull\bk*

That's right, an Excel plot. The horror. To create this plot, we used SEA /DATA:RE to write the roll RAO (and all other RAOs) to a CSV data file for easy import into Excel. Alternatively, one could read the CSV files back into GHS and create a custom plot, as evidenced by this run file.

Support for other types of appendages will be released soon. And of course, the theoretical methods used to predict damping and appendage effects will be published in the SK User's Manual, perhaps in time for John Bonn's holiday reading.

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

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