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

(Requires GHS version 16.82A or later with SeaKeeping)

Last week we offered a glimpse at the power of the /COUPLE parameter and alluded to the level of development effort that went into that one parameter. This week we tell you how to throw all of that out the window.

The /UNCOUPLE parameter, as you probably guessed, uncouples the equation of motion. Since COW158 explained the power of a fully-coupled system, you may be wondering what utility an uncoupled system could offer.

All by itself, /UNCOUPLE will do exactly that: fully-uncouple the system. While fully-uncoupled motions are sometimes needed as input for certain rule-based analyses and other computational tools, a fully-uncoupled system is not necessarily why the /UNCOUPLE parameter is so useful. Uniquely, /UNCOUPLE allows you to specify one or more modes to individually uncouple from the system. This is quite powerful, because it grants you the ability to "turn off" certain degrees of freedom, opening the door to all sorts of creative analysis.

If you have a condition where it is reasonable to model the vessel as a reduced DOF system, you can turn off the modes that are not free. For example, if we wished to compute the motions for a structure which can be assumed to be free only in heave, sway, and yaw, we uncouple surge, roll, and pitch from the system using:


The report layout will remain virtually the same, with the exception that the indicated modes (1,4,5) will be displayed with asterisks noting that they are uncoupled from the other modes.

Note that any modes that are not included in the /UNCOUPLE parameter remain fully coupled to each other. So, in the above example, the sway-heave-yaw motions are fully coupled within their system and the surge, roll, and pitch modes are individually uncoupled.

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