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

Acceleration Transform Wizard

One of the joys of working with the GHS command language is creating tools that automate repetitive and time-consuming calculations. One such task of tedium is computing the vector components of the total acceleration acting on a cargo. Typically, the absolute dynamic acceleration is known as the longitudinal, transverse, and vertical components in some external reference frame, i.e. the waterplane.

However, it is customary when designing foundations, sea-fastenings, or lashings, for example, to compute the total accelerations perpendicular and parallel to the ship deck. A possible "worst case" scenario would occur when the vessel is also heeled to the maximum expected roll angle, meaning the deck is no longer parallel to the waterplane. Now we start dusting off our trigonometry skills...and textbooks. Oh boy.

But wait, there's a better way. Without getting too deep into the mathematics, the new Acceleration Transform Wizard implements a three-dimensional rotation matrix (read about it here). There is no need to derive trigonometric equations, although there are some here in the office who quite enjoy the challenge, and would do so anyway.

Now, by simply entering the known acceleration components and entering the maximum expected roll angle (with a 3D rotation matrix, you may enter pitch or yaw angles too, for a more generalized approach), the run file will transform the accelerations into the components parallel and perpendicular to the deck by multiplying the acceleration vector by the rotation matrix. Here's a look at the wizard:

But what about gravity? Ah, just a single check box. Since gravitational acceleration only acts in the z-direction when in the waterplane reference frame, the additional acceleration component is simply added in before the transformation occurs.

After the transformation, you have the x, y, and z acceleration components in the new reference frame. Also given are the resultant deck-normal and deck-parallel accelerations.

The Acceleration Transform Wizard is available for download here and does not require the SeaKeeping Module, although all of the required input data can be easily obtained from said module.

Questions, comments, or requests?
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