General HydroStatics Ship Stability Software

Command of the Week (New or interesting aspects of GHS that you may not know about)

Early Design Seakeeping (Requires GHS with SK)

An email popped up in our inbox over the weekend--and while we may not recommend checking email over the weekend--the sender was Bonn, John Bonn, and when you get an email from Mr. Bonn you tend to pay attention, for reasons entirely unclear. John's email was mostly related to his recent bicycle tour--he even attached some nice photos of an eagle. However, near the end he mentioned that the shipyard has begun construction on his client's vessel and he thought it might be a good time to check the vessel's seakeeping characteristics. Mr. Bonn, may we kindly suggest to do this kind of work at an earlier date? Too often a vessel's seakeeping characteristics are either entirely ignored, or considered far too late in the design to be properly considered. In many cases, seakeeping design requirements are not specified at all, but the seaworthiness and comfort of a vessel are still important considerations. In a preliminary sense, variations in principle dimensions, such as beam, length, or depth, can have a marked impact on responses. Other parameters of interest could be draft, VCG, roll and/or pitch gyradii. In John's case, the only parameters he could still change were related to the loading condition, and even then his options were limited by operational requirements. Fortunately, in this case, his vessel was reasonably seaworthy, but this was by no fault of his own. As an example, we investigate the effect of changing the beam of the FV and compute its RMS roll and pitch responses. First, we make a macro which enters Part Maker and uses the SCALE command to scale the vessel's beam.

macro scale_beam  read fv.gf  enter pm   scale 1,%1,1   write fv.gf1  qu `pm /

Then, we make a macro to run seakeeping on the scaled geometry. In this example, we keep the seaway at a constant significant wave height of 3.5 feet.

macro vary_beam  b:=b+%2  .scale_beam {b}  .condition  wave (spe) p2 3.5  sea /speed:6 /head:135 /data:st /brief /noplot  .sk.getstatsdata "SK-STATS.DAT"  vari RMS4%1={sk.RMS4_CG_P}  vari RMS5%1={sk.RMS5_CG_P}  vari B%1={woa} /

Note that the second macro vary_beam calls the first macro scale_beam. We can loop over the vary_beam macro to test a range of beams. Each time the beam is updated, the SEAKEEPING command is issued and SK.LIB is used to collect and store the RMS roll and pitch responses in user variables. Starting the whole process looks like this:

run sk.lib /call /quiet vari b=0.6 .vary_beam(10,1)1 0.1

Now that we have the RMS data conveniently stored in user variables, we can make a custom plot of the RMS responses versus beam.

The complete run file may be found here. Hopefully Mr. Bonn doesn't use this as a reason to design a 90-foot-long fishing vessel with a beam of 39 feet.

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