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

Roll Period
(Requires GHS version 17.00 or later with SK)

With one last silent thought of motivation, you drop in over a steep ridge on 26 inches (66 cm) of new snow, hit the wrong edge and click... your right ski ejects and you bury your head in the snow. Upon recovery, you can't find your ski and eventually determine it continued merrily down the rest of the face without you--100 feet down.

In skiing, ankles, and ships, rolling is bad no matter how you look at it. Minimizing (or designing for) roll motion is often a topic of ample consideration.

In our quest to quantify roll motions, we sometimes need to calculate the "roll period" of a vessel subject to a certain condition. But roll period is decidedly ambiguous, so exactly which roll period are we seeking and how do we compute it in GHS?

Average Roll Period in a Seaway
wave (spe) br 6.3 3.1
 sea /head:90
report close /preview

When using a wave spectrum, a response statistics table will be included in the report automatically. Here you will find the roll periods listed among other response statistics:

What is the difference between the Zero-Up-Crossing period (PERZUC) and the period between maxima (PERMAX)? Find the answer here, along with other seakeeping related Q&As.

Natural Roll Period
macro NaturalPeriod
 `%1 is physical mass
 `%2 is added mass
 `%3 is stiffness
 `%4 is modified variable name
 vari R1
 set R1=%1 PLUS %2
 set R1=%3 DIVIDE {R1}
 set %4=2 TIMES 3.14159 DIVIDE SQRT {R1}
 clear (R1) vari

wave (spe) p2 2.3
run sk.lib /call /quiet
set sk.nwaves=60
vari (string) skparams="/samp:{sk.nwaves},0.5,3.0 /resp:p"

 sea /head:90 {skparams} data:hy /period
 .sk.gethydrodata "SK-HYDROS.DAT"
 vari Tn
 .NaturalPeriod {sk.Ixx} {sk.A44_{sk.nwaves}} {sk.C44} "Tn"
 \\ Roll Uncoupled Natural Period: {Tn:3} Sec \
report close /preview

This returns an uncoupled natural roll period of 4.263 seconds. Looking at the location of the peak in the roll position RAO (noting that parameter /PERIOD was used to plot the RAO against wave period instead of frequency) we observe the expected agreement.

Questions, comments, or requests?
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