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) |
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
report 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" report sk.pf 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. |
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