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

Dynamic Stability
(Requires GHS version 17.5 or later with DS, SK, and AF)

The year 2020 will undoubtedly be remembered by all for its abundance of DB COWs. Even John Bonn said he never left his house, no doubt waiting for the next COW.

Until the addition of the SEAKEEPING command in 2018, GHS had operated in a primarily "static" world. Since then, SEAKEEPING has grown from a fledgling command, to a powerful analysis module adopted by users around the world.

Hydrostatics and hydrodynamics are obviously quite useful in their own right, but leveraging and exposing the synergy between them has always been a major development priority since the addition of SK. A perfect example of this synergy is the new DYNSTAB command: a complete solution for the newly adopted IMO Second Generation Intact Stability guidelines.

If you aren't familiar with these guidelines, here's a recent article that we wrote in collaboration with the folks at the American Bureau of Shipping (ABS).

In short, the new criteria address dynamic stability failure, which is unfortunately far too common. To this end, the IMO SGIS guidelines consider five different dynamic failure modes: Dead Ship, Excessive Acceleration, Pure Loss of Stability, Parametric Roll, and Surf-riding/Broaching. Each mode may be analyzed using a level 1 or level 2 analysis, with level 1 being a conservative vulnerability check, and level 2 being a more complex analysis. DYNSTAB makes it very easy to run a complete analysis on all five modes and all necessary levels with only a single command (and a few others...):

read tanker
enter pm
deckedge `DYNSTAB requires a deck edge
write tanker.gf
quit `pm
we 6400 47f 0 5 /gyr:gf,gf,gf
so

crt "EA-CREW" 15f,0,15 /noflood

vari res="-0.04441,0.077841,-0.02772,0.004242,-0.00018"
vari thr="0.2244,-0.2283,-0.1373"
prop 3.5,1.885f,0,1.756 /thrust:{thr} /wf:0.156 /td:0.142

report dynstab.pf /box:color
 status we di in
 crt report
 prop report
 roll imo /s /ak:10
 dynstab /speed:12 /res:{res} /sum
report close /preview

The result is a compact summary of vulnerability status for all modes of failure:


Dynamic stability has been termed the "second generation" of intact stability analyses. GHS has already been around for one generation; welcome to the next. Happy New Year!

Questions, comments, or requests?
Contact Creative Systems, Inc.

support@ghsport.com

USA phone: 360-385-6212 Fax: 360-385-6213
Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday

Mailing address:
PO Box 1910
Port Townsend, WA 98368 USA

www.ghsport.com

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