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

SPECIAL WPLUP


The SPECIAL command in GHS can do several things, none of which are needed outside of very special circumstances.

Some of the special things are definitely not what you would expect GHS to do. Like what it does in SPECIAL WPLUP mode, making all waterplane values (e.g. KM, BM, GM, FSM, WPA, etc.) be for zero heel regardless of the actual heel. This applies in Load Editor as well as for waterplane system variables:


Thus SPECIAL WPLUP fulfills a certain demand for showing loading conditions at zero heel even when the heel angle at equilibrium isn't really zero. (Credit for this goes to the expectations of a certain regulatory authority.)

We might even argue that GHS should not do this because SPECIAL WPLUP can lead you to present information that's very wrong. To put it kindly, it's dangerous. To minimize this danger, STATUS WPL, HS, and GHS reports are available only at zero heel.

Here is an exaggerated example, illustrating in principle the kind of deception that could happen in SPECIAL WPLUP mode.

The ship has a large interior deck that we are assuming is flooded. The condition we're looking at has the waterplane just below the interior deck level.

We called for hydrostatic properties at equilibrium and got this:


As you can see, in order to satisfy the restriction imposed by SPECIAL WPLUP we set the heel angle to zero then solved for draft and trim to correct for any differences caused by nullifying the heel. The GM turns out to be very healthy indeed, so we might conclude that the ship has great stability in this condition.

But of course we know enough to always check the righting-arm curve:


Obviously there's a problem. The equilibrium at zero heel has a tiny range, and the GM there is deceptive. For practical purposes the equilibrium is at 9.45 degrees.


Here is the zero-heel situation with the interior space slightly above the waterline.


And here is the equilibrium that occurs after the proverbial mouse walks across the deck:


We do not recommend falsifying equilibrium. If the heel angle is very close to zero, it would probably be harmless, but you might be surprised.

If you're under some authority who demands that it be done the SPECIAL WPLUP way, be sure that the real equilibrium angle in every case is not much different.

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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