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General HydroStatics
Ship Stability Software |
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Command of the Week
(New or interesting aspects of GHS that you may not know about) |
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GMTMMT
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A young naval architect name Eileen contacted us recently. She was fresh out of college and was preparing for her first ever, live, in-the-flesh stability test. This was a "big deal" and she was doing everything she could to make sure it would go smoothly. She had already read through guidance documents such as ASTM_F_1321, but wondered if we could recommend anything specifically related to GHS that would help.
"Well, you should definitely review HELP GMTMMT," we offered. "That's a command, what kind of cryptic name is that?" she replied. "It stands for GM transverse moment." "What?" "You know one of the results of the inclining experiment is that plot of the known heeling moments versus the tangent of the observed heel angles?" "Oh yeah," she said, "you mean like this example from a previous stability test?" |
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"That's the one. You see, the slope of that plot is the GMT moment, and you enter it into GHS via the GMTMMT command. From that value and the supplied draft readings that you entered, GHS solves for the vessel weight and CG location."
"Sorry, did you say draft readings, with an S, as in plural? I've only ever used DRAFT with a single value." "In that case, you should review HELP DRAFT as well. There is a form of the command where you can enter several drafts and the longitudinal locations where they were taken. This sets the draft, trim, and even deflection in a single command." "That will be very handy; it looks like I'm supposed measure the freeboard at 5 locations along the vessel and I was wondering how to enter them into GHS. Getting back to GMTMMT, are you telling me one single command finds vessel weight as well as the LCG, TCG and VCG? I needed a whole spreadsheet to do that when I was taking Intro to Naval Architecture. If I'd known about this GMTMMT command back then, I could've gone on that weekend sailing trip!" We further suggested that she take our example run file and work it up with as much information as she could before going to the vessel. Then, during the experiment, she could get a reality check on the predicted displacement and CG locations. The Inclining Wizard is a good alternative to the run file. You can find it in the GHS pull-down menu under Wizard\Light Ship\ and Wizard\All\. Our parting shot was to suggest, "If you are really worried about getting this right, it only takes a moment to simulate the whole inclining with a short run file. Use the best weight estimate you have for the vessel and that will give you approximate heel angles to look for once the real experiment gets started." |
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read fv.gf
we 250 -2 0.01 8 add "Inclining WT" 1 20 0 14 solve macro shift `%1:TCG offset, include "*+" or "*-" before the offset to indicate direction add "Inclining WT" 1 20 %1 14 solve st / status .shift *+5 .shift *+5 .shift *-10 .shift *-5 .shift *-5 |
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Questions, comments, or requests?
Contact Creative Systems, Inc. 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 Click here for an index to this and previous COWs |