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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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SPECIAL HMLPA
(Requires GHS version 18.82A or later) |
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What is the SPECIAL command and why is it important? Here is an example.
Calculating inclining effects due to wind forces can be quite a complex subject. Even with methods that involve fairly substantial simplifications, there can be nuances that need to be attended to. Consider the following simplified crane barge with 3 deductions: |
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The overall barge is 50x20x10; two of the deductions are 4x20x1, and the third is 4x4x10. Should the deductions reduce the projected lateral area of the hull?
Well, it turns out that you, the user, have full control over this. The HMMT command has parameters such as /NONEG and /YESNEGBELOW which change the default behavior for deducting displacer components. But wait, what if we want to treat the deduction on the upper left differently from the moonpool deduction on the right since it wouldn't actually reduce the windage? That is possible using SHAPE FACTORS! You didn't think they were reserved just for MODU regulations, did you? Setting the shape factor of the moonpool deduction to 0 eliminates any possibility of it reducing the vessel's lateral plane area. The STATUS LPLANE command isn't affected by these HMMT parameters or shape factors unless SPECIAL HMLPA mode is turned on. It is off by default to maintain continuity with the legacy behavior of STATUS. If you put this command in your library file (see HELP MAIN) or at the top of every run file, then the methods used by STATUS LPLANE (and system variables LPA1, LPA2, HCP2, LPAARM) are adjusted to align with HMMT behavior. The handy table below clarifies whether deductions are treated as negative areas or ignored. |
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| While SPECIAL HMLPA has been available for many years, in version 18.82A, the STATUS table header was improved to indicate that areas include the effect of shape factors. |
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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 |