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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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WIND Moment Methods - variation with heel (review)
(Requires GHS version 18.88 or later ) |
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We outlined the three methods used to calculate wind areas in a previous COW. The methods are referred to as the lateral plane method, the band method, and the newest is the panel method. Previously, all three methods were shown to produce the same heeling moment for an upright vessel where shielding between parts is ignored.
When the same vessel is inclined, there will be areas where the main hull and the towers overlap. See the 4 dark red squares in the image below. Ideally, the projected area of these overlaps shouldn't be counted. As you'll see, a small shielding distance can be used with HMMT /Panel to eliminate any double counting of wind area. |
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You may recall that the four towers are modeled as separate parts in the model so that they don't shield each other when using default HMMT /BAND settings. If we add the /Combine parameter, then windward towers will completely shield the leeward towers.
The following HMMT REPORTs show the resulting moments for each method at 30 degrees of heel. Note that the lateral plane, band, and panel methods all give the same result when there is no shielding between parts. |
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When you plot the results of the different heeling moment methods for a range of heel angles, then a pattern develops.
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HMMT /Band /Combine produces lower heeling moments (even at zero heel) than any of the other methods because the leeward towers are shielded completely.
Note that the HMMT /Panel curve matches the Lateral Plane method when the shielding distance is zero. When the shielding distance is 1, however, the heeling moment gradually transitions to a value similar to /Band /Combine. It turns out you get nearly the exact same result as the /panel:1 calculation if you prohibit shielding of the leeward two towers with HMMT /Band /Combine /NoShield:twr_fs,twr_as. This is because the duplicate area where the bottom of the leeward towers overlaps the hull immerses at a heel angle of about 20 degrees before its projected area amounts to much. The band shielding between the windward towers and hull eliminates any duplicate areas, giving the same projected area as HMMT /Panel:1. |
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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 |