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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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WEIGHT
(Requires GHS version 17.00 or later) |
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Although the WEIGHT command can represent light ship and solid cargo together, more typically
the parameters issued with WEIGHT are for just the light ship weight and center of gravity
while other non-liquid weights come in via ADD commands.
If the light-ship weight is to be assembled from several items, it can still be represented in a single WEIGHT command using a series of semicolon-delimited segments in its parameter sequence. If elements of the light ship have names to be preserved, they can be presented by special ADD commands where a "+" prefixes the name, causing them to be reported as belonging to the light ship. When deciding whether to use WEIGHT for the bulk of the light ship or divide it among ADD commands, remember that the weight entered directly through WEIGHT is the weight manipulated by MAXVCG and by default the weight addressed when doing SOLVE MAXVCG and SOLVE MAXWEIGHT. Only one WEIGHT command is effective at a time since no name other than "WEIGHT" or "Light Ship" is associated with it. But any number of ADD commands can contribute weight items by assigning them unique names. When weight distributions are needed, the "weight" of the weight information (not that information has any weight) becomes greater, of course. WEIGHT and ADD commands have that capability as well because they can take longitudinal weight-density curves. GHS provides several tools for managing and verifying weight information. One of these is WEIGHT REPORT, which presents a graph of the longitudinal distribution (requires the LS module). |
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| For cargo weight items that must be distributed, the ADD_WEIGHT3 wizard is useful. Its interface looks something like this: |
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See more about ADD_WEIGHT3 here
There are parameters like /LEN:x1,x2 available with the ADD command which provide handy alternatives to a density curve if the distribution is linear. Particularly interesting are the cases where the weight is not attached directly to the hull girder but is cantilevered at some distance from its base. The weight distribution of such an item may include a negative area. An example of this is a crane where its weights are supported within the span of its base. Shown below is a Longitudinal Strength plot. The red line is the weight density curve, the blue or cyan line is the sheer curve, and the green line is bending moment. |
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More discussion about this is at COW 013
For motion calculations, the gyradii of weight items become important. By means of the /GYRADIUS parameter, gyradii about the three axes can be specified. See COW 071 for a presentation of this feature. DB AddLSWt is the Data-Base version of the WEIGHT and ADD + commands. DB AddFixWt is the Data-Base version of the ADD command. For example: DB AddLSWt /WEIGHT/ 3000, 48F, 0, 4.0 DB GetLSWt /WEIGHT/ is equivalent to, WEIGHT 3000, 48F, 0, 4.0 DB PutLSWt /WEIGHT/ For weights not in the light ship: DB AddFixWt "Crew and Effects" 4.0 1.0F 0 11.5 DB GetFixWt "Crew and Effects" is equivalent to, ADD "Crew and Effects" 4.0 1.0F 0 11.5 DB PutFixWt "Crew and Effects" But this is only the tip of the iceberg. DB AddLSWt and DB AddFixWt provide extensive templates for entering parameters. For example, DB AddFixWt "Store Top" brings up this template: |
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Nothing more than the Weight and CG fields are required in addition to the name,
but all of the many parameters offered by the ADD command
are covered as well.
As mentioned above, the ADD command can take a series of segments. For example, ADD "Store Top" 1.0, 22F, 0, 12.0; 1.5, 23F, 0, 12.5 The two segments are two weights with their CG values separated by the semicolon. Here is what it looks like in the AddFixWt template: |
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The DISTRIBUTE button brings up options for distributing a weight
longitudinally.
A good example for that is the light ship distribution showing here in the AddLSWt template. |
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All of the distribution options available with the ADD command are
represented. But here we see the direct input of the weight-density curve.
Check the "Edit in Table Form" and get this: |
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| If the distribution happens to be in "blocks" (constant weight density over lengths), a block-oriented version of the table takes blocks of weights directly. |
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| The Display button brings up a display showing the location, size and extent of the weight. Weights do not have shapes; the circle is representative of a sphere proportional to the weight, and the yellow bar shows the actual longitudinal span of distributed weights. |
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In this main template the sum of the segments is shown. In the
segments template the display which the button brings up shows
each segment with a cross at the center.
The lower part of the main template is divided into three sections that cover all of the other parameters that are available in the ADD command. Clicking the button reveals the details. |
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This is a brief survey of the Data Base weight-entry tool. Some features are left for you to discover.
If weight information is available in another format, the file I/O features and string handling in GHS can be used to do the translation. The result would be a Run File containing WEIGHT and ADD commands. |
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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 |