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

(Requires GHS with Multi-Body)

Wind heeling moments are easily derived from the geometry model by means of the HMMT WIND command. But what if various deck loads make a significant difference? You can turn SAIL parts on and off with the TYPE command, but anticipating every deck load that might come along is not practical. You could add to the Geometry File, but if you would rather not mess with the basic geometry, there is an alternative.

By means of READ /APPEND you can bring in additional geometry that combines with whatever geometry is present from the original READ. Another advantage of this is you can keep your deck-load models in separate Geometry Files that can be applied to any ship.

If you have never used Multi-Body, this might raise several questions:

Q. Can I read in the same Geometry File more than once? You see, I went ahead and modeled a certain piece of deck cargo and have stored it in a separate Geometry File. I would like to read it in two or three -- or however many -- times in order to place several copies of it on the deck of my vessel.

A. Sure. Go ahead. Bring 'em all in.

Q. All right. But since duplicate Part names are not allowed, how can I read in a Geometry File more than once since it will try to bring in the same Part names?

A. No problem: the /RENAME parameter allows you to change the names of Parts as you read them in.

Q. Ok. Good. Of course I need to put each one at a different place on the deck. How do I do that?

A. You can locate each one anywhere. Just include the /SHIFT parameter. By the way, if you happen to be loading something that has tanks, or if you're being clever and are filling some object as if it were a tank in order to get its weight and C.G., the tank loads get shifted too.

Q. I know that. It's obvious that the tank loads would be where they are. But here's one I bet you haven't thought of: What if I want to bring in just a certain Part out of all the Parts in the file? For example, maybe I have several deck-load objects on the same file.

A. Just happens to be a parameter for that: use /PART followed by a list of the Part names you want to bring in.

Q. Good deal. Looks like Multi-Body may be useful. Can you give me an example?

A. Our Supply Vessel wanted to get into delivering windmills. So we threw together some cylinders and made something that resembles a windmill. We needed three blades, but after making the first blade it occurred to us that we could READ /APPEND one blade three times.

Q. Won't the blades be oriented all in the same direction? I think you had better not be so lazy and make all three blades in Part Maker. It isn't that hard with cylinders, you know.

A. They're only SAIL parts.

Q. Sure. So what?

A. You can do fancy things with SAIL parts -- even orient them in any direction and change their colors.

Q. I tried that, and it didn't work.

A. Did you surface them?

Q. You mean in Part Maker with the SURFACE command?

A. Right. The TYPE command moves and reorients surfaced sails. But you will need to use HMMT WIND /BAND to pick up the correct locations of any SAIL parts that have been moved or reoriented. ...

Here is a macro that makes the complete windmill. It uses READ /APPEND to bring in first the base Part and then the blades one by one -- all from the same Geometry File.

Q. Why didn't you bring in the whole thing on the first READ /APPEND? Then you would need only two more to pick up two more blades?

A. You have a point, but the macro is prettier this way.

Q. Wouldn't it demonstrate another option? Somebody might think you have to bring in only one part at a time.

A. It's too late to change it now.

Q. So this macro makes one windmill. I see you're using several macro parameters. The first one adds something to the Part names, so there will be no duplication of names. The second one gives the longitudinal location. Where did that gf.towerht variable come from?

A. That's the "tower height" from the windmill's base to the center of the rotor.

Q. I see that, but where did it come from?

A. Well, from Part Maker.

Q. You mean you can access variables that were defined in Part Maker?

A. Just like that.

Q. Tell me more. Are the variables carried in my Geometry File?

A. It's off the subject. I'll get back to you.

Q. In another COW, I suppose?

A. Probably. There's more new stuff in Part Maker that we need to explain, somehow.

Q. By the way, how did the heeling moments come out? Did you get the contract for hauling those windmills?

A. No, unfortunately we need a bigger boat. Here's the righting-arm and heeling-arm curves, and that's with 30 knots of wind, only.

Q. Why don't you guys get a bigger vessel for your work here instead of always using that old offshore supply vessel? All you have to do is pop into Part Maker and scale your old one to a new size. It would take half a minute!

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