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

(Requires GHS with Condition Graphics)

The purpose of the BOOM command is to position a weight by spherical coordinates rather than the typical rectangular LTV coordinates of the ship's system. Commonly it is used to set the weights associated with a crane, where you specify the boom length and the azimuth and elevation angles.

Now suppose you want to know exactly where on the deck the hook would drop when the boom is at a certain topping (elevation) angle and swing (azimuth) angle. The trig functions are available, which you could use to calculate it, but an easier way is to use ADD /ACCESS.

For example:

BOOM "Wt name" {WtValue}, {Lbase}, {Tbase}, {Vbase}, {BoomLength}, {SwingAng}, {TopAng}
ADD "Wt name", *, Lhook, Thook, Vhook /ACCESS

ADD /ACCESS sets your variables Lhook, Thook, Vhook to the LTV coordinates you were wanting. Note that in the /ACCESS form of the ADD command, you access values that were previously given.

The reason this works is that BOOM is really a fancy ADD. In other words, BOOM creates the same kind of ADDed weight that you get with the ADD command. So ADD /ACCESS works with weights that were created by means of the BOOM command.

In case you're still unsure what to make of the BOOM command shown above, note that all of those names would be variable names that someone has dreamed up and set to values pertaining to the location and size of the boom. The curly brackets surrounding the names cause their values to go into the command. "Wt name" is a literal name, not a variable, though it could be a variable name like the rest.

We put this technique to good use after our fishing vessel, FV.GF, sank. We needed heavy-lift equipment. So we hurriedly built this husky crane barge:

BOOM commands are good for positioning not only the weight of the hook load but also of the boom and other rotating elements. But that massive ram cylinder needs special treatment because its elevation angle is not easily derived from the boom's elevation angle. With the BOOM + ADD /ACCESS we get the end points of the ram, which enable us to calculate the angle, length, and CG of it. For the visual model we made the crane elements as "SAIL" parts, which allows us to position them to match the crane orientation.

To see the salvage operation in action, here is the run file. If you have GHS and Condition Graphics, try it out! If not, you can still look over the run file. Even if you do have GHS you may discover something you had not seen done before.

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