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

CONTENTS (part 3)

How close is GHS to the real world?

The answer is always the same: close enough.

For example, regulatory stability calculations pertain to an abstracted "real world." GHS models ships in a manner which is quite "close enough" for that purpose.

Likewise GHS seakeeping calculations predict accelerations that are statistically "close enough" to what can be expected in the real world and are therefore quite useful.

Salvage situations are different. The real world stands at the door in real time and must be dealt with by matching the GHS model to certain details. GHS modeling addresses the particulars and makes predictions that are "close enough" for the salvage engineer's purposes.

But none of this was acceptable to Fred.

Fred likes to stop by the office and give us a hard time about "making things up." It seems he can't quite grasp how invented computer code can predict the behavior of a real ship.

You must understand that Fred is an old-school flight instructor. He demands the utmost precision when his students are performing maneuvers. There is no "close enough" with Fred. If your line of sight to the pylon deviates only by the width of a coke can at the wing tip, Fred demands that you get it down to one inch, preferably less. During your pre-flight inspection he expects that you mark down the precise reading in gallons when you draw the dipstick from the fuel tank.

Someone asked him, "Fred, how do you account for the fact that the actual amount of energy in your fuel tank depends not only on the volume the dipstick indicates but the temperature too?"

It took Fred by surprise. It would have been against his nature to say that the difference was insignificant in general aviation.

"GHS takes temperature into account!" our man boasted before Fred could reply.

"No kidding," said Fred. "I'd like to see that!"

Soon the computer next to him was showing how a tank might be initially loaded with aviation gasoline at a temperature of 80 degrees and then by lowering its temperature, the sounding would go down. He was familiar with the API degree method of representing density, so he had no problem with the 70.44 API referenced to the standard 60 degrees.

tank wt1.s
contents "oil @ 80F", 70.44 API60
load snd:5.0
load status
contents "oil @ 40F", *
load status

"Okay, now tell me this," said Fred. "How does your fancy software account for the fact that one guy has a computer that runs hot and another one runs cool? Doesn't that throw off your calculations?"

That one took us by surprise. It appeared that our demonstration was completely lost on Fred. Then he laughed. He understood software well enough to make a joke about it. He just hated to admit it.

Questions, comments, or requests?
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USA phone: 360-385-6212 Fax: 360-385-6213
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Port Townsend, WA 98368 USA

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