Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)
John Bonn called and offered to fix the problem featured in last week's COW—
the unexpected flat deflection that we said was reported by the captain
of the chemical tanker TANKER.GF.
John said he would be glad to perform a light-ship survey on TANKER.GF and
update their GLM to match observed drafts.
We passed the offer on to Capt. Chemical who assured us that he had already done that. But he wanted to see if he could get the hog/sag prediction which comes out of the LS (longitudinal strength) command to match observed deflections. It would be an additional check that the light-ship weight curve was still producing real-world bending moments.
When we remarked that it seemed it had not taken him very long to do that, he let us in on his method. It turned out that he had not done a physical light-ship survey but instead had used GHS to supplement the light-ship weight curve to bring the drafts into very close agreement (after accounting for the permanent sag). He sent us his Run file, thinking it might interest John Bonn.
We took the liberty of running his Run file, and here's what we got. The first status report is before the correction and the second is after the weight-curve correction and deflection calculated by LS.
The captain explained: Over the years he had made small weight additions in the GLM to
keep draft and trim agreeing exactly with observed drafts. He said these weights accounted
for "growth" primarily at the ends of the vessel. As expected, this slightly increased the hog deflection
predicted by the LS report in his GLM. But he had never checked
the deflection by making careful draft measurements along the length of the vessel
as he did last week. The loading he had on that day was concentrated in the middle of the
ship, so he expected that the hog prediction would be small or even zero. The fact that
it came out exactly zero was a rare accident of small random
errors in the measurements that happened to cancel each other almost perfectly.
(His consternation about it was somewhat of a joke.)
Capt. Chemical decided to make good use of that draft survey, which was done under ideal conditions. He wanted to see if he could verify that the deflection prediction from the LS command was on target. He knew that the original light-ship LS calculation had predicted a hog that was greater than the observed deflection by 3.0 cm. So that 3.0 cm was presumed to be the built-in sag. "She's about as straight as they come," he said.
In other words, if the ship were loaded such that LS predicted zero deflection, then a careful draft survey should show 3.0 cm of sag.
Now with the loading last week that flattened out the built-in sag, he would expect the LS calculation to show a 3.0 cm hog. But instead it predicted 3.9 cm hog, which meant the adjustments he had made to the fixed weight may have been too far toward the ends of the vessel or maybe other small changes had accumulated or perhaps the aging structure had changed slightly. Air and water temperatures were the same, and the survey was done early in the morning before the sun heated the deck.
So to correct this, whatever it was, Capt. Chemical augmented the weight curve by distributing a compensating weight along the middle of the vessel at the VCG and tapered at its ends to avoid introducing shear. He wrote a Run file and used his GHS program to find the size of the compensating weight that would correct the deflection calculation.
Of course he had to scale back the light-ship weight distribution slightly to compensate for this in order to keep the displacement from changing, which is readily done with the command
SOLVE WEIGHT, LCG
after setting the observed waterline.
John Bonn was indeed interested, but not for the reason we expected. He wanted to know why the captain of a ship did not stick with his GLM and how he was able to use GHS and why in particular he had taken it upon himself to modify the light ship in the GLM by a nonstandard method since all that sort of thing is supposed to be done by a naval architect.
We had to apologize to John—and now everyone—for not mentioning earlier that Capt. Chemical is a naval architect. He decided he did not want to spend his career working in an office, so he got his captain's license and went to sea.
John remarked that he had never heard of a marine engineer so keen on getting numbers to match up perfectly, but now that he knows that Capt. Chemical is a naval architect it all makes sense.
Click here to get Capt. Chemical's Run file.
Questions, comments, or requests?
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Port Townsend, WA 98368 USA Click here for an index to this and previous COWs