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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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HEIGHT
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An acquaintance from the local waterfront rushed into the office
demanding that we tell him if his sailboat would stay upright or
lay over as the tide went out. He said she had touched bottom
while at anchor, and the water level would drop two feet
at low tide. He was working on the engine at the time it happened
and had no way to get her into deeper water.
"Haven't you ever had her go aground before?" we asked. "Oh, yes," he replied. "But it's uncomfortable staying aboard. Since the tide is supposed to drop only two more feet, maybe she'll stay upright. I need to know if I can continue working aboard." "Go see Tim the naval architect," we told him. "We can't do anything without a GHS model of your vessel." "I just happen to have it on me," he said, and produced a thumb drive. "The depth was seven feet one inch at midship when she grounded. Can't you tell me real quick whether she will heel over?" We gave in and made a simple GHS run for him. He estimated the trim at one degree aft, so after reading in his vessel model and observing that the baseline was at the 7-foot design waterline and midship 30 feet aft of the origin, we put a critical point at the ground and used the HEIGHT command to set the water depth (negative height) at midship where he had taken the sounding: |
trim=1.0 `estimated 1 degree
crtpt (1) "ground" 30.0, 0, -7.0 `critical point at the ground
HEIGHT (1) = -7.083 `observed height of the critical point
ground "bottom" 0, 30.0, 0, -7.0 `establish a ground point to take the weight
vcg=-1.0 `his estimate
solve weight, lcg
HEIGHT (1)=*+2.0 `Increase (decrease negative) height for the tide drop
solve trim
status ghs
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The STATUS report showed a disappointing GM that was only slightly positive.
We told him the GM was only as good as his VCG estimate, so we couldn't
guarantee that his boat wouldn't tip.
"Thanks, but I could have guessed that myself," he said. We told him if she leans over as the tide goes out to observe the heel angles and water depths. Then he would have something to take to Tim who would be able to get him an accurate VCG, which may be useful next time. "But we're heading down the coast as soon as I get the engine running," he said. "What if the naval architect doesn't have GHS?" "They all do -- at least all the good ones use GHS." No sooner had he left than another local waterfront guy burst into the office. He said the boat he designed that had been 20 years building just got weighed for the first time and had turned out to be heavier than he had calculated. He was afraid it would submerge the engine exhaust ports if it had full fuel and empty water. He was desperate to find out and had to know now! We recommended he go see Tim. He said this was all he needed. He presented a diskette and said, "I need a one-inch clearance at the exhaust port." We found an old computer with a diskette drive, brought up GHS, read in the geometry file, and proceeded to employ the HEIGHT command to set the waterplane. "What is your design draft?" "Five feet." |
draft=5.0
vcg=5.0 `we knew he wouldn't have a VCG, so didn't ask
solve weight, lcg
load (fuel*)=0.98
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"Where is the exhaust port, exactly?"
"45 five feet aft, 4.5 above baseline." |
crtpt (1) "exhaust port" 45,0,4.5
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"Ok. We'll need recursion to find the weight and trim."
"What's recursion?" "We're setting up a loop to recalculate trim since it affects the port clearance." |
macro findwt `will pass the desired clearance as a parameter %1
HEIGHT (1)=%1
solve weight
solve trim
variable ht
HEIGHT (1) ht /ACCESS `retrieves the actual height after trim solving
set ht={ht} minus %1
set ht=abs {ht}
if {ht}>0.01 then exit findwt %1
/
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"One inch you say?"
"Right. One inch." |
.findwt 0.08
status ghs, crtpt
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"So there you are. Is that weight budget something you can live with?"
"It will be tough to meet." "What about your LCG? Is that on track for level lightship trim? Because that's what we assumed." "LCG? What's that?"
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