Page 14 - WestlawnMasthead20_Dec11

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On Calculation of Maximum He
ue to Off
Center Loading
(This Issue’s Question is on Page 13)
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
The 75-foot, aluminum expedition motorcruiser
Green Hawk
is fitted with a pair of identical 17-foot RIB
launches on her sundeck. They are deployed and retrieved using a crane davit, with a cable hoist, located on
the centerline and configured to launch either RIB off either side of the vessel. The arrangement is as shown
the drawing next page.
Green Hawk
is 66.5 feet on the waterline, 22.5 feet beam overall, and 21.5 feet wate
line beam. She displaces 77.23 tons, and her GM—as equipped and outfitted with launches and normal cruis
gear, crew, fuel, water and supplies—is 4.74 feet.
Assuming the case for maximum heel, and that each RIB launch weighs 2,500 pounds with their outboard, f
gear and equipment aboard, what is the largest angle that
Green Hawk
will heel to when launching or retriev
one of these RIBs?
The Winners Are:
Once again, we received nearly a dozen answers to the September 2011 Know It All question. Only two of th
were correct, however. These were from naval architect and marine engineer Alan Gilbert and from Michael
Ales, a professor in the Department of Marine Engineering at the U.S. Merchant Marine Academy. As you wo
expect, with experience and backgrounds such as this, both Gilbert and Ales are clearly too smart for their o
good. With their now proven status as brainiacs, we had no choice but to award each of them the official titl
“Know It All.” Henceforth, naturally, they should be addressed as such. Of course, Westlawn T-shirts, caps an
Know It All certificates are speeding their ways to our winners.
And the Solution Is:
Green Hawk
’s heel angle during boat launching is a two-step process. This is because when a weight
hosted on a crane using a cable, the effective center of gravity of the lifted weight immediately rises to the
point of the upper attachment of the cable. In this case, the vertical lift is 13.67 feet.
To find the vertical rise in the center of gravity resulting from this lift in the position of the RIB’s weight:
The change in location of CG is found from:
CG shift, ft. = (w x d
w = weight moved, lb. or kg
d = distance moved, ft. or m
W = boat displacement, lb. or kg.
The boat is lifted 164 inches, which is 13.67 ft.
CG shift, ft. = (2,500 lb. x 13.67 ft. lift
173,00 disp. lb.) = 0.197 ft. rise in CG of boat
The rise in CG reduces GM (metacentric height) as follows:
4.74 ft. GM – 0.197 ft. CG rise = 4.54 ft. GM
The reduced GM can now be used to find the heel angle:
Continued next p