Page 15 - WestlawnMasthead20_Dec11

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The maximum heel takes place with the largest transverse shift in the RIB’s location. Accordingly, we need t
look at the case where one of the RIBs is launched on the opposite side from which it is stowed, say, the port
RIB launched over
Green Hawk
’s starboard side.
The transverse distance the RIB is moved is 244 in. or 20.34 ft.
The heel angle from shifting weights already aboard is found from:
Heel angle, degrees = arctan(w x d
W x GM)
w = weight moved, lb. or kg
d = distance moved, ft. or m
W = boat displacement, lb. or kg.
GM = metacentric height, ft. or m
arctan = The arctangent of X is an angle whose tangent is X, often notated as tan
. It is not the same as 1/t
Heel angle, degrees = arctan(2,500 lb. weight x 20.34 ft. shift
173,000 lb. disp. x reduced 4.54 ft. GM)
Heel angle = 3.7 degrees
Of course—as Gilbert correctly pointed out in his answer—properly speaking, the vertical rise in the center of
gravity of the articulated crane itself should also be included as the vertical shift of its mass further reduces
GM. In the case of this particular problem; however, the difference is very small. Keep this in mind, though,
should you do this calculation for situations with larger, heavier cranes and/or cranes with a greater vertical
In this example—if you neglect the vertical rise in CG—the difference in heel angle is not too large. Boats, ho
ever, have capsized because this instantaneous effect of the CG raising to the attachment of the hoist cable
high up was overlooked. This can occur, for example, on fishing boats such as purse seiners, which hoist ma
tons of fish out of the water in their nets. If there is already a load of fish and equipment on deck, when a bo
is rolling in heavy seas, the additional large extra weight of fish being hoisted out of the water and the result
large rise in CG can suddenly reduce GM to the point where the boat can capsize at the next wave-induced ro
to the hoisting side!