Which technique could be used to give a more comfortable roll to a stiff vessel?
A. Concentrate weights on upper decks
B. Add weight near the centerline of the lower hold
C. Move weights lower in the ship
D. Ballast the peak tanks
A. Concentrate weights on upper decks
B. Add weight near the centerline of the lower hold
C. Move weights lower in the ship
D. Ballast the peak tanks
A
ReplyDeleteThe best answer is A, because if the vessel is "stiff" its GM is large and to reduce roll angle and increase rolling period for a smaller roll we need to reduce GM. This can be done by stowing concentrated weights on the upper deck. You should note it can also be done by concentrating weights port and starboard to the sides of the vessel to create more "Transverse Mass Moment of Inertia" the same way a tight-rope-walker used a weighted pole to help walk the wire with a steady motion.
ReplyDeleteLets see why I would not choose B, C or D. Either B or C would only make the GM bigger and increase rolling angle and shorten rolling period. Choice D would only really affect the vessel's trim and pitching motion by increasing the "Longitudinal Mass Moment of Inertia".
Let me clarify one thing a large GM has a short rolling period and a large rolling angle just like a bell buoy. That is how the bell get's rung by wave action. The stiff buoy actually is trying to stay perpendicular to the surface of the waves. On the other hand a small nor almost non-existent GM will have a very long rolling period and a very small angle of roll. A floating log is a good example. it will not respond to a wave and most likely the wave will wash over the log.
BIG GM: Large angle of Roll and Short Roll Period that produces greater angular accelerations. Greater accelerations produce greater forces on the ship, crew and cargo. F=MA
SMALL GM: Small angle of Roll and Long Roll Period that produces smaller angular accelerations thus smaller forces on the vessel. Again F=MA rules.
Sadly if you ask most people about this they think a small GM produces larger angles of roll and a large GM has a small angle of roll, which is not true!
A. Moving weight higher will lower the GM
ReplyDelete