Here's last week's poll and its results (at right). Two people got it right – the correct answer is the longitudinal center of gravity.
Many moons ago, this little fact was vividly demonstrated to me, in a memorable way. I was serving on a U.S. Navy ship, the U.S.S. Long Beach. This was a nuclear-powered guided missile cruiser, a one-of-a-kind experimental platform for surface nuclear propulsion, electronic systems, and missiles. It was also, oddly enough, the tallest ship in the Navy – about 210' above the waterline if memory serves me right.
On the day of my center-of-gravity demonstration, we were cruising off the southern California coast. A storm moved in, quickly and a bit unexpectedly – and we were subjected to the fiercest winds and largest waves I'd yet experienced. In various maneuvers the ship was subjected to winds and waves that tipped it far over – 48°, according to the “barfometer” (clinometer) in my working space. The really remarkable thing was how long it took to straighten back up – and then swing the other way – it seemed to take forever, but I timed a couple of the recoveries at over 50 seconds (the time between maximum tipping and being back straight up).
Both the large angle of tipping and the slow recovery were caused by the ship having an unusually high longitudinal center of gravity (LCG): just a foot or two below the waterline. The further below waterline a ship's LCG is, the more stable it is.
But what is an LCG? The easiest way for me to visualize it is this: Imagine a ship with a shaft all the way through it, lengthwise. Then imagine this shaft is located so that the ship could be rotated around it while balanced. That shaft would be directly through the LCG.
Now imagine that same ship (with the magic LCD shaft) in the water. If that shaft were exactly at the water line, then it wouldn't matter to the ship whether it was right-side up or up-side down – it would neither right itself or flip up-side down. If the shaft were above the water line, the ship would immediately flip over, up-side down. And if the shaft were below the water line (as it would be on any real ship), the ship is stable in an upright position.
Naval architects (that's what the engineers who design ships are called) pay very close attention to the LCG; it is one of several critical design parameters for any ship. I was told that the U.S.S. Long Beach was initially designed with an LCG considerably lower than it was when I was on board. The LCG was raised when additional superstructure was installed in the 1960s, along with large amounts of additional electronic equipment – some of it located far above the waterline – in the last 1960s and early 1970s (I served on board in the mid-1970s). Whether that raised LCG was accidental or intentional, I couldn't say.
But it sure was uncomfortable!
