- #1
DaveC426913
Gold Member
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This is naive question, born in a half-conscious imagination, being rocked to sleep.
On a (very) recent vacation, I took a cruise aboard the MSC Orchestra, almost 300 metres long and almost 100,000 tons.
During medium-high seas, I lay in my bunk (midship, near top deck, 120 feet up) and felt the ship rocking up and down (pitching). The freq was about 2Hz (i.e. two rocks per second) and felt like it could have been a centimeter or more*.
*On another cruise, during heavy seas, I set up a pendulum in my cabin to see if I could record the gyrations, a la, whatever that toy was in the 70s - can't remember what it was called. But I digress...
It often stopped quite suddenly. i.e. bobbing for several minutes then stopping when the right cross wave threw off the motion. Yet still no creaks.
What exactly was involved in transmitting that from the waves to my bed?
I'm trying to imagine how a 300m, 100,000 ton mass could have such a motion transmitted through it at the speed of sound in steel without every beam and bulkhead creaking and groaning till its rivets popped. (It would take 1/20th of a second to traverse the length of the ship)
Was each wave transferring its energy from hull bottom to me? Or did the initial waves start up a resonance that, after a while caused a resonant bobbing? Would each motion traverse the ship at the speed of sound, so the ship was deforming with every hit? (i.e. a push at the bow ripples through the length of the ship till it reaches the aft, a few fractions of a second later?)
Or did the ship gain the resonance slowly until it was self-resonating - i.e rocking as a single unit (at 2 Hz)?
Is there a natural resonance for an object of this size/mass? Could it be in the 2Hz range?
On a (very) recent vacation, I took a cruise aboard the MSC Orchestra, almost 300 metres long and almost 100,000 tons.
During medium-high seas, I lay in my bunk (midship, near top deck, 120 feet up) and felt the ship rocking up and down (pitching). The freq was about 2Hz (i.e. two rocks per second) and felt like it could have been a centimeter or more*.
*On another cruise, during heavy seas, I set up a pendulum in my cabin to see if I could record the gyrations, a la, whatever that toy was in the 70s - can't remember what it was called. But I digress...
It often stopped quite suddenly. i.e. bobbing for several minutes then stopping when the right cross wave threw off the motion. Yet still no creaks.
What exactly was involved in transmitting that from the waves to my bed?
I'm trying to imagine how a 300m, 100,000 ton mass could have such a motion transmitted through it at the speed of sound in steel without every beam and bulkhead creaking and groaning till its rivets popped. (It would take 1/20th of a second to traverse the length of the ship)
Was each wave transferring its energy from hull bottom to me? Or did the initial waves start up a resonance that, after a while caused a resonant bobbing? Would each motion traverse the ship at the speed of sound, so the ship was deforming with every hit? (i.e. a push at the bow ripples through the length of the ship till it reaches the aft, a few fractions of a second later?)
Or did the ship gain the resonance slowly until it was self-resonating - i.e rocking as a single unit (at 2 Hz)?
Is there a natural resonance for an object of this size/mass? Could it be in the 2Hz range?