B Resonance in large objects, say a cruise liner

[DaveC426913]

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?

 

[anorlunda]

Sure, why not. It is common for boats at sea to change speed up or down to get a smoother ride. The ship likes to rock like a pendulum, and that can resonate with waves. It not exactly on the frequency, you get a beat effect. The waves themselves have sequences of positive and negative interference. We call that a confused sea.

If you want something really uncomfortable, I once made the mistake of booking a stateroom down near the waterline. It was much cheaper. When the boat hit the ice pack while I was sleeping it sounded like I don't know what. There was only a few inches of steel between the ice an my ears.

 

[tech99]

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?

The period of ocean waves tends to be around 7 to 10 seconds, so it is not that directly. 2Hz seems too high to involve the length of the ship, but of course, the structure of the ship will have innumerable modes of vibration, and it is not just the speed of sound in steel we must consider. The density and springiness of the vibrating structure will determine the frequency. I wonder if the mode you experienced is the floor going up and down - something I noticed in my house in our UK "hurricane" of 1987.
The various vibration modes might be excited by noise-like inputs such as the engine/propeller, the breaking of waves against the hull, turbulence etc.
I have not noticed 2Hz vibration from the sea alone in various sailing ships, both wood and steel.
One interesting effect I do notice is that the gentle rolling of the ship at anchor rises and falls over several cycles; when it falls to zero it undergoes a 180 degree phase shift.

 

[DaveC426913]

I have not noticed 2Hz vibration from the sea alone in various sailing ships, both wood and steel.

It is definitely a direct effect of the particular seas, wind and heading we were experiencing at that time.

If you want something really uncomfortable,

(BTW, it wasn't uncomfortable; I have never had a problem with sea motion.)

I once made the mistake of booking a stateroom down near the waterline. It was much cheaper. When the boat hit the ice pack while I was sleeping it sounded like I don't know what. There was only a few inches of steel between the ice an my ears.

I neglected mention that I've learned that trick over many cruises.
First cruise we berthed on Deck 4, Next cruise, Deck 6, then 9 (We learned to pay for better digs. movin' on up! dah dah dut daah!). This time we booked on Deck 12. We thought we were golden. Alas, Deck 12 is right below the pool deck. And our cabin was right below the Party Bar, which kept their infernal whippersnapper caterwalin' music* up till all hours of the night.*Both kinds: Boots n Cats and uhn tss.

 

[Nidum]

The old wooden sailing ships used to flex a great deal in rough seas . Apparently enough to be quite visible sometimes and the noise level below decks has been described as being unbearably high . Endless creaking and groaning of the timbers accompanied by loud 'crack' like sounds at frequent intervals .

 

[CWatters]

2hz is 120rpm which I think is close to the typical engine/prop rpm for a large ship.

 

[DaveC426913]

It's not the engines. Not unless they run them at that speed for a few minutes and then kill the engine, then run them again a few minutes later, pointing in a different direction.

But my question is not what is causing the pulsing, but how it is being transmitted through the structure.