Posted August 17th, 2018, 02:54 PM
Vibrations can be caused by a few things. One is the infamous "azipod shimmy", which is caused primarily when the ship is in following seas, and to keep a constant heading, the azipods that steer will "sweep" back and forth, port to starboard and back, as the following seas swing the stern one way or the other. This constant sweeping of the propellers causes the water flow into and out of the propellers to sweep across the flat hull above the pods, and the ship vibrates horizontally. More noticeable the further aft you go. This problem was so severe on the QM2 at sea trials, that they had to take the ship back into drydock and add a "skeg" or vertical portion of hull between the port and starboard azipods for more lateral stability.
The other vibrations are because the ship is a large "sounding box" like a violin or guitar. Externally produced vibrations, like the blades of the propellers passing the hull at the top of their rotation, or periodic waves striking the ship, can cause the ship's hull to vibrate. Due to the complex nature of the hull structure, various areas will have different natural frequencies, and so will start to vibrate at different times when different frequencies are induced. So, combinations of propeller speed, wave period, wave direction relative to ship's course, and the like will vary the input vibrations, and consequently an area might vibrate in the morning and not in the evening, if the environmental conditions change. Similarly, one cabin may vibrate at one speed, in a given weather condition, but not another cabin down the passageway, but if the propeller speed changes by a few rpm's, the second cabin may vibrate while the first does not.
Good insights chengkp75 ...
Harmonics are mysterious and wondrous things!
We recently had a new-build small vessel that kept destroying drive belts to the alternator PTO, and we were investigating misalignment, bad bearings, all kinds of things. Turned out in the end to be a faulty fuel injector that introduced a harmonic at certain engine RPMs which manifested in a local vibration that destroyed the drive belt. Seemed very counter-intuitive to me.
I also sailed on an icebreaker that would experience harmonics in shallow waters at certain engine rpms, where vibrations made the whole ship feel like it was driving over a corduroy road or a series of speed bumps. Vary the speed by a few revs and the problem would disappear.
I'm joining the QM2 next week in Brooklyn, but I don't anticipate too many vibrations to bother me on Deck 12 forward. Should they occur, I'm only a scant few metres from the wheelhouse so I'm guessing the OOW will feel them too and make the appropriate adjustments.