Rough Seas

Last night was a rough passage for a landlubber like me, with strong winds and 10-foot swells. The night before, the ship was rocking, but last night it was pounding. The captain apologetically announced that conditions would be a bit uncomfortable, but our situation wasn’t bad, and just enough to remind us that we were really at sea.

Of course, it can get much worse — here’s a youtube video where it was substantially worse — you can hear an authentic example of the emergency signal (7 short blasts, then a long one).

The rocking motions are roll, pitch, and yaw, like an airplane. Yaw wasn’t noticeable, but roll and pitch were strong enough to make it impossible to walk in a straight line. Roll and pitch are rhythmic vertical motions, with the ship’s buoyancy trying to return to level. Roll had a period of a few seconds; pitch had a longer duration. (On the other hand, yaw is a horizontal rotation with no vertical component and no passive returning force. A yaw motion is essentially a tiny course change that requires autopilot or human correction to undo.)

Of course, all these motions combine unpredictably. The ship moves as a rigid body in a complex dance, and as long as it’s not too bad, you can enjoy it.

However, when conditions got slightly worse, the water slapped the hull explosively, and a new form of motion manifested. Rather than moving as a rigid body, the ship vibrated like a colossal gong, with a pitch of about 1.5Hz. This ringing sometimes persisted for several seconds (though generally less than 10 seconds). I suspect this vibration came from the waves striking the ship’s bow at an angle.

When the ship slammed even harder, though, sometimes there was a higher frequency vibration, maybe 5Hz, that decayed within 2-3 seconds — a violent shaking motion that rattled small objects.

These vibratory modes must be from the flexing of the ship itself, and the frequencies probably reflect the natural resonant frequencies of the ship.

Leave a Reply