"Gordon" wrote
There is a sailboat on my pier that always seems to be rocking a little
when none of the others around it are moving.

Rolling is most pronounced when the period of the wave matches the natural
rolling period of the boat. The configuration of the harbor may favor or
amplify wave lengths that have a period that is the same or a multiple of
that vessel's natural roll period. What you are seeing is a function both
of the location and the boat. It doesn't mean the boat will be a roller in
other conditions.

There used to be an H28 on a mooring in Boothbay Harbor that would be
rolling in what appeared to be flat calms when every other boat looked like
it was set in cement. I wondered for a few years what was going on and
finally learned when I started studying boat motions.

--
Roger Long