Also, a pendulum is only accurate if it's stationary (in the sense of not being interfered with via inertia). Ships of that day were very much not stationary, but oscillated with the motion of the water. Even the huge ships of today are not sufficiently stationary to not interfere with pendulum periodicity.
Written by Bob Koure
Retired software architect, statistical analyst, hotel mgr, bike racer, distance swimmer. Photographer. Amateur historian. Avid reader. Home cook. Never-FBer