I remember watching "Master Commander; The Far Side of the Earth"

And when the Brits werre chasing the French in the storm, and the Capt,
was wanting a speed report, I rememeber the guys at the back tossing a
line over the stern, and as the line reeled off they counted the
"knots" agains the time of an hr. glass. So to Google we go, and I
found this:

http://www.maryrose.org/lcity/pilot/tnotes.htm

Interesting to say the least.