Hourglasses were invented around the 12th century. By the 15th and 16th centuries they were in
general use on board sailing ships, and were actually used more to help with the timing
of the ship's watches,
than for navigation. Dead reckoning (guessing based on windspeed and weather conditions)
was then the main way of establishing where the ship
actually was. Small sandglasses were used to help with the log, and to find the speed of the ship.
Typically, large ship's hourglasses were only made in thirty minute versions. Antique hourglasses of
sixty minutes or more were mostly used to time religious sermons.