Revised by BoatUS editors in April 2012
In the old days, few sailors could afford time pieces. So, at the beginning of each watch, the Quartermaster upended an hourglass containing 30 minutes of sand. When the sand ran out, he struck one bell and turned the glass. Another bell was struck each time the glass was turned. Thus, half-hours were marked by an odd number of bells, and an even number of bells marked each hour. Eight bells signaled the end of every four-hour watch-either 4, 8, or 12 o'clock.
Today, bells are used as signaling devices when visibility is restricted. Efficient sound producing devices are required by the Coast Guard on vessels less than 12 meters (39'). Vessels over 12 meters, but less than 20 meters (65') must carry a power whistle or power horn, but are not required to carry a bell. The whistle must be audible for 1/2 nautical mile. the mouth of the bell must be at least 200mm (7.87") in diameter.
See Navigation Rules Online including in particular Part D