Bell Time
Because most sailors did not own watches, time was kept by ringing a bell. When keeping bell time, each bell rung equaled one half an hour. When the bell was rung on the hour, the number of strokes sounded would be even. When the bell was rung at the half hour, the number of strokes would be odd. Bells were always rung in pairs. (A continuous ringing of a bell signals a dangerous situation such as "man overboard" or "abandon ship".)

Check out these sites to learn more about bell time: http://www.boatsafe.com/nauticalknowhow/shipbee.htm
http://www.antique-horology.org/_Service%20pages/Shipsbell/shipsbell.htm
http://www.captainsnautical.com/resourceCenter.asp?a_id=1