I have no idea if this works. i found this via google.
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Download Ship's Clock 2.0.0 Free - Rings a bell every half an hour in the style of a ship's clock - Softpedia
Ship's Clock description
Rings a bell every half an hour in the style of a ship's clock
Ship's Clock is a free and open source application that will ring a bell on every half-hour in the style of the standard ship's clock.
That standard has its roots in centuries of sailing ships around the world where clocks were impractical, and is still used in present-day navies.
Ship's Clock is based on four-hour watches: each watch was on duty for four hours starting at the hour of 12, 4, or 8 (am or pm).
Each half-hour during a watch was marked by a single bell and each hour by a double bell. For example, starting at 12 o'clock:
Here are some key features of "Ship's Clock":
∑ Audible reminder of the current time every 30 minutes, using the nautical four-hour watch tradition.
∑ Starts automatically when the computer is rebooted (service on Windows, cron daemon on Linux, launch daemon on Mac)
∑ Simple configuration through a graphical tool on Windows.
∑ Integrated smoothly into System Configuration on Mac.
∑ Small footprint.
∑ Code is fully documented.