The Ship's bells reference article from the English Wikipedia on 24-Apr-2004
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Ship's bells

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Ship's bells are a system to indicate the hour by means of bells, used aboard a ship to regulate the sailors' duty watches.

Unlike civil clock bells, the strikes of the bell do not accord to the number of the hour. Instead, there are eight bells, one for each half-hour of a four-hour watch.

The classical system was:

Number of bells Middle
watch
Morning
watch
Forenoon
watch
Afternoon
watch
First
dog
watch
Last
dog
watch
First
watch
One bell0:304:308:3012:3016:3018:3020:30
Two bells1:005:009:0013:0017:0019:0021:00
Three bells1:305:309:3013:3017:3019:3021:30
Four bells2:006:0010:0014:0018:0022:00
Five bells2:306:3010:3014:3022:30
Six bells3:007:0011:0015:0023:00
Seven bells3:307:3011:3015:3023:30
Eight bells4:008:0012:0016:0020:000:00

Most of the crew of a ship would be divided up into three or four groups called watches. Each watch would take its turn with the essential activities of manning the helm, navigating, trimming sails, and keeping a lookout.

The hours between 16:00 and 20:00 are so arranged because that watch (the "dog watch", which is cur-tailed) was divided into two. The odd number of watches aimed to give each man a different watch each day.

Some "ship's bell" clocks use a simpler system:

Number of bellsHour (a.m. and p.m.)
One bell12:304:308:30
Two bells1:005:009:00
Three bells1:305:309:30
Four bells2:006:0010:00
Five bells2:306:3010:30
Six bells3:007:0011:00
Seven bells3:307:3011:30
Eight bells4:008:0012:00

The number of bells may be used to refer to the hour so indicated.