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

Unlike 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.

Bells would be struck every half-hour, and in a pattern of pairs for easier counting, with any odd bells at the end of the sequence.

The classical system was:

No.of Bells	Middle Watch	Morning Watch	Forenoon Watch	Afternoon Watch	First Dogwatch	Last Dogwatch	First Watch
One Bell	00:30	04:30	08:30	12:30	16:30	18:30	20:30
Two Bells	01:00	05:00	09:00	13:00	17:00	19:00	21:00
Three Bells	01:30	05:30	09:30	13:30	17:30	19:30	21:30
Four Bells	02:00	06:00	10:00	14:00	18:00	.	22:00
Five Bells	02:30	06:30	10:30	14:30	.	.	22:30
Six Bells	03:00	07:00	11:00	15:00	.	.	23:00
Seven Bells	03:30	07:30	11:30	15:30	.	.	23:30
Eight Bells	04:00	08:00	12:00	16:00	.	20:00	00.00

Most of the crew of a ship would be divided into two to 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 were so arranged because that watch was divided into two. The odd number of watches aimed to give each man a different watch each day. It also allowed the entire crew of a vessel to eat supper, the normal time being at 1700 with First Dog watchmen eating at 1800.

Some ship's bell clocks use a simpler system: