A Ship's Bell is usually made of brass, and has the ship's name engraved on it.
Strikes of a ship's bell are used to indicate the hour aboard a ship and thereby 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. 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.
Most of the crew of a ship would be divided up into between two and 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 curtailed) was divided into two. The odd number of watches aimed to give each man a different watch each day. It also allows the entire crew of a vessel to eat an evening meal, the normal time being at 1700 with First Dog watchmen eating at 1800.
The term "Eight bells" can also be a way of saying that a sailor's watch is over, for instance, in his or her obituary. It's a nautical euphemism for "finished".
Ship's bells are also used for safety in foggy conditions, their most important modern use.
A ship's bell is a prized possession when a ship is broken up, and often provide the only positive means of identification in the case of a shipwreck.