Cats at SeaTo the sailor of old, "the cat" was not necessarily a pleasant sight. The word "cat" could refer to the cat 'o nine tails, a fearsome whip used for punishment. That aside, feline cats were normally welcome on board ship, albeit as working animals rather than pets.
The chief reason for having cats at sea was primarily practical. By catching mice, rats and other pests the ship's cat was performing a valuable service. If left to spread, these vermin could seriously damage the ship's supplies and possibly cargo. Having your food stocks destroyed by rats when in the middle of the ocean is not something to be taken lightly.
A good ratter was thus highly popular on board ship. Sailors believed that unusual polydactyl cats - those with more than the normal number of toes on their feet - were better at catching pests. This may be connected with the suggestion that the extra digit gives a polydactyl cat better balance when at sea.
Polydactyl cats have sometimes been called "ship's cats" even when on land.
LuckBeyond their practical benefits, sailors also believed cats on ships to be lucky. This might in part be due to the simple logic that a catless ship overrun by rats was definitely unlucky! However the belief that cats at sea brought luck is likely to be connected with the cat's long standing reputation for being a lucky creature in general.
For sailors one of the most important influences for good or bad fortune was the weather - calm or storms? Cats were sometimes said to be able to predict the weather. Sometimes it was said that they could actually control the weather, using their tails to call up winds for good or ill depending on their mood.
It was therefore important to treat your ship's cat well. Some sailors went as far as to believe that if the ship's cat fell overboard then it would retaliate by calling up a fearsome storm to sink the ship.