The weight of the cannons made it so they were hard to move, so they had to be fired from the deck of  the ship. 

The cannons were used while battling with another ship especially from long ranges.

If a cannonball hit a ship it would not likely sink it, but it could bring down riggings preventing the ship from getting away.  It could also send up deadly splinters that might kill or confuse crewmen.  This confusion allows enemy pirates to have time to board and capture the ship and crew.

The cannons were made from heavy steel or iron materials so that they could take the blast when fired.

Large ships could hold up to twelve cannons, while smaller ships were not strong enough to hold any.