i hit space, no fire of guns ? ammunition doesn't go down ? do i need a master gunner ? thanks

Due to a bug in the forums I see that you have 4 replies, but can't read them to see if you're issue has been solved.

In order to fire the cannons you need to be in first person perspective, meaning that you are viewing the scene from the deck of the ship. If you are viewing things off the ship, press Q. Then turn around so that you have a silver targeting circle aimed at where you want to fire. If you don't have a circle when you're aimed at your target, the target is out of the cannons' firing arc, turn the ship so that it is in the arc.

I hope this helps.