What was the main purpose of the navigation acts ?

They were a succession of laws by the British to limit trade of foreign ships for trade between any sea vessels with the exception of England.  It reflects the principles of mercantilism where all the wealth is kept within the British Empire.  Because of this, the colonies were limited as to who they can trade with. Apart from that, it fueled resentment among the colonists especially the merchants whose livelihood was affected by these Acts.  Eventually this helped fuel the start of the American Revolution.