At the beginning of the play, Act 1, Scene 1, the Master, the Boatswain, and the mariners are doing their best to steer the ship through the storm. Alonso and Antonio ask the Boatswain where the Master is. The Boatswain tells them to get back to their cabins. They are not sailors and they are not helping, so at this point they are only interfering with the crew's work. The Boatswain tells Antonio, "Keep your cabins; you do assist the storm." They are only making his and the crew's jobs more difficult. Then he criticizes Gonzalo, saying that he can take over the duties of steering the ship if his authority as a king's councilor gives him the power to control the weather:
You are a councillor;
If you can command these elements to silence and work
peace of the present, we will not hand a rope more. (I.i.18-20)
Sebastian, Gonzalo, and Antonio return. The Boatswain is once again furious because they are just getting in the way. Sebastian, the brother of the king of Naples (Alonso), can not believe that a mere sailor would speak so defiantly to members and councilors of the royal family. This is why he calls the Boatswain an "incharitable cur." Basically, the Boatswain is trying to get things done and doesn't have time to be humble and obliging to the king and his men.