Why was Ralph angry with Jack after the ship passed by the island? What did Jack give as an excuse?
As the previous post mentioned, the conflict is between Ralph who is angry that the fire was not kept going, and Jack who felt that going to hunt was more important. But it is also symbolic for the things that they are both representing. Ralph is concerned for the future and knows that strict organization is the best way to take care of things like shelter and a signal fire but Jack knows that the boys are hungry and that the hunt and the fire and the meat will please them immediately. The hedonistic allure of the hunt and the meat wins over so many of the boys that eventually it is only a fire set to smoke Ralph out of hiding that brings a ship to the island.
The boys built a fire on top of the mountain to signal any passing ships of their presence on the island. Jack and his men were supposed to make sure that the fire remained ablaze, but Jack decided to go on a hunt for a pig instead. His excuse - they had to eat. This angers Ralph because he saw the ship and then realized that the fire has gone out and they would not be rescued. The ship represents the return to civilization for the boys and the hunt represents a return to the primitive life for the boys. this scene foreshadows what happens between Ralph and Jack - the clash between civilized man and primitive man.