This comes near the beginning of the chapter. Ralph has awoken to find himself dirty, & really sees the decay for the first time. He's upset by his long hair, his stiff clothes, his fingernails bitten to the quick. He sits to gaze at the sea, & Simon joins him. Simon remarks that he thinks Ralph will be rescued, but Ralph doesn't believe him:
Ralph was puzzled and searched Simon’s face for a clue. “It’s so big, I mean—”
Simon nodded. “All the same. You’ll get back all right. I think so, anyway.”
Some of the strain had gone from Ralph’s body. He glanced at the sea and then smiled bitterly at Simon. “Got a ship in your pocket?”
Simon grinned and shook his head.
“How do you know, then?”
When Simon was still silent Ralph said curtly, “You’re batty.”
Simon shook his head violently till the coarse black hair flew backwards and forwards across his face. “No, I’m not. I just think
you’ll get back all right.”
The bold question is Ralph's sarcastic response. He's feeling quite unsure of himself, & doesn't understand how Simon can be so confident. His declaration of Simon as "batty" connects to the hallucinations Simon experiences. Simon's passionate denial foreshadows his inability to articulate his feelings and ideas to the other boys. It is what will eventually cause his death.