Why is David taken to the forecastle?

Riach tells the Captain to move David to the forecastle. He believes the boy is dying. Mr. Riach told Hoseason that if he wanted to be a paid murderer then he would have nothing to do with it. Outraged at this claim, Hoseason bid him do as he pleased. Riach, a bit drunk, defiantly cut David from his shackles and brought him to a bunk in the forecastle.