Why does long john silver pretend not to know black dog in Treasure Island?

Long John Silver pretends not to know Black Dog in order to maintain his cover as a loyal member of Captain Smollett's crew. By denying his past connections with Black Dog, Long John Silver avoids suspicion and protects his own interests, as Black Dog is associated with piracy and illegal activities. This deception allows Long John Silver to advance his own plans and gain the trust of the crew.