Hi John S.
As you say a ships top man was the Master, I suppose a Master of his trade, I doubt seamen would have called him Master though, maybe Skipper or sometimes if warranted something starting with a B, Captain was easier on the tongue than Master and not so subservient to the seamen under him. Like Rod the only Captain proper I sailed with was on the Wave Baron, he was RNR, always the gentleman. But I always looked at the Captain on [most] ships I sailed on with respect, as they had earned it.
Cheers Des