Dear All
Thanks to the wonders of the interweb, I have found out that a stopperman is another name for a Bittman, and older, experienced but less active seamen who did not go aloft, so called as they expertly handled the ropes. The ropes were wrapped around bitts, or posts on deck to tie up or belay anchor. Bitting was also the verb for wrapping the ropes, hence bitt-man.
Sorted!
Andrea