this is the place to ask that question.... but I wonder
the shanties were sung onboard & at the dock, so everyone would hear them....
so it would be the more musical members of the crew that would retain the tunes & words. But how did these newcomers then get the opportunity to lead a shanty? I suspect that multiple concurrent tasks necessitated a shanty leader for each work group, and smartly run ship would make sure that each group had a capable singer assigned. just an opinion, though