Consider this: At the start of a voyage, with a new crew who may or may not know the standard shanties, the shantyman would have been obliged to 'keep things simple'. After a few months at sea the crew would be more likely to instantly recognise which shanty was to be sung almost before the 'nightingale' had uttered a word. So then, and only then, would he introduce subtle differences (or the obscene verses)into his chosen lyrics in order to keep the crew amused or on their toes, so cementing his position at the head of the gang.
There also seems to be an assumption that shantymen knew the words to every shanty and that they had an inexhaustable repertoire. I challenge this. If it were so, the collector would have been able to get all his information from one source, and clearly that was not the case. The often heard repetition of a verse line is, to me, a sign that either the shantyman didnt know the correct next line, or that he had forgotten it and was unable to make one up on the spot, or that the fault lay with the collector.
I am not denigrating the shantyman here. It is hard enough to sing a shanty in a warm comfy folk club, without having to shout it out into a howling wind with the ocean doing its level best to drown me, thanks.
As for the earlier suggestion that capstan and pump shanties were not performed in port as often as they were at sea (where the public could not hear them) - rubbish.
On leaving port the capstan would invariable come into use for warping within the docks, for squaring up the yards ready to take sail or heaving the vessel up to its anchor.
On finishing a voyage the pumps would be used for the last and possibly only time to leave the ship dry before the crew were paid off.
The opposite view holds more credence, as tacks & sheets, halyard and stamp & go shanties would be used throughout a voyage in order to trim the sails and yards for the frequent changes in conditions encountered, or at the whim of the deck officer.
I am certain than rude verses would have been sung, but they were variations on standard shanties, to be sung when conditions were right and not otherwise.