The reason that information is so sketchy is because there are no hard and fast rules worth relying on.
For instance, although the description given by Gibb Sahib is correct for short drag and for long drag shanties, Haul Away Joe fits the long drag better than it does the short drag - even though given as an example of the latter.
Several lists have been made by supposed 'experts' in the genre, listing which shanties fitted which task.
They are all different and rarely agree on anything. (well, ALMOST rarely agree).
Try making a recording of someone singing the chorus ONLY, to the accompaniment of a metronome or a regular beat, then try to work out where the shantymans part would be sung.
I feel that would give you a better understanding of the songs as used for work. Todays shanty singers have got it wrong as far as I am concerned because they come at it from the other way round. They seem to think that the lead singer (shantyman) actually led the shanty, whereas it was the JOB and the effort required that did this.
THAT is where the shantyman came into his own as he was the man who knew the job, knew the state of the crew, knew the amount of effort required under the prevailing conditions, and so set the right tune for that job. Sometimes the mate would disagree and overruled him by wanting a faster tempo, and what the mate said was what was done.
I noticed just the other day that when I was singing one particular shanty I came in with the verse both before the end of the chorus AND on some verses hanging over the start of the chorus. I cant for the life of me remember which shanty it was, but did make a note to myself that I had always done it this way without realising it.
The song 'Haul on the Bowline' is one where I definatley sing the first line of the verse to coincide with the last line of the chorus, and as both words are mostly the same (Haul) nothing is lost.
Try it sometime.
P.S. How many times do you hear the words 'Timme' or 'To me' sung by the modern shanty crew at the start of the chorus?
And how many times do you think it makes me want to throw something sharp and pointy at them for doing it? :-)