I think what little evidence exists point towards your conclusion, Nordhoff particularly clear. The logical conclusion to me is the chanty is a more complex development of the stevedore method. Let us hypothesise the transfer from shore work to shipboard.. are there any tasks aboard, c1830-1840, that would necessitate a similar exertion to what was happening ashore? Perhaps the rowing songs are closer to chanty form in that more pulls per line?