Hi Donald, I think more of the harmony that's heard on field recordings of work gangs on the southern prison farms. The leaders weren't required to have a great voice only that they could keep the timing & be heard. Yet there's a raw harmony here as well, with voices of all abilities grouped together.
I've been finding, lately, that Blacks in pre Civil War had quite the network going throughout the northern Alantic Rim. Many were the pilots when entering a port, the steveadores when ships were taken on or off cargo, the fishermen inshore & whalers offshore often making up a good percentage of the crews & sometimes making up the total crew. The plantations often had their own vessels that were maned by the same hands that worked the fields. I agree that they had an influence on style & content & a greater portion would've been around & prior to the so called time of the golden age of shanties. So I see their influence as being far greater than thought of in the past. Barry