I have a 40 tiller (2006 merc 4 stroke, 3 cylinder)on a 16'6" Alumacraft Yukon. It'll run 31 mph with only me in it with a 12 pitch 3 blade prop turning 5600 rpm- if I trim it up any more it loses speed. I get 27 mph out of a 10 pitch 4 blade prop turning 5850 rpm.
For an 1860, I'd go with a 75...