Boating Magazine did a test of the 20' Seafari in 1969 with the 140 Mercruiser and published a very good report on it with lots of detail on the construction, laminate schedule, etc., including a complete performance report.
http://www.classicseacraft.com/broch...cle/index.html I think this is a jpg file so it's pretty hard to read, but I have a good .pdf file of the original article that's much more readable. I have your email address, so I'll send that to you. I also have a couple of the factory brochures from '69 and '72 which I can send if I haven't sent them already. In that test, they ran a 15x21 prop; it was a 5 mpg rig, which will pretty much match a modern 4-stroke outboard!