I'm thinking that boat is a
19'2" O'Day Mariner 2+2. A nice boat with many having been built. The hull came from the O'Day Rhodes 19. The
original Mariner had a "bump" in the coach roof, shown in the Wiki picture, to give more headroom. Supposedly the boat had four berths, but that would be a stretch in real life use. Two adults and maybe two small kids.
Four horsepower is fine for that boat. I have seen Six horespower used but it's really not necessary.

NB
http://en.wikipedia.org/wiki/O%27Day_Mariner