In the early 1970s the inboard powered sailboats we sold almost all were powered by 20 to 30 horsepower gas engines.  Some were Palmer but most were the ubiquitous 30 horsepower universal atomic 4.  There were the occasional Gray marine gas inboards too.  Diesel inboards were mainly confined to larger sailboats 40 feet and up. They were more expensive, but they were also available in higher horsepower versions.  The big change began in the late 1970s and early 80s with the move to inboard diesel engines on all sizes of vessel.  That time in our history we sold sailboats exclusively so the change to diesel propulsion was mainly limited to sailboats and most power boats were and still are powered by gasoline.