I like King Gambit's answer. He's right about HP being based on how you're going to use the boat, and how attitudes have change.
If I leave port and go 50 miles north, stay a few days, and have to return home in 5-6 footers with the wind straight on my bow, I'm not at all shy about leaving the sails down, putting the diesel at 70% of rated RPM, and powering straight into a South wind to get home. I view my boat as being a sailboat AND a slow powerboat, as opposed to a sailing auxiliary. I like to be green and I don't like paying for fuel, but I run the diesel anytime I want, and still probably spend more on ice than diesel.
Oh, and as far as selecting the correct prop for a 13HP engine so it will be efficient, you'd still have to compare it to a 21 HP engine with the correct prop. Just because my boat has a big engine, doesn't mean I'm running the wrong prop.
If I leave port and go 50 miles north, stay a few days, and have to return home in 5-6 footers with the wind straight on my bow, I'm not at all shy about leaving the sails down, putting the diesel at 70% of rated RPM, and powering straight into a South wind to get home. I view my boat as being a sailboat AND a slow powerboat, as opposed to a sailing auxiliary. I like to be green and I don't like paying for fuel, but I run the diesel anytime I want, and still probably spend more on ice than diesel.
Oh, and as far as selecting the correct prop for a 13HP engine so it will be efficient, you'd still have to compare it to a 21 HP engine with the correct prop. Just because my boat has a big engine, doesn't mean I'm running the wrong prop.