Well that's a tough one to answer, in the old days it was measured at the power head, then it changed when CG got involved it changed to prop shaft HP. In the really early days it was just what the manufacturer decided to call it. That old 20 hp merc was probably pushing close to 30. Today's motors are getting more HP out of smaller displacement engines as a result of better tuning internally, more efficient electronic ingnition systems and better more effect fuel delivery systems. Beter prop designs and gear ratio's. Just like your truck with a V6 getting much better fuel mileage, higher torque and HP than the older big block engines. Your old motor is probably close to the 15 rating as far as performance. Still be better than a new 10. If you get into the smaller motors under 25 and in 4 stroke you will see some performance drop off compared to a 2 stroke. So if I had a 15 and wanted to upgrade to a 4 stroke I would go to a 20 or 25.