Not to the point. Going under sail at 10 knots and adding motorpower that without sails up gives 5 knots will not result in a boatspeed of 15 knots.
Also going faster than 10 knots under sail is a piece of cake for racing sailboats.
Interesting question. The answer is a plain NO however, no way you are ever going to reach 15 knots ! I own a 31 foot sailboat, that weighs 3.2 tons and has an inboard diesel of 18 hp. In flat water with no sails up I make about 5.5 knots with the engine running at about 2000 rpm. Under sail...