Michigan wheel used to have, and may still have a chart that gives you a vague idea as to what prop you need for a given engine on a general boat of a certain type. Otherwise somebody has to respond that has an old catalog or the like. How about 10x13 for a swag...10" dia, 13" pitch. That is the middle of the more recent 40-50 hp Merc Classic sales brochure. If you are running a heavy hull drop the pitch and if a light hull increase it....range was 8-19".