I am ready to re-install and inboard to my Classic Sailboat despite the 15HP Mariner is working fine (up to 5.5 knots cruisng speed) but it just does not contribute to the looks of the boat. I've gone through several sites but still have many doubts on the size of engine and prop, as well as reduction ratio I should be looking for. Overall displacement is about 11,900 lbs., LOA 31' and beam 8'. My guess is that with a 30 hp engine I should be able to do better speed (say 7 knts.) but I wonder what RPM and prop size I should be looking for. Many thanks,