I am in the market for a four stroke engine. I currently have a 40 hp 1990 mercury 2 stroke. I have a 18.5 foot center console gregor aluminum boat. Weight of dry hull 560 pounds. I only need to max speed at 20-25 mph and use it mainly for trolling at low speeds. I want to know if you think that a 25 or 30 hp 4 stroke would accomplish what I need. I don't want to spend the extra money for the 40 hp if I don't need it. Thanks in advance for your advice.