I have a 78 seray 18' with mercruiser 140 the boat weighs around 2500 lbs. Just bought the boat it had a 15x17p prop on it but it was in bad shape. The guy gave me a 15x19p prop that he said he put on but it never ran as good so he put the other back on. I changed the prop then took it out on the lake the boat took a long time to get on plane, and when it did rpms at wot were only 3700 @ 28 mph (speedo not gps). Would like to put on a 17 pitch to try, my dad has a 13.25x17p he would give me but what would the smaller diameter do? Also the outdrive has a "doel-fin" on it should i take that off? thanks guys