I have a 70 hp merc. on my 18 foot cobia center console. The boat goes very slow and barely gets on plane with two people in it with no fishing gear or coolers. It has those rubber dolphin fins on the motor, I guess to help it plane easier. The serial number on the motor is 6382744. I want to know the year of the motor and what the prop size is supposed to be. The motor runs good. Doesn't miss, doesn't over rev, although I don't have a tach. on it. It sounds like it runs good. I also have a 16 foot bow rider with a 1973 evinrude 70 hp on it. That prop is almost twice as big as the merc.'s by looking at it. The bow rider goes about 34 mph according to my gps and speedometer. The merc. 70 hp 18' boat goes about 15 mph on flat water. I was told by a guy who claimed to work as a boat mechanic who was buying my bow rider that the prop on the merc. is supposed to be 13.5x17.5. The only number on the prop I found on the merc. was 48 85632 11p. It was on the outside of the prop. So I don't even know what size I have. This so called mechanic also said that the 18 footer was lighter than the sixteen footer and should go much faster than it does. He claims all I must do is change the prop. I told him I figured the prop was correct because the motor doesn't seem to over rev. Any input would be appreciated.