I'm looking for possibles. I have an 1986 26'loa 10' beam 350 w/ Mercriuser alpha outdrive. A month ago at launch it ran at 20 mph at 3000 rpm; now I get 15mph at 3200 under the same load. Is this the beginning of a spun prop or is there some coupler problem that I'm not aware of due to the boat's age? Any thoughts on the possibilities before I call the mechanic will be appreciated.