I have a new Yamaha 15 Hp 4-stroke pushing a new 14 foot aluminum. This motor trolls slow (1.5 MPH) and when I need it, gets up and goes (25 MPH).
I fish all over Puget Sound and large lakes and have never had a problem. The key to a happy motor is changing the oil religiously, especially if you troll alot. Plus they are really easy to work on. If you can tune up your car you can maintain a Yamaha 4 stroke. smile
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Carl C.