Originally Posted by SIBRider
Yamaha 90 is one big motor for F470.
Well it is and it isn't. HP yes, on paper, no question. Weight, not so much. It went down like this... I've run up through 55hp motors on these boats and was left wanting a little more power. With the hard deck the F470 is rated for 65hp, which isn't very common so I began looking for good deal on a Yamaha 70 hp 2stroke. After searching the better part of a year I came across a great deal on the 90, which is pretty much the same as the 70 (size, weight, etc) just more HP. In fact the Yamaha 70/90 weight is about the same as the Johnson 55hp Enforcer and my previous favorite Honda BF50...around 230#...so the 90hp motor isn't making it any more stern-heavy than the other commonly-used motors. So I figured I'd "try" it. If it turned out to be too much, I'd sell it and go back to plan A. Plus I could always install a throttle stop, right? (BTW, psychologists call this process "rationalizing" lol)
The newer F470s like mine have a straight transom with SUBSTANTIALLY beefed up mounting to the tubes (Zodiac calls them "Ovoid transom reinforcement patches"), furthermore I have all the components of the CMK installed: the stainless transom cap, the one piece solid stringers and the thrustboard reinforcement plates. I also added fittings to attach torque rods between the transom and aluminum hard deck to prevent excessive stress on the transom attachment. Finally, I run SeaStar full hydraulic steering. Long story short, the 90 is working fine on this particular F470. I've been running this setup for about 4 yrs now and other than the aforementioned transom backwash issue, I haven't observed any issues: No excessive fatigue on the transom or boat hull and no "scary" behavior—even at WOT in rough water.
Despite those assurances, I'm sure there will be those that disapprove of my setup. For those people, take solace in the fact I do plan to repower in the future. Yamaha finally came out with a lightweight (~250#) 70hp 4-stroke last year that I would like to eventually throw on the back on this boat. Then I can hang the 90 off the back of a 5.4m RIB
All this is probably better located in my build thread...as soon as I get around to starting it I'll move it there.