Question about outboard mounting height

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djcamera

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I recently repowered my 1975 Starcraft 16 SS. I put on the exact same motor as the one that died: a Mercury 75HP made in 1986. Identical in every way, down to the model number.

All the years I had the old motor, I had the feeling that it was mounted too low. I had no problem getting on plane, but I thought the splashing on the corners of the stern was a bit much, and it did feel a bit boggy at lower speeds. I didn't let it bother me and would rather be on the lake than go through the trouble of tinkering with the motor height. But when I hung this new motor a couple of weeks ago, I saw it as an opportunity to raise it one hole. Just one hole, about an inch on the bracket. A straight edge on the cavitation plate goes right to the point of the bottom of the v-hull.

Now I'm back up and running again, but the trim dynamics are dramatically different! Now when I come out of the hole, the bow immediately rises sharply like it never did before, and I start porpoising much sooner if I trim up at the same rate I'm used to...and I never had porpoising issues to speak of. I also have a hydrofoil mounted.

One other thing: the motor is not mounted directly to the transom, it's on a CMC tilt/trim rig that is about six inches wide. I don't use it, but it was on the boat when I got it. See picture attached.

So, questions: did re-positioning the motor ONE HOLE UP change the trim characteristics that significantly, or is something else throwing my trim off, like perhaps the energy provided by a better-performing motor? And should I take into consideration the extra distance from the transom that the CMC tilt/trim gives me, and maybe raise or lower the motor to compensate?

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Set back from the transom can increase the amount you can raise a motor. You may actually be able to raise it more. Yes more power can also cause the sharper bow rise and porposing. Same prop as the old motor? Top speed and rpms are good tools to record while tuning motor positioning so a tach is helpful if you don't already have one. I also like to use a GPS speedometer app on my phone. Does it cavitate when turning sharply?

Sent from my CLT-L04 using Tapatalk

 
The prop is not the same as my old one. I never thought that could be the cause of the different behavior. I'll compare the two next time I'm at the boat. Good idea.

It's amazing to me how 3/4" in motor height or 1" in prop pitch can impact how a 3,000 pound machine acts in the water.
 
Need to make sure you have a tach and GPS speedo. And record rpm and speed as well as behavior of the boat, then only change one thing at a time.

Sent from my CLT-L04 using Tapatalk

 

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