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motor mounted too low? 1999 100 hp

mutineer

New member
I have 99 Alumacraft 170 tournament pro with a 100 hp Merc on it I bought the boat used from the original owner. The problem I'm having is that when I get it up on plane water comes up between the motor mount and the transom and sprays into the back of the boat. I looked and it seems like the motor maybe mounted too low, any thoughts? I was also thinking of putting on one of those fins on the bottom of the motor to see if that helps.


Thanks in advance
 
Re: motor mounted too low?

Just for starting purposes, the motor should be mounted so that the cavitation plate on the motor is level or just slightly lower (1/2 to 3/4 inch) than the bottom of the hull. Check this and let us know what you find.
 
Re: motor mounted too low?

Ok I checked and the lowest plate is 1.5 inch below the lowest point of the hull. Should I move the motor up one hole on the mounting bolts?
There are two plates, is the bottom on the cavitation plate? I'm assuming that it must because I could never get the upper plate that low.
 
Re: motor mounted too low?

I have a question that I just thought of, are you trimming the motor up once you get on plane, cause if you are not that could be the whole problem.
 
Re: motor mounted too low?

Yes I am trimming the motor, I've tried to trim it up and down this does help some but then it pushes the front of the boat down so that it is riding in the water more.
 
Re: motor mounted too low?

Trimmed up it should be better, but as timebandit said set it level first if you can. Then go try it and see what happens.
 
Re: motor mounted too low?

How much to lift depends on a couple of factors.
1: The V-angle of bottom. The deeper V, the more lift
2: Your prop type and Rake.
No Rake (blades 90-degree on prop-shaft) the less lift. Alu props normally have little Rake, SST props more. The more Rake in a prop, the more it lifts the bow, the less need to trim out. Switching from Alu to SST (raked) may often allow to lift another bolt hole. The optimum top speed is when engine is only trimmed until prop-shaft is parallel to the water (when running). If you need to trim more, get more Rake!
If running at 17 Degree bottom angle and alu prop, I would estimate about 1/2 to 1" lift above bottom. (Trim cav plate parallel to bottom and measure). A SST raked prop, you may even be able to go 1,5 - 2 ". If 21 degree V, you may add another 1/2 ".
 
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