Calling all motor mounting/RPM experts?

[dpski]

Hello, I've been reading with great interest the posts on max WOT RPM's and motor mounting/prop selection.<br /><br />I have a 1997 90hp V4 Johnson on a 17.5' bow rider with a 14x17 prop. The boat hole shots incredibly well but I'm only able to turn about 4900RPM. I'm thinking of going to a 13+3/4 by 15 prop. However after reading all the other post here about motor height I decided to check mine. Best I can tell my cavitation plate is about an inch lower than the bottom of the hull.<br /><br />Here are some pics I took;<br /><br />

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<br /><br />Thinking of moving the motor up a hole or two, before I switch props. Not sure if the flat section in the center of my hull would be any reason to leave the cavitation plate at the height it is now. Any thoughts?<br /><br />Thanks,<br /><br />DP

 

[red10]

Re: Calling all motor mounting/RPM experts?

does that motor have power trim and tilt

 

[umblecumbuz]

Re: Calling all motor mounting/RPM experts?

Hi DP,<br /><br />Very clear pics.<br /><br />Looks like you will get some benefit from moving it up. If you do, check that you're still pumping water OK, and watch for prop 'blowout' on turns.<br /><br />(Oh yeah - and if you want to be exact, it's an AntiVentilation plate. Cavitation is a different ball game).

 

[Dhadley]

Re: Calling all motor mounting/RPM experts?

Youre on the right track. Work the set up. Going to a smaller pitch is always a last effort deal.

 

[dpski]

Re: Calling all motor mounting/RPM experts?

Originally posted by red10:<br /> does that motor have power trim and tilt

Yes the motor has power trim and tilt. I tried to adjust it so that the anti-ventilation plate was parallel to the bottom of the hull. The pics showing the lower unit were taken with the top of straight edge held flush against the bottom of the hull, resulting in that same edge being higher than the anti-ventilation plate on the lower unit.

 

[ob15]

Re: Calling all motor mounting/RPM experts?

You can definitely raise the motor up. I would try 2 holes & see how it handles in the water. As Stillfising said make sure the water is flowing & you don't blow out in turns. Keep raising it until it blows out & then lower it a hole.

 

[red10]

Re: Calling all motor mounting/RPM experts?

i would start by raising 1 hole and see what happens, then if you really want buy a stainless prop.

 

[MarkHPCA]

Re: Calling all motor mounting/RPM experts?

If you are going to play with the motor height I would strongly recommend you put in a water pressure gauge. They are inexpensive, a kit is about $35 and easy to install. A motor rebuild from having low water prssure would be much more $$!