1979 25 hp evinrude mounting height question!

[spacemonkey77]

I have 1979 25 hp evinrude model 25904r that's in excellent working order an I'd like to keep it that way. My question is that currently when mounted on my 1975 Lowe line Jon boat the anti ventilation plate is about 2 inches below the bottom of the boat. I'm under the impression that the anti ventilation plate or cavitation plate should be just about flush with the bottom of the boat, so I purchase the th marine mini jacker to raise it up! But before I do so I want to make sure I'm not making a mistake. The water intake on this model is on the fin/exhaust exit that sticks down below the vent plate behind the prop, but I noticed that there is a plate with three holes just above the anti ventilation plate. From the schematics it's called a water bypass plate. Upon interior inspection it is tied to the intake that's below the ventilation plate. Does this plate need to be submerged when the boat is on plane or at wot? Is this motor designed to be mounted with the anti ventilation plate an inch or two lower than the bottom of the boat?

See picture below!

 

[spacemonkey77]

Here is a picture of the plate

 

[spacemonkey77]

Here is s picture of the plate

 

[oldboat1]

Holes above the AV plate would be for operating in reverse (there are sometimes tiny reverse-facing scoops in the cover plate).

Can't say much about mounting height, other than to say the usual starting point is even with the bottom of the hull (like I think you suggest). If you are 2 inches below the bottom of the hull, there would presumably be some drag. Think I would try that, though, then move to level and see what difference you find.

I'm not a performance guy, and there are better sources for suggestions on mounting height.

 

[F_R]

First, a word about that motor. That lower unit was designed in 1951, when most boats were made of wood, with keels. The motor was designed to run with the cavitation plate well under water to avoid cavitation caused by air bubbles ahead of it. Furthermore, it wasn't exactly a high speed design.

OK with that in mind, it is ok to jack it up. But you will find that cavitation may become a problem as you raise it. The general rule is run it as high as you can without objectionable cavitation.

The water intake: Yes, if the bypass plate is above water, the pump will suck air. However, in neutral or reverse it probably is well underwater. At planing speed, the main inlet behind the prop is being force-fed by prop thrust and forward speed. So, water is actually flowing OUT of the holes in the bypass plate at high speed.

All this is speaking in general terms. Your experience may and will vary. Personally, I don't think you will gain enough speed to make it worthwhile. But go ahead and see what you think.

 

[spacemonkey77]

Ok thank you for the information. That all makes since. I'm not at all concerned with gaining speed and performance from the boat but more after shallower draft. That long motor seems to always drag tha bottom in areas ihunt and fish.

I'll start with mounting it flush. If cavitation is an issue I can always drop it an inch if I have too.

Thanks again.