Outboard cavitation

[Blackiron99]

I have a custom fabricated 16 ft duck hunting boat. It's like a v-hull, but flat bottom out the back. I bought a 1984 25hp evinrude to power it. The guy I bought the boat from said it was built for a long shaft, and my motor is a long shaft. I looked it up by the numbers, and it says long shaft. However it seems like an extra longshaft. Anyway, I raised the height of the motor, based on other conversation, so the cavitation plate on the motor is roughly 1" higher than the bottom of the boat. I took it out yesterday, but the motor removed real high, and the boat didn't want to plane. It just seemed like the prop was cavitating. I lowered it about an inch, now it will plane, but it takes about 2-3 minutes, and if I try to turn it starts to cavitate and I loose all power. So does it need to be lowered more? Was the information I got saying the cavitation plate should sit higher than the bottom wrong? I need help. I had to raise the motor 4" over all, but there was no way it could stay the way it was when I originally mounted the motor. There was way too much motor in the water being dragged and it would throw tons of water up on the back of the boat.

 

[racerone]

The boat transom should be 20" high.-----That should put the cavitation plate BELOW the bottom of the boat.

 

[Chris1956]

Tot be precise, the motor is likely ventilating. That is, it is sucking air from the surface of the lake. The flat plate (antiventilation plate) on the motor should be even with the water surface, when cruising. If it is above the surface, air can be sucked into the prop.

 

[roscoe]

Actual measurements would help.
And a "custom fabricated" boat may not perform anything like a traditional boat.
Photos would help.

 

[Scott Danforth]

a short shaft motor transom is 15"
a long-shaft motor transom is 20"
an Extra Long shaft motor transom is 25"

your AV plate should be even with the bottom of the motor

post pics of the side of the boat and the motor. and when you take the pic, stand back 15' from the boat

 

[racerone]

You meant ---even with the bottom of the boat-------I believe.

 

[Blackiron99]

Here are a couple pictures of the motor since someone asked. I do believe I was wrong in calling it cavitation, after reading more into it, the guy who described it was ventilation was probably spot on. It's bad until the boat planes, which takes forever, and if i try and turn more than 10 or 15 degrees the motor revs high and i loose all power.

 

[racerone]

Your motor is mounted too high.-------See the wee plate above the cavitation plate ?------Has to do with motor cooling.

 

[Blackiron99]

Your motor is mounted too high.-------See the wee plate above the cavitation plate ?------Has to do with motor cooling.

Sorry, what is a wee plate?

 

[Blackiron99]

Does anybody know what he means by "wee plate"

 

[Mohawkmtrs]

In picture #2, see that little plate on the port side of the motor just above the anti-ventilation plate that is fastened by two screws? That's the water intake to cool the engine and has to be in the water (not above it).

That's the "wee" (meaning "little") plate Racerone is referring to.

 

[Sea Rider]

Blackiron 99,

Being a long shaft OB, need to visually determine at which lower leg height is water flow (wf) passing at speed as wild guessing will take to much precious boating time along frustration.



Ideal for any OB to achieve the best prop thrust is while water flow at plane is passing right under small water plate (yellow lines) under WF area as in picture. Go for a wot spin with deck weight well balanced, sit a boater up front if you like, trim OB to 90 deg (fully vertical) and play with different OB heights till that cond is visually met and boat nicely forever...

Bear in mind that if actual water flow is passing close to lower AV plate will produce excessive prop aeration on non calm flat water cond and at tight turns. OB will cool much less as water intakes are located right over AV plate..

Once there prop right OB tiorun middle to max wot rpm range factory stated as usually loaded...

Report your findings..

Happy Boating

 

[Faztbullet]

Needs to be dropped about 2 inches as that ice runner in front of lower will cause turbulence...

 

[racerone]

The main water intake is in the exhaust snout just behind the prop !!-----Drop this motor at least 2 "