Johnson 25hp 1975 prop question

[mja2770]

I have a this engine on a 15.5' custom boat that was built for a go-devil. I have tried to go up and down with the engine but it keeps cavitating on me. I cannot tell what size prop that is on it now. Could the prop be causing this problem and if so what size prop do I need? Oh yea the prop that is on there now apears to be in good condition.


What size prop is recommended for this engine?

 

[steelespike]

Re: Johnson 25hp 1975 prop question

Check on the prop for the size. -- X -- or perhaps a part number.
Is there anything unusual about the boat.How high is the transom.
Where is the antivent plate (just above the prop)in relation to the bottom of the boat.

 

[mja2770]

Re: Johnson 25hp 1975 prop question

The prop does not have any markings on it. The transom is 17". The cavitation plate sits about 2" below the keel, as it sits now, but I have raised it higher and still same problem. I just went pick up a tiny tach and im going to hook it up and run the boat again tommorow just to see what RPM i am at before it starts cavitating.

 

[walleyehed]

Re: Johnson 25hp 1975 prop question

Is there anything, such as a transducer, speedwheel or pitot tube, etc, mounted on the transom with-in 12-14 inches of center?
If you remove the prop nut and washer, you should be able to look closely and read the size and/or P/N.

 

[whywhyzed]

Re: Johnson 25hp 1975 prop question

....

 

[steelespike]

Re: Johnson 25hp 1975 prop question

I wonder if you may have a spun hub??I'm surprised the antivent plate is
2 inches below on a 17 inch transom unless you have a long shaft 20" motor
and then 2 inches wouldn't add up.A short shaft would put it 2 inches above which could cause venting.
You can check the hub by drawing a line across the underside run the motor and check the line.

 

[mja2770]

Re: Johnson 25hp 1975 prop question

OK well today I cut 1/2" out of the transom and the boat does not cavitate anymore. But my next question is, I am running about 4500 RPM 27mph. Is this normal? I cannot find any markings on the prop, I took it off and cannot find anything.

Oh yea this motor is a long shaft which has me scratching my head on why I had to lower it even more.

 

[steelespike]

Re: Johnson 25hp 1975 prop question

69-76 25s are rated at 5,000 to 6,000 rpm.If you feel your setup is ideal and motor in good tune you may need to decrease your pitch.If you have a prop shop check your pitch we can tell much better what you need to do.It is critical for the health of your motor to get the rpm up into the higher end of your specs. It doesn't make sense for the ativentilation plate to be 2 inches below the bottom and need to drop 1/2" inch to stop ventilation.Is there something odd about the bottom?Keels possibly disturbing the water?

 

[BF]

Re: Johnson 25hp 1975 prop question

are you sure it is a long shaft?? I bet it is not. Here is a pict of my 1970 short shaft 25 hp. Notice on this type (that don't have through the prop exhaust) there is a water pump pickup above the anti-ventilation plate. Most transoms for short shaft motors are 16", which should put the water line on plane about 1/2 way between the anti-ventilation plate and the little splash deflector plate above it.

Sometimes people measure the distance to the anti-ventilation plate on these motors and are confused that it is something like 18".... but that's because they should be run deeper than newer ones. Don't go by what a previous owner told you... often they don't know what they are talking about.

So, I'm not surprised you needed to trim your transom a bit if it was 17", and you are using a short shaft motor.

Also, it's possible that you're running a prop from an older, larger HP OMC motor. With mine, I got a spare prop that is a 13" pitch, fits on and works, but I'm pretty sure it is off an older 28, 33 or even 40 (?) hp motor. The blades and hub are a bit thicker than the correct prop for this motor. It would get me home, but the 25 hp does not have the power to get to proper WOT rpms. I've been using a 9x9 on mine, but the standard prop (for my motor anyway) was 9x10".

good luck.