2000 johnson 50hp cavitation but not prop

[Tim Frank]

Re: 2000 johnson 50hp cavitation but not prop

no it doesn't jerk or anything, I'm thinking its cavitating air somehow, after looking again today. the prop doesn't slow down on rpm's it increases. im trying to upload pics now.

My guess, from a distance, is ventilation.
Motor shaft is just on the edge of being too short.
As the pontoons get further up on plane as you get near WOT, the shaft is no longer far enough into the water.

Try a run with near maximum load.

 

[Scurvy Knave]

Re: 2000 johnson 50hp cavitation but not prop

So, to recap. As you increase RPM above basically 4500 RPM, you lose (all?) power (as felt by a noticeable deceleration)? When this occurs, does it sound like the engine is "surging"? Can you see the prop still spinning? If you leave it WOT in that condition, does the boat continue to go forward or does it coast down (to a certain speed)?

When you throttle back, do you perceive that you are re-gaining power (as felt by a noticeable acceleration)? When this occurs, do the RPM drop sharply and recover partly?

You mention that there is not "jerk" when the condition occurs or ceases to occur. Are there any abnormal noises or vibrations?

Any time your stop loading the engine (shift out of gear or prop hub spins out or major cavitation) the RPM's will have a tendency to surge at constant throttle.

 

[bcasurvey]

Re: 2000 johnson 50hp cavitation but not prop

Tim - this might be the problem what would be the fix?

Scurvy - when it breaks free the engine over revs with no traction, im finding that the prop is still spinning with the motor at higher rpm's, the boat coast down but the prop is still spinning as best I can tell,


((When you throttle back, do you perceive that you are re-gaining power (as felt by a noticeable acceleration)? When this occurs, do the RPM drop sharply and recover partly?)) yes the rpms drop and recover

((You mention that there is not "jerk" when the condition occurs or ceases to occur. Are there any abnormal noises or vibrations?)) no just over reving

((Any time your stop loading the engine (shift out of gear or prop hub spins out or major cavitation) the RPM's will have a tendency to surge at constant throttle.)) no surging

I now belive its a ventilation problem but how do I fix it?

 

[Scurvy Knave]

Re: 2000 johnson 50hp cavitation but not prop

sure sounds like a spun hub. Did you try drawing/scribing a line across the prop and hub and seeing if remains aligned after breaking free?

edit: ventilation is possible, but I think there would be visual signs

 

[Tim Frank]

Re: 2000 johnson 50hp cavitation but not prop

Tim - this might be the problem what would be the fix?

Scurvy - when it breaks free the engine over revs with no traction, im finding that the prop is still spinning with the motor at higher rpm's, the boat coast down but the prop is still spinning as best I can tell,


((When you throttle back, do you perceive that you are re-gaining power (as felt by a noticeable acceleration)? When this occurs, do the RPM drop sharply and recover partly?)) yes the rpms drop and recover

((You mention that there is not "jerk" when the condition occurs or ceases to occur. Are there any abnormal noises or vibrations?)) no just over reving

((Any time your stop loading the engine (shift out of gear or prop hub spins out or major cavitation) the RPM's will have a tendency to surge at constant throttle.)) no surging

I now belive its a ventilation problem but how do I fix it?

See post#2. You need to lower the motor, or get one with a longer shaft.
Your pics show the motor to have the correct length if you use the bottom of the V-plate as a datum, but every pontoon I've seen has the tubes' bottom-point lower than that.
i.e. you are right on the edge. That pontoon probably needs a 25" shaft motor.

If you know the 'toon manufacturer's name, call the and ask what length motor shaft they suggest.

To confirm the problem, you could try loading the boat up to almost capacity...balanced a bit to the stern...and see what happens.

 

[Tim Frank]

Re: 2000 johnson 50hp cavitation but not prop

sure sounds like a spun hub. Did you try drawing/scribing a line across the prop and hub and seeing if remains aligned after breaking free?

edit: ventilation is possible, but I think there would be visual signs

OP has already replaced the prop.

 

[bcasurvey]

Re: 2000 johnson 50hp cavitation but not prop

See post#2. You need to lower the motor, or get one with a longer shaft.
Your pics show the motor to have the correct length if you use the bottom of the V-plate as a datum, but every pontoon I've seen has the tubes' bottom-point lower than that. could you explain this alittle more in detail please
i.e. you are right on the edge. That pontoon probably needs a 25" shaft motor.

If you know the 'toon manufacturer's name, call the and ask what length motor shaft they suggest. yes its a bentley by smoker-craft will call them tomorrow

To confirm the problem, you could try loading the boat up to almost capacity...balanced a bit to the stern...and see what happens.

can I buy a 25" lower unit to add to this motor?

 

[Tim Frank]

Re: 2000 johnson 50hp cavitation but not prop

can I buy a 25" lower unit to add to this motor?

You can probably make the conversion, but first you need to confirm your problem:

To confirm the problem, you could try loading the boat up to almost capacity...balanced a bit to the stern...and see what happens.

Your pics show the motor to have the correct length if you use the bottom of the V-plate as a datum, but every pontoon I've seen has the tubes' bottom-point lower than that.

could you explain this alittle more in detail please

If you set the whole boat on a large dead-flat surface, would it be sitting firmly on the tubes, or would it be resting on the "v-plate" at the back?

If you know the 'toon manufacturer's name, call the and ask what length motor shaft they suggest. yes its a bentley by smoker-craft will call them tomorrow

That will answer the question. The previous owner may have`replaced the motor before selling it to you.

 

[bcasurvey]

Re: 2000 johnson 50hp cavitation but not prop

I will load the boat before going any further

if setting on the ground the v-plate would be 8" off the ground (that sounds like I need a 25" shaft???)

the previous owner was a little old lady and doesn't have a clue.

 

[James R]

Re: 2000 johnson 50hp cavitation but not prop

Get a few people on board and have them sit at the back. This generally shows if it is cavitation or spun prop.

 

[bcasurvey]

Re: 2000 johnson 50hp cavitation but not prop

ive never messed with a jack plate before, does anyone know if a jack plate will lower the motor and do you think it would help?

 

[Joe Reeves]

Re: 2000 johnson 50hp cavitation but not prop

I replaced the prop with a brand new one and it didn't help.

One of the pictures sort of looks like the engine has the wrong (tilted in too far) angle of attack. It may be an illusion but if it actually exists, would cause the bow to plow... and as the speed/rpms increase, the engine would continue to raise higher, near the surface of the water.

With the rig on a trailer, back off to the side so that you can view the bottom of the boat's hull in relation to the cavitation plate just above the propeller. That cavitation plate should be as close to parallel with the hull as possible. If at an angle, the cavitation plate acts as a "dive" or "surface" trim plate, depending on which way it's angled.

 

[bcasurvey]

Re: 2000 johnson 50hp cavitation but not prop

good point joe, it may be setup that the trim is to far in but that could be corrected by trimming out a little.
any ideas on a jack plate?