Thought I had a motor issue but now suspect cavitation

sbklf

Petty Officer 2nd Class
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Oct 26, 2006
Messages
190
98 23' Bayquest (V-hull with a slight tunnel) with Merc 200 W/jackplate. At times I notice that the motor RPM will increase like it was missing before and then decided to start hitting on all 6. This weekend we took a long drive in varying water condtions. Going out in smooth water it ran steady 42 at 5500 RPM. After the water roughed a bit, during a long run, the motor RPM jumped up to about 5900 and the speedometer dropped to about 33. The boat might have slowed slightly but I am sure it did not slow down that much. While still runing I jacked the motor up, down, trimmed up and down and could not get it to grab again. I could stop and start over and it would run steady 42 at 5500 for a bit then do the same thing (RPM jump up and speedometer drop). The speedometer pickup is on the leading edge of the lower unit on this motor. That is why I suspect cavitation since it is effecting the speedometer. Once before I was expiermenting with the jack plate and it overheated with it up a bit. Since then I always run with the jack plate all the way down.

I hope it is not a hull design issue that cant be easily solved with an anti cavitation plate. Any thoughts?
 

Bondo

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Apr 17, 2002
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Re: Thought I had a motor issue but now suspect cavitation

Ayuh,... If it IS cavitation,...

There should be cavitation burns all over the prop....
 

sbklf

Petty Officer 2nd Class
Joined
Oct 26, 2006
Messages
190
Re: Thought I had a motor issue but now suspect cavitation

Looked at the prop and did not see any thing but not sure what to look for. It is a stainless prop, will it still show up and if so what should I look for?

Ayuh,... If it IS cavitation,...

There should be cavitation burns all over the prop....
 

lncoop

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Apr 18, 2010
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Re: Thought I had a motor issue but now suspect cavitation

Suspect you actually mean ventilation, which is more common but is often confused with cavitation.
 

oops!

Supreme Mariner
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Oct 18, 2007
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12,932
Re: Thought I had a motor issue but now suspect cavitation

chek to see that the anti vent plate of the motor is at the same height as the bottom of the hull.....also check your hub
 

sbklf

Petty Officer 2nd Class
Joined
Oct 26, 2006
Messages
190
Re: Thought I had a motor issue but now suspect cavitation

If it is ventilation, how do I cure it? The motor on this boat is already as low as it can go due to the hydraulic steering cylinder fittings hitting the top of the transom.

Suspect you actually mean ventilation, which is more common but is often confused with cavitation.
 

lncoop

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Apr 18, 2010
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5,147
Re: Thought I had a motor issue but now suspect cavitation

Since the motor is already as low as possible let's hope it's a spun prop. That's certainly a possibility. Do you know the sharpie diagnostic trick?
 

sbklf

Petty Officer 2nd Class
Joined
Oct 26, 2006
Messages
190
Re: Thought I had a motor issue but now suspect cavitation

No but I will guess you paint the prop with one and see where the water rubs it off? What is the trick?

I talked to a local propeller shop here and he feels confident the cup on the prop is gone and just needs to be rebuilt.

Since the motor is already as low as possible let's hope it's a spun prop. That's certainly a possibility. Do you know the sharpie diagnostic trick?
 

lncoop

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Apr 18, 2010
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5,147
Re: Thought I had a motor issue but now suspect cavitation

No but I will guess you paint the prop with one and see where the water rubs it off? What is the trick?

I talked to a local propeller shop here and he feels confident the cup on the prop is gone and just needs to be rebuilt.

Well, the trick is very technical, so try to stay with me here.:rolleyes: Joking aside, you just mark all the way across the hub, go for a spin (no pun intended), then trailer the boat and take a look. If your one mark has become multiple marks, get thee to the prop shop (or iboats for a new prop).
 

Chris1956

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Mar 25, 2004
Messages
27,036
Re: Thought I had a motor issue but now suspect cavitation

My 21 Seaswirl runabout has a flattened keel area that acts like a pad. When the water is smooth, and I am going straight, I can trim the motor all the way up, and max out speed and RPM, and run on the pad. When running like that, if I hit a wave or turn or the water gets rough, the boat slows and the RPMs increase. The cure is to only run under max trim, when the water is smooth.

If that is your issue, the cure is likely to lower the motor on the transom a bit.
 
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