Cavitation? what are some possible symptoms?

[ml1991]

I was told that it looks like i am getting some cavitation and that is why i am having a hard time getting on plane. Would that also cause the motor to start cutting out and act like it is going to die as i am hitting the throttle trying to get on plane?
Wouldnt i also have cavitation when i am at higher speeds?
What are some of the symptoms of cavitation?

By the way, i have an 02' merc optimax 225 on a 21ft bass boat.
Thanks guys,
Mike

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

You need cavitation planing off if you want good top speed, if the boat's not too heavy and sluggish for the motor: jack the motor up until the cavitation plate's at least 1/2" above the bottom of the boat, and cup the prop. If the motor cavitates at top speed then either the prop's not cupped enough, or the motor's too high. You can tell by the feel of the boat, once you experience the difference.

Motor dying or cutting out has nothing to do with cavitation. Motor hesitating under acceleration may indicate that the carb butterflies are opening too fast relative to spark advance, and can be due to clogged jets.

I was told that it looks like i am getting some cavitation and that is why i am having a hard time getting on plane. Would that also cause the motor to start cutting out and act like it is going to die as i am hitting the throttle trying to get on plane?
Wouldnt i also have cavitation when i am at higher speeds?
What are some of the symptoms of cavitation?

By the way, i have an 02' merc optimax 225 on a 21ft bass boat.
Thanks guys,
Mike

 

[Chris1956]

Re: Cavitation? what are some possible symptoms?

Mike, Sandhammer is giving misleading advice.

If the motor revs up while trying to plane, but speed does not increase, your prop is likely ventilating (not cavitating). If the motor doesn't speed up while trying to plane, but slows down, you are having engine problems, possibly the carbs are out of adjustment or dirty. Of course on your Optimax, you do not have carbs, and the cause/cure for anything motor related is out of my area of expertise.

Repost with a better description

 

[CharlieB]

Re: Cavitation? what are some possible symptoms?

Cavitation is the prop loosing bite or traction with the water and the motor reving.

It can be caused by too high of mounting the motor, irregularities in the hull directly in front of the drive, chips, dings in the prop leading edges. Anything that can create too much of a low pressure area in the water causing an air bubble to form.

 

[ml1991]

Cavitation repost

Cavitation repost

I posted last night about cavitation and was told to repost with more info so here it is...

When i go to take off, the motor bogs down and will only go 9-10mph. It wont get on plane and that is with full throttle engaged. It sputters and cuts out a couple times then all of the sudden, its like an injection of nitrous and the boat leaps forward and jumps right on plane and runs perfectly.
The shop told me he believes it is cavitation because the dyno doesnt show anything wrong.
I have tried different take offs, starting out at 7mph then giving it throttle...giving it partial throttle to get on plane...idling at 5mph then hitting the throttle to get on plane. No matter what i try it has the same symptoms.
It has gotten on plane twice like it should. Once in the early morning after idling past the buoys, and the other in midday while moving to another fishing spot.
I dont know if this is the info you were looking for so if not let me know what else you would like to know. Thanks for the responses.

Mike

 

[Don S]

Re: Cavitation repost

Re: Cavitation repost

I moved this to your original thread, in the future, additional information should be posted to your original post as a reply, not by starting a new thread. Makes it impossible to follow with a new thread.

 

[Chris1956]

Re: Cavitation? what are some possible symptoms?

Mike, It sounds like your shop doesn't know the difference between cavitation and ventilation, which is not good.

If the motor runs good on the dyno, but bad on your boat, what are the differences between the two? Does the shop use it's own fuel supply? Do they use a different prop? Both of these differences could cause the difference in performance.

A stupid question. Do they really have a dyno, or are they just funning you?

 

[QC]

Re: Cavitation? what are some possible symptoms?

Cavitation is the prop loosing bite or traction with the water and the motor reving

Just to back up what Chris is saying, you are describing Ventilation. Cavitation is extreme low pressure on the trailing edge causing the water to literally boil and resulting in pitting on the blade.

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

You get your prop information at Sears?

Mike, Sandhammer is giving misleading advice.

If the motor revs up while trying to plane, but speed does not increase, your prop is likely ventilating (not cavitating). If the motor doesn't speed up while trying to plane, but slows down, you are having engine problems, possibly the carbs are out of adjustment or dirty. Of course on your Optimax, you do not have carbs, and the cause/cure for anything motor related is out of my area of expertise.

Repost with a better description

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

What you describe has nothing to do with cavitation ('ventilation'), in that case the motor winds up and the prop 'doesn't catch. When the motor bogs down, as you describe, it's the opposite, RPM too low. Can bog down if the plugs are too oily from idling too long (gapless plugs with extra oil or too rich carb setting), or if the carb butterflies open too fast as the spark is advanced. there can be other causes as well. The advice I gave you earlier is correct, I would ignore postings by 'experts' who want to nit pick over terminology.

I was told that it looks like i am getting some cavitation and that is why i am having a hard time getting on plane. Would that also cause the motor to start cutting out and act like it is going to die as i am hitting the throttle trying to get on plane?
Wouldnt i also have cavitation when i am at higher speeds?
What are some of the symptoms of cavitation?

By the way, i have an 02' merc optimax 225 on a 21ft bass boat.
Thanks guys,
Mike

 

[CATransplant]

Re: Cavitation? what are some possible symptoms?

You get your prop information at Sears?

Just a hint: You're not making any friends here with your sarcasm and mistaken information. The two people giving advice in this thread know what they're talking about and have been giving good advice here for a long, long time.

 

[QC]

Re: Cavitation? what are some possible symptoms?

I would ignore postings by 'experts' who want to nit pick over terminology.

Hmmmmm . . . Not sure how else to accurately convey a point in writing. I guess we should give up and just start posting pictures

We try and run both an informative and accurate forum here. If we can help each other with terminology that helps eliminate some ambiguity, then we usually do. With that said, I am having trouble with the thingy that makes my nose go high when I am trying to go faster cause I push on the gas but then my screw thing that pushes goes faster and it's noisey. Better?

 

[JB]

Re: Cavitation? what are some possible symptoms?

It is definitely NOT carb butterflies opening too fast and getting ahead of the ignition advance, though that would be a reasonable suspect in a carbed engine. This is an Optimax, "Professor". There are no carbs.

I think you are making an error in disrespecting the expertise of established helpers here, and I know you are making a serious error in insulting any member. That is a banning offense here.

The "advice" you gave in your first post is incorrect, not just the terminology but the substance. In spite of your credentials I think you are over your head in this one. I suggest you take a couple of humble pills, read and learn.

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

You don't seem to have anything to say. With only unfriendly remarks and insults, you're in danger of making a fool of yourself.

Just a hint: You're not making any friends here with your sarcasm and mistaken information. The two people giving advice in this thread know what they're talking about and have been giving good advice here for a long, long time.

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

Science and good mechanic work aren't based on respect for excessive self-appointed 'authority'.

It is definitely NOT carb butterflies opening too fast and getting ahead of the ignition advance, though that would be a reasonable suspect in a carbed engine. This is an Optimax, "Professor". There are no carbs.

I think you are making an error in disrespecting the expertise of established helpers here, and I know you are making a serious error in insulting any member. That is a banning offense here.

The "advice" you gave in your first post is incorrect, not just the terminology but the substance. In spite of your credentials I think you are over your head in this one. I suggest you take a couple of humble pills, read and learn.

 

[Sandhammaren05]

Re: Cavitation? what are some possible symptoms?

You're right that 'cavitation' is used loosely where another term might be better, but propeller theory hasn't advanced much in understanding problems with reentry, or 'sucking air', the problems are too hard to formulate mathematically. By cavitation we understand well enough that the water boils locally on the low pressure side of the blade (not merely on the trailing edge), forming bubbles that collapse. That happens when the velocity over the blade is so high that the pressure drops to the vaporization point (Bernoulli equation). We colloquially' say that a prop 'cavitates' if the prop 'slips', spins at high RPM when planning off, and then 'catches'. This is loose terminology and it's probably true that this differs from cavitation, so the word ventilation was introduced. It's easy to understand qualitatively how a fully submerged prop works from hydrofoil theory, but surfacing props and 'ventilation', are not well understood mathematically. Then there's the term 'supercavitation' in the older literature (ca. 1960) in early attempts to understand surfacing props.

Added later: Have checked via google, it seems that both 'ventilation' and 'supercavitation' have become standard technical terms in recent years (my propeller theory studies date to ca. 1980). Here's a recent quasi-technical but fairly qualitative article:

www.smbf.info/smbf/tech/527_2.PDF

The author writes down the Navier-Stokes equations (physics of fluid flow) but only uses them to identify the various dimensionless numbers of interest in describing different flow regimes (Reynolds number, Froude nr., and so on). I think it's about as nontechnical and fairly informative an article as you may find. There's no theory of prop performance in the article, most of what's described is quasi-empirical. Most of what we know in practice comes from trial and error, and that's where racing has contributed enormously to our knowledge.

Just to back up what Chris is saying, you are describing Ventilation. Cavitation is extreme low pressure on the trailing edge causing the water to literally boil and resulting in pitting on the blade.

 

[CharlieB]

Re: Cavitation? what are some possible symptoms?

CHILDREN........Play NICE!!



To get back to the posters original problem, It's NOT the prop, evidenced by the fact that 'occasionally' the boat accelerates correctly and runs at rated RPM.

The DIFFERENCE in experiencing the problem AND the shops 'Dyno' runs is the submersion of the drive in the water and being 'on the hose' in the shop.

It may be Either the Throttle Position Sensor (TPS) is adjusted a hair lean or the intake vacuum sensor may have a drop of oil restricting it, if the Opti uses an intake vacuum sensor (small hose) from the intake to the processor

 

[CATransplant]

Re: Cavitation? what are some possible symptoms?

You don't seem to have anything to say. With only unfriendly remarks and insults, you're in danger of making a fool of yourself.

I offer suggestions only in areas where I have experience. No doubt I have made a few mistakes here, but I try to check my information before offering it.

One of the areas where I do have significant experience is in online forums. That was the area of expertise I consulted when offering advice to you. You may, of course, take it or leave it. I won't be offering it again to you.

 

[ml1991]

Re: Cavitation? what are some possible symptoms?

Thank you everyone for responding. Also i am sorry about the posting process, it is new to me so i am trying to get the hang of it. I talk to the shop today and will hopefully find out more. Now i have a few things i can bring up when i talk to them so hopefully it can help to diagnose it and solve the problem. Thank you again.

Mike