Cavitation cause and solution - looking for some advice

jeep8589

Cadet
Joined
May 19, 2004
Messages
23
I have a 1998 Maxum 17ft that I just bought. Has the 3.0l mercruiser and alpha one. It is equipped with an Apollo SS prop. I took it out on the water and torque steer was horrible. So I put a anode trim tab made for a SS prop. It appears they removed the original tab since it would hit the prop. The prop clears the new trim tab, but only by about 1/8 of an inch. It is very close. It is also angled to fight the torque steer. I ran it again and it now has ventilation / cavitation issues. (I do know the difference, just so many people mix the terms) It seems to kick in around 4000rpms. Is the tight clearance between the prop and tab causing the issue? I dont think it is the gear case slippping or the prop hub (which I checked) because the most resistance is when you dump the throttle, which it is pulling just fine off the line. Starts to happen when you get onto plane. Also, did not do it without the new tab. Any boat experts out there that have seen this? The torque steer is bad enough that I would like to have the trim tab on there to help with it. Would grinding some clearance between the prop and tab fix this???
 

IanFS

Petty Officer 2nd Class
Joined
Apr 21, 2009
Messages
188
Re: Cavitation cause and solution - looking for some advice

"It is equipped with an Apollo SS prop. I took it out on the water and torque steer was horrible. So I put a anode trim tab made for a SS prop. It appears they removed the original tab since it would hit the prop."

This happens quite a lot when the original Aluminium propeller is replaced by a Stainless job. I usually buy a normal tab but grind it down. The Apollo is quite raked, so a little ground from a normal tab will do the trick and gove you a little more surface area to play with.

"I ran it again and it now has ventilation / cavitation issues."

"(I do know the difference, just so many people mix the terms"

Ventilation is as it sounds. Water surrounding the propeller under operating conditions is allowing air to add to water thus venilating the propeller. Basically the prop is not powering through water but instead is powering through fizzy water and has no grip.Cavitation on the other hand is a little more complicated but simply put it is immediate implosion of cavities in water. It is caused by low pressure and badly formed propelllers, unusual unless a propeller is damaged.

"......seems to kick in around 4000rpms......Starts to happen when you get onto plane........"

What exactly starts to kick in at 4,000rpms? I'm guessing you get propeller slippage??


"Also, did not do it without the new tab.........The torque steer is bad enough that I would like to have the trim tab on there to help with it."

What tab did you use before replacing it with a Stainless prop tab?
 

jeep8589

Cadet
Joined
May 19, 2004
Messages
23
Re: Cavitation cause and solution - looking for some advice

Yes, it is ventilating. I just put both terms so that some people who refer to it as cavitation would grasp my meaning. Yes, it is ventilating around 4000 rpms. I just bought the boat, and it did not have a trim tab. I am guessing that they removed it since it was hitting when they (who ever had it before me) put the SS prop on it. Is the ventilation being caused by the close tolerance between the prop and tab, or the angle for countering the torque steer. I would like to have a tab to help with torque steer as I dont have power steering.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,464
Re: Cavitation cause and solution - looking for some advice

Ayuh,.... Ya got a spare prop,..?? if so, Try it...

It's entirely possible that, That prop don't like yer hull...
 

jeep8589

Cadet
Joined
May 19, 2004
Messages
23
Re: Cavitation cause and solution - looking for some advice

Come one guys.... I do my best to help people, and have answered hundreds of questions for people on forums. If I know the answer, then I put it down. I read their question and answer it to the best of my ability. I appreciate the effort english mariner, but half the questions you asked me where answered in my question to begin with.

"Ventilation is as it sounds. Water surrounding the propeller under operating conditions is allowing air to add to water thus venilating the propeller. Basically the prop is not powering through water but instead is powering through fizzy water and has no grip.Cavitation on the other hand is a little more complicated but simply put it is immediate implosion of cavities in water. It is caused by low pressure and badly formed propelllers, unusual unless a propeller is damaged."

I even told you I knew what ventilation was versus cavitation and yet, you still try to explain it to me.

And bond-o tells me to try another prop and english mariner tells me that SS props do this a lot..... but as I told you in the question, it did not do it until I added the new trim tab. Then, someone moves me to the prop forum. I am looking for someone that has had the same issues as I have, and then sorted it out in the past. I would think I would want someone that has experience with alpha ones and their design can help me more than someone that has knowledge of propellers. Maybe I am wrong. I am not trying to be a jerk, but I am trying to get to the root of the problem. Is ventilation caused by the fact that there is only 1/8" of clearance between the prop and tab, or is it that the tab is offset for the torque steer causing the issue? Is it that the oversized SS prop does not like trim tabs period?
 

sschefer

Rear Admiral
Joined
Nov 13, 2008
Messages
4,530
Re: Cavitation cause and solution - looking for some advice

Are you experiencing prop walk where the boat feels like the engine is turning the boat? That can be caused by too large a diameter prop. Sometimes folks believe you can change the diameter of a prop without changing the pitch. A 1 inch change in diameter can equate to a necessary 2" reduction in pitch. It all depends on the design of the prop.

I would start by finding out what the recommended prop diameter and pitch in stainless are and then comparing it to the one you have. Because of the clearance issue with the trim annode I'm suspecting that the prop was changed without a lot of research. I'm just guessing so you need to do the research and confirm things.
 

IanFS

Petty Officer 2nd Class
Joined
Apr 21, 2009
Messages
188
Re: Cavitation cause and solution - looking for some advice

I am not trying to be a jerk,

Well you're doing a pretty good job of it so far.

I apologise I misread the bit about "I do know the difference", I thought you had written "I don't know the difference". I was trying to be helpful to you, I have a fair bit of experience with racing props etc.

The tab could cause a little of what you are describing but it is unlikely that is what you have.

You post was moved quite rightly by Admin because it is a prop question. I'm out now I'm afraid, don't like wingeing ungrateful people.
 

Bondo

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Staff member
Joined
Apr 17, 2002
Messages
70,464
Re: Cavitation cause and solution - looking for some advice

And bond-o tells me to try another prop and english mariner tells me that SS props do this a lot..... but as I told you in the question, it did not do it until I added the new trim tab. Then, someone moves me to the prop forum. I am looking for someone that has had the same issues as I have, and then sorted it out in the past. I would think I would want someone that has experience with alpha ones and their design can help me more than someone that has knowledge of propellers. Maybe I am wrong. I am not trying to be a jerk, but I am trying to get to the root of the problem. Is ventilation caused by the fact that there is only 1/8" of clearance between the prop and tab, or is it that the tab is offset for the torque steer causing the issue? Is it that the oversized SS prop does not like trim tabs period?

Ayuh,.... You have a Prop issue, Not a Drive issue....( Yes, I moved yer thread)

We're tryin' to help ya find the Cause....

If ya Don't want my help, 'n ya Don't wanta try to find the REAL issue,...

Fine,.... Yer on yer own, 'n I'm outa here....

Generally speakin', so long as the prop Don't hit the tab, it Ain't the issue....
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Cavitation cause and solution - looking for some advice

I have 63 years of boating When you have an I/O that vents if it hasen't been messed with there isn't much to check,mostly logical stuff.
You did the hub,vent plate height should be factory though you could check it and report back Is there anything on the transom or the bottom that could disturb the water in front of the prop.Are there any trim tabs?
Have you tried a different more routine prop?Is the hub of the prop the right size to fit the gear case?Does the boat respond to up trim.
Does it lay down like its glued to the water?
Have you tried it with the tab removed again?.Is the tab one of the offset tabs.If not; try one or grind some off.
 
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