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View Full Version : Rudder Cavitation Burn


rockcrwlr72
07-28-2012, 10:33 PM
Anyone ever scene anything like this? This is from one weekend! The rudder even bent under the heat. Anyone know what causes it, or how to fix it? I think it might be the strut bearings are so worn out, it is making the prop cavitate.

jfw432
07-29-2012, 12:32 AM
Definitely some cavitation going on there. I can see just from your photo that the driveshaft isn't sitting centered in the strut. I'd be willing to bet that your prop and driveshaft have excessive play that is causing a lot of problems. Does that strut have heat damage? I can't tell what that is.

rockcrwlr72
07-29-2012, 01:13 AM
The strut seems to be fine, just has general surface corrosion. Do you know if the brass rudder can be repaired by oxy / acetele torch and brass rod?

jfw432
07-29-2012, 08:26 AM
I would think if you try to melt brass on a brass rudder, you're going to also melt the rudder and really mess it up. Some solders can be used for brazing brass. My rudder has some cavitation burns on it but not all the way through like yours. I used JB weld last year and sanded it smooth. Some areas have flaked off but most of the JB weld is still in place.

Ron Grover
07-29-2012, 11:28 AM
JB Weld is the boaters "duck tape"

FrankSchwab
07-29-2012, 07:58 PM
Prop shops add brass all the time to prop blades, so I'm assuming that they could do the same to the rudder. Don't think I'd try it myself, however.

cptskier15
07-30-2012, 06:52 AM
We tried to repair our rudder with adding brass, 80% of rudder eneded up at bottom of lake after 2 hours! Had to use wakeboard to get home. Got new mastercraft rudder and steering cable and was A for away

Hrkdrivr
07-30-2012, 07:56 AM
Cavitation caused the damage to the rudder? Wow.

aquaman
07-30-2012, 08:04 AM
If its not a casting flaw.....I might think it was Electrolysis?....because the strut also has pits all over.

Added any new electronics lately or re-wired ?

CCAnderson
07-30-2012, 09:09 AM
the under water gear is not brass but Nibral which an alloy of nickel bronze and aluminum. you have to use the correct alloy rod to repair it correctly or you will introduce a hole set of new problems.

There are many variables that cause cavitation including water temp (density) but is all comes down pressure and velocity. The faster the boat moves meaning the faster the prop spins the more likely the occurance of cavitation. If you have a heavily loaded boat and are spinning a ton of RPMs to hold a speed that can cause it. if you have a damaged prop - that can cause it because the damage areas cause higher then normal velocities. as the velocity increases the pressure decreases. if velocity goes too high the water separates into a vapor aka steam (not hot but cold steam)

The cavitation damage which sometime called "burn" is caused by the vapor bubbles caused by the underwater gear collapsing on the rudder. you are not actually burning away material but more like sand blasting it away. I can go into the physics if you want but it a long boring lesson.

The navy has done extensive testing on this and come to the conclusion the cavitation cannot be avoided so they design the gear for "normal operation" when they run their boats outside of the "normal" range like wide open for hours on end they know they will be replacing props and rudders.

gatorguy
07-30-2012, 10:02 AM
I think a better question is if this is actually from cavitation, why are you getting so much cavitation? No one else is on here complaining of burning through props and rudders.

TRBenj
07-30-2012, 10:42 AM
Our Barefoot Nautique came to us with similar damage on the rudder and rudder port. The gel and glass on keel of the boat aft of the prop was also in pretty tough shape.

Our best guess was that it was caused by a combination of minimal prop-hull clearance, and the banged up prop... which could have been run in its damaged state for years.

aquaman
07-30-2012, 10:46 AM
I think a better question is if this is actually from cavitation, why are you getting so much cavitation? No one else is on here complaining of burning through props and rudders.

Agree......the strut also has generalized pitting.

Wonder if this boat has been running in salt water ?

Phntmski
07-30-2012, 11:02 AM
Interesting thread. I've never really had an issue with cavitation before but a few weeks ago I was teaching my wife to ski. We were both out on the boom on 2 skies. The friend driving the boat got a lot of cavitation because he was turning hard away from (rt) the drag the 4 skis were putting on the boat. As he straightened out the cavitation decreased. Later in the day I was driving the boat under similar circumstances. I kept the rudder straighter, sooner and didn't have near the cavitation. I'm not that experienced using the boom so this was a learning experience. Luckily I didn't have any burn issues.

CCAnderson
07-30-2012, 11:24 AM
That is definitely cavitation damage. I see this all the time in the valves manufactured by the company i work for. Can we get some pics of the prop, shaft nut, strut, etc?

Also is this boat in fresh or salt water. Nibral holds up very well to salt water but you could also be experiencing flow accelerated corrosion which is where the corroded material is blasted away by the cavitation because it is weaker than the base material.

I have damage on my prop and rudder but not to this extent. I talked with Eric @ OJ. he said that every damaged prop they see has cavitaiton damage as a result of the dings.

jfw432
07-30-2012, 04:36 PM
This is what's worrying me. This may not be the entire problem but I can't believe that gap difference is normal. The strut bearing looks to either be non existent in some places or severely worn.

CCAnderson
07-30-2012, 05:56 PM
yup the bearing needs attention but I doubt it is the culprit.

cptskier15
07-30-2012, 06:12 PM
The whole cavitation effect is really interesting!! On our old prop that took a beating before we got the boat it had same "cavitation damage" as the pictures posted. We just thought previous owner had done something to cause it.

What about polishing up rudder and prop from the state in photos to an almost mirror finish? You guys seem to know your stuff about this, worth it or does it look pretty and dampen performance?

rockcrwlr72
07-31-2012, 02:02 AM
That is definitely cavitation damage. I see this all the time in the valves that manufactured by the company i work for. Can we get some pics of the prop, shaft nut, strut, etc?

Also is this boat in fresh or salt water. Nibral holds up very well to salt water but you could also be experiencing flow accelerated corrosion which is where the weaker corroded material is blasted away by the cavitation because is is weaker than the base material.

I have damage on my prop and rudder but not to this extent. I talked with Eric @ OJ. he said that every damaged prop they see has cavitaiton damage as a result of the dings.


Yup, getting new strut bearings, the discoloration on the strut is not pitting. Not sure how to explain it, its not pitted, the surface is actually raised.

rockcrwlr72
07-31-2012, 02:12 AM
Here are some more

rockcrwlr72
07-31-2012, 02:15 AM
Tell me what you guys think, could the extremely worn or non-existant strut bearings allow the driveshaft to vibrate badly enough to cause excess cavitation. For those readers who had not followed any of my other posts, this is my first inboard but and the damage was from the first time out. So I am on a steeeeeep learning curve here.

petermegan
07-31-2012, 04:32 AM
It would not surprise me if the strut bearings are worn that badly that it is allowing air to feed down the hull and maybe sucked through the strut and the prop is throwing the air bubbles onto the rudder. Pretty far out but you have to replace the strut bearings anyway. Give it a go. Those strut bearings are completely hammered.

CCAnderson
07-31-2012, 09:09 AM
Here goes. loose strut bearing cannot introduce air because it is submerged constantly and air actually lessens the effects of cavititation. It acts as a cushion. The company will sometimes recommend introducing air to a valve to limit the damage potential.

I suppose excess vibration may cause the caviation but i still suspect there are more issues to be uncovered.

The prop and rudder have been replaced recently. The strut, prop, rudder, and rudder support are all the same material but in the pictures they show a different degree of discoloration. Nibral ages like copper since copper is ~80% of the alloy. This begs the question why have they been replaced. Also I do not recognize the brand of prop. Federal? maybe. and where is the cotter key. My gut tells me that the previous owner unloaded the boat after they caused an issue and now you get to deal with it.

My recommendation is to take to the dealer. have them replace the strut bearing, check the drive shaft for straightness, check engine alignment, fix the rudder, and finally call OJ or ACME and get the right prop for the boat.

While this seems like an expensive endeavor the upfront cost will be much less than letting it go and causing more extensive damage in the future.

CCAnderson
07-31-2012, 09:13 AM
Actually replace the rudder. It too looks suspicious as the overall casting quality does not look appropriate for a MC.

CCAnderson
07-31-2012, 09:24 AM
This is getting the best of me. If you can hold a straight edge in front of the damaged spot on the rudder and parallel to the hull all while shooting a picture form the side we should be able to tell where the cavitation is generated. (see the really bad MS paint sketch)

aquaman
07-31-2012, 09:36 AM
Well, from the looks of the trailer i think the previous owner did a bit of salt water boating.

Fresh water typically does not make struts look like that.

rockcrwlr72
07-31-2012, 01:30 PM
That is my suspicion that it is more from the bearings than the prop. The prop does have one small bend in a bade, it is in one of the pictures. As far as the rudder being replaced, I don't think it was ever replaced. I just bought the boat and cleaned everything out and greased the steering cables, and rudder. While I did that, I am pretty sure that rudder had never been removed judging by the age, grease and dirt buildup on the components. I am the third owner and I know the original owner lived in Oklahoma, then the guy I bought it from Lived in Apple Valley, Ca and had the boat registered in Az. According to his testimony he only used the boat at lake Mojave or Silverwood lake by his house.

Also the cavitation burn only is on the right side of the rudder.

rockcrwlr72
07-31-2012, 01:33 PM
Pics before damage.

My plans as far as fixes are:
-Replace strut bearings
-either replace or have rudder repaired, comes down to money
-Replace the prop
-Rebuild trans and replace trans mounts

rockcrwlr72
08-08-2012, 01:56 PM
Well guys I got all my parts ordered. The only thing I did not order new is the rudder, I am going to have it repaired. I will keep this thread updated on the repairs, tranny rebuild, strut bearing install and the new stock dimension prop.

aswinter05
08-08-2012, 02:37 PM
I copied this from "outboard-boat.com". Very interesting. I've not heard much if anything about cavitation before seeing this thread.

Cavitation is when you get lots of pitting on your propeller. At the early stages it will be noticeable by pitting in the paint. Eventually it will eat into the blades of the propeller and in extreme cases the blades can be weakened to the point that they break off. The cause of the pitting on the blades is from quite a violent reaction under the water.

Cavitation is caused from low and high pressure areas in the water. When water is pressurised it raises the boiling point of the water. This concept is used in household pressure cookers, when water is pressurised it can be heated above 100 C without boiling and turning to steam. Water also works the other way if you lower the pressure below atmospheric pressure it lowers the boiling point.

As the boat moves through the water, if the bottom of the hull has imperfections, it creates low pressure areas. These low pressure areas cause the water in that area to boil and turn to steam. These pockets of air travel backwards through the water. When they hit the propeller which is creating very high pressure areas on the blades the steam implodes back into a liquid form with a very violent reaction literally eating away the material of the propeller.

petermegan
08-08-2012, 06:24 PM
Good research. Well done

rockcrwlr72
08-08-2012, 06:35 PM
Interesting, I have not heard about the hull imperfections causing low pressure areas. As for propeller the only thing wrong with mine is the bend in one of the fins, no pitting.

ahhudgins
08-08-2012, 10:17 PM
I'd bet that it's more of an issue related to the PO's use of the boat and/or neglect. Rudder box, strut and prop nut all look corroded in the "before" pic. No cotter pin on the prop nut and the strut bushing is either just worn out or the engine needs to be aligned. If it were me, I'd gut everything under the hull and start with fresh parts rather than waiting for each one to drop off one at a time. But that's just me.

jakethebt
08-08-2012, 10:48 PM
What is this? It looks like an anode that is used on salt water boats?

mikeg205
08-08-2012, 11:54 PM
This boat must have been in at least brackish water....way too much corrosion even it is sat in fresh water on the strut, and rudder... +1 on the underwater gear replacement.

rockcrwlr72
08-09-2012, 06:38 PM
I'd bet that it's more of an issue related to the PO's use of the boat and/or neglect. Rudder box, strut and prop nut all look corroded in the "before" pic. No cotter pin on the prop nut and the strut bushing is either just worn out or the engine needs to be aligned. If it were me, I'd gut everything under the hull and start with fresh parts rather than waiting for each one to drop off one at a time. But that's just me.

Yeah, I just bought a new prop, strut bearings, rudder box seals, Tranny rebuild kit and Tranny couplers. I am going to keep this thread updated with all the work.

rockcrwlr72
08-09-2012, 06:40 PM
What is this? It looks like an anode that is used on salt water boats?

There is a long very thick stainless steel bar that attaches to that bracket on the inside of the boat and then attaches to the rear lifting ring.

ahhudgins
08-09-2012, 06:51 PM
Yeah, I just bought a new prop, strut bearings, rudder box seals, Tranny rebuild kit and Tranny couplers. I am going to keep this thread updated with all the work.

A very wise decision. I think you will be happier with your results in the long run. Judging by the gap in the strut bushings, I believe a motor alignment is needed. I'd junk that rudder....just polising a turd. :D

rockcrwlr72
08-21-2012, 01:11 AM
Well guys I told you I would keep you guys informed of work and progress, so here are some tranny tear down pictures. The outside is just plain ugly, rust, rust and more rust. Some of the bolt heads were so gone I had to cut them off and take them out later. My suspicions were wrong however, I thought the tranny was slipping due to worn clutch packs. When I tore it apart I found the clutch packs and bearings look great. But there was evidence of water or moisture in the tranny. I found a good amount of sludge, where the water, oil and filth had mixed together in the Forward/Reverse adapter assembly. I suspect this was not letting the pump, well pump! Other than that, the tranny insides looks good. But i already bought the rebuild and various parts before the tear down, so I am going to replace it anyways. The case and various parts are off to get hot tanked.

rockcrwlr72
08-21-2012, 01:13 AM
some more

curtish
08-22-2012, 01:42 AM
Dang that trans looks nasty.

aquaman
08-22-2012, 08:39 AM
No question about it now....that's a Saltwater boat.

You were lucky to get the trans. off at all.

Phntmski
09-06-2012, 10:39 AM
Generally speaking, how close to the hull bottom can the prop be before cavitation burn is a concern? How does it affect performance?

Phntmski
09-06-2012, 11:30 AM
My situation is this; I have a 91 PS 190 with a slot. Right now I'm running a 4 blade that is the smoothest thing I've ever experienced. When I shift to a 3 blade version of the same prop I get a little vibration out of the hole. The blades on the 3 are much bigger than the 4 which puts them closer to the rudder. In an effort to alleviate that a little we bore the prop hub a tiny bit in order to slide the prop forward a little. This puts it further from the rudder and closer to the hull. Just don't wanna risk any hull damage.

The 3 blade takes such big bites I don't know if it's possible to remove all vibration from an aggressive hole shot.

Thanks for any input.

DooSPX
09-06-2012, 11:34 AM
My situation is this; I have a 91 PS 190 with a slot. Right now I'm running a 4 blade that is the smoothest thing I've ever experienced. When I shift to a 3 blade version of the same prop I get a little vibration out of the hole. The blades on the 3 are much bigger than the 4 which puts them closer to the rudder. In an effort to alleviate that a little we bore the prop hub a tiny bit in order to slide the prop forward a little. This puts it further from the rudder and closer to the hull. Just don't wanna risk any hull damage.

The 3 blade takes such big bites I don't know if it's possible to remove all vibration from an aggressive hole shot.

Thanks for any input.

What props are u running?

EJ OJPROP
09-06-2012, 12:03 PM
The 3-blade will make more noise that the 4-blade especially with the larger diameter 3-blades on the gear reduction transmission. Might not be the best idea either to overbore the propeller. Did you use a tapered reamer and then re-balance the prop?

Phntmski
09-06-2012, 12:03 PM
Acme 817 for the 4 blade. Acme 1749 for the 3 - 13.5 x 17.5, 150 on the cup. Was thinking about backing off a little cup on the 3.

Phntmski
09-06-2012, 12:08 PM
Actually sent the prop back to Acme for the ream. It's really just an experiment now. Just don't want to do any damage.

TRBenj
09-06-2012, 12:19 PM
Ive run props within 1/4" of the rudder for short periods and never noticed any problems (vibration, etc). In regards to hull clearance, the general rule of thumb is 10% of the prop diameter should be maintained for clearance... but infringing on this a little bit seems to be ok (~1" on a 12-13" diameter prop).

Phntmski
09-06-2012, 12:34 PM
Ive run props within 1/4" of the rudder for short periods and never noticed any problems (vibration, etc). In regards to hull clearance, the general rule of thumb is 10% of the prop diameter should be maintained for clearance... but infringing on this a little bit seems to be ok (~1" on a 12-13" diameter prop).


TR -
So, 1.3" should be fine?

The 3-blade will make more noise that the 4-blade especially with the larger diameter 3-blades on the gear reduction transmission. Might not be the best idea either to overbore the propeller. Did you use a tapered reamer and then re-balance the prop?

Eric-
I've wondered if it's possible to rid the noise completely with the 3 blade. My boat mostly runs 32 - 34 mph in a slalom course. I love the 4 blade but wouldn't mind bringing the rpm down a little to save gas. To be honest I don't know the tach #s for the 3. Haven't kept it on long enough to find out. Seems like the effeciency of a 3 blade would be a positive. I can tell you WOT is about the same rpm with the 3, maybe a few less. Top speed is about 1 mph faster.

ahhudgins
09-06-2012, 12:57 PM
Dang that trans looks nasty.

After seeing those pics, I'm sticking to my story.:D
I believe the problem is related to usage and neglect by the PO, not cavitation. Of course, my opinion and a dollar will get you a cheap cup of coffee.

DooSPX
09-06-2012, 01:35 PM
On my 91 with the stock engine and powerslot trans, the OJ CNC 4 blade 13.7 x 17.5 .110 made it feel like a different boat. It is smooth, quiet, a lot more pulling and holding power, brought my rpm's down by about 200 or so at ski speeds, so between 32-36 the rpm's match my speed with normal boat load and I lost NO top end, in fact, I swear I gained 1 mph over the factory SS 14 x 18 3 blade. So, I have gained a bit of fuel efficiency at my speeds too!

rockcrwlr72
09-07-2012, 04:48 AM
I couldn't agree more, the PO. Just used and it. I will have some pictures to upload in a few days of the tranny rebuild.

Phntmski
09-07-2012, 10:55 AM
Hope it didn't look like I was trying to hijack this thread. I really thought it kinda ran it's course. Thanks for the helpful input.

TRBenj
09-07-2012, 11:34 AM
TR -
So, 1.3" should be fine?
Yessir.

Phntmski
09-19-2012, 01:22 PM
:steering:

OK, to finish up. Finally got the 3 blade on for a ski day. Don't have measurements in front of me but everything was fine. Measured to rudder, hull and strut.
The 3 blade is still a tiny bit noisy compared to the 4 but still pretty smooth. Both handle very well and wake characteristics seem the same. That 4 blade is like a hot knife cutting through soft butter. From hole shot to WOT. I mean 0 vibration. What did surprise me is that Stargazer baselines are virtually the same. 4 blade is actually 5-10 rpm less across the board.
Love both props but the 4 is gonna be my primary for sure.

kjohnson
09-19-2012, 02:45 PM
I had a new strut bearing installed and still got prop cavitation on a stainless steel OJ prop. I sent it to a repair shop and they buffed it out. Still don't know why I was getting it though.

Table Rocker
09-19-2012, 08:50 PM
As to your corrosion, Oklahoma has lots of salt/saltwater. I have seen trailers that looked like they were from the coast. Shovels or implements left in the ground will rust like you cannot believe. My parents lived in Stillwater and had a well dug for irrigation. The water was so salty it couldn't be used even for irrigation - Boron salt. It tasted like ocean water. I know you wouldn't expect a landlocked state to be so salty, but Utah has a pretty salty lake as well.

On the cavitation burn, I had the same thing last year. I had one blade on a three blade that was bent so little you wouldn't notice it. At some rpm's I could feel a vibration, at others it was smooth. I sent the rudder and prop to Eric @ OJ's last spring and had no issues this year.

You had a pretty good ding in your prop. I'm glad you are getting it all sorted out.