Prop burn on rudder?

[BB marine]

Good to Know. When we are cruising for fuel range and economy 1950rpm. When we are in open water moderate sea's 3200 2 mpg, but 3500 gets the boat up and it rides better and like I said faster 2mpg. I never got an answer if above 80% was ok. I did read in the operating manual do not operate above 90% for extended time. 3500 may be my new number a little more often.
Brian Brown
26 Cutwater
PORT-A-GEE

 

[Red Raven]

This is a follow up regarding my rudder cavitation erosion adventure discussed above.

Since the last post...

I contacted Ranger Tugs. They said they had only seen this once before. They contacted the rudder manufacturer (marine Hardware) on my behalf. The rudder manufacturer said it was indeed cavitation erosion and not galvanic or electrolytic corrosion. Neither RT nor Marine Hardware could say what was different about my boat to cause this. We were all stumped. RT suggested having the propeller checked. The propeller manufacturer looked at detailed photos of the propeller and said it was fine and not even worth testing. I went ahead and purchased a new rudder. I don’t want to put it on the boat until I find a solution or I will just use up another rudder.

I contacted SubSea Industries the makers of EcoShield in the Netherlands (see article linked to above). This product is used on large container ships to protect against cavitation erosion. It is a glass fiber impregnated epoxy. Unfortunately it is not stocked in the US and the shipping cost they gave me was nearly as much as a new rudder!

I chose to attempt my own repair using West System epoxy impregnated with micro fibers. Based on everything I had learned from the EcoShield folks I thought this was worth a shot and cost almost nothing as I had the epoxy materials already. Since I alrapeady have a new rudder as a backup I thought it would be interesting to try and couldn’t hurt. I reamed out the cavities and roughed up the surface with a grinding bit on a dremel tool first like a dentist would do with a tooth. I then painted the surface with epoxy without filler for increased bonding strength then filled with epoxy impregnated with micro fibers with a consistency of peanut butter. After curing I sanded it all down and coat with a final coat of epoxy without filler. I then painted it all with cold galvanizing paint for antifouling and to match the rest of the rudder.

The photos below show the before and after. I won’t know how well it actually works until after next season. I will post pictures in the spring.

Curt

Before:


After epoxy and before paint:


After painting with cold galvanizing:

 

[Automaticman865]

I've been having some good luck using the blue water marine barrier paint, its grey in color. I put at leat 2 coats on the rudder and the prop. Then when dry I put on 2 coats of regular bottom pain on top of it and usually black. So as time progresses you get to see a color change if things are getting removed too fast. Have the same, trim zincs, prop zinc, thruster zincs, and a diver down hull zinc bonded to the ground terminals.

Stuart

 

[BB marine]

Looks Good ! I hope I don't get the same cavitation burn on the rudder next year. I'm going to try picking my cruise speed up to 3500 Rpm. Do you think the continuous extra RPM is causing that ?
Brian Brown
Cutwater 26
PORT-A-GEE

 

[knotflying]

Looks good. It will be interesting to see how it holds up. We are staying tuned in!

 

[Red Raven]

Looks Good ! I hope I don't get the same cavitation burn on the rudder next year. I'm going to try picking my cruise speed up to 3500 Rpm. Do you think the continuous extra RPM is causing that ?
Brian Brown
Cutwater 26
PORT-A-GEE

I don’t think so but can’t rule it out. Acme, the prop manufacturer said they have never seen it on boats that run much faster than ours. Hardware Marine did suggest to try a different rpm cruise speed as there may be some sort of resonance going on. I think it would be hard know without doing more controlled testing.

Curt

 

[Hydraulicjump]

Since I started this thread so very long ago, I thought I would weigh in. In a way, I was "pleased" to see that cavitation is the likely cause of the pitting on the rudder. It was my working hypothesis when I saw this on our R-27. But I could find nothing on the prop that I would blame for excessive cavitation. I did the same procedure of cleaning everything up with sand paper and a dremel tool to catch any nicks or irregularities. But I never ruled out one other possible source for the problem. I kept my 27 in a hot marina that burned through prop anodes pretty fast, even with a galvanic isolator on the boat. What was so striking about the anodes is that they did not erode evenly. I often had "horns" develop on them due to irregular wear. Just the kind of shape that at high RPM's (I cruised at 3600) would throw bubbles that would then pop on the rudder, causing cavitation.

I never ruled this potential cause out, but if you notice your latest post, you have "horns" on the eroded anode in your before picture.

I can think of lots of reasons this is probably wrong: (1) flow separation around the shaft area of the prop puts the anode in a relatively low velocity location, making it less likely to throw cavitation bubbles and (2) the erosion of the rudder is more in line with bubbles being generated at the tip of the prop blades). Still, worth some idle speculation.

Good luck with your fix. BTW, I have not seen any indication of this problem on my R-29CB and was looking for it right away.

 

[Red Raven]

Since I started this thread so very long ago, I thought I would weigh in. In a way, I was "pleased" to see that cavitation is the likely cause of the pitting on the rudder. It was my working hypothesis when I saw this on our R-27. But I could find nothing on the prop that I would blame for excessive cavitation. I did the same procedure of cleaning everything up with sand paper and a dremel tool to catch any nicks or irregularities. But I never ruled out one other possible source for the problem. I kept my 27 in a hot marina that burned through prop anodes pretty fast, even with a galvanic isolator on the boat. What was so striking about the anodes is that they did not erode evenly. I often had "horns" develop on them due to irregular wear. Just the kind of shape that at high RPM's (I cruised at 3600) would throw bubbles that would then pop on the rudder, causing cavitation.

I never ruled this potential cause out, but if you notice your latest post, you have "horns" on the eroded anode in your before picture.

I can think of lots of reasons this is probably wrong: (1) flow separation around the shaft area of the prop puts the anode in a relatively low velocity location, making it less likely to throw cavitation bubbles and (2) the erosion of the rudder is more in line with bubbles being generated at the tip of the prop blades). Still, worth some idle speculation.

Good luck with your fix. BTW, I have not seen any indication of this problem on my R-29CB and was looking for it right away.

Yes, I also have had the same thought. Acme, actually immediately focused on the large fender washer that RT supplies with the anode thinking that it was creating increased turbulence. When I stated that as far as I know all RT boats are provided with that washer they dropped that angle. Another point against the anode besides the two you make is that the erosion has been occurring at roughly the same rate each year but the anode has different wear each year and has even been gone for part of 3 out of 4 years. Regarding the fender washer it is only exposed to the flow after the anode significantly degrades and it has also been missing for part of each season (except the last). Having said this, you will note that the fender washer is missing in the last photo. I have gone back to the cap bolt and toothed washer that the anode manufacturer supplies with the anode.

On a side note I think the “horns” and other uneven wear on the anodes is caused by surface contaminants on the anode that slow the material sacrifice in those areas. In one instance There was a small piece of scotch tape left on the anode creating a large horn like the one you point out. In other cases I have seen it from a small piece of the price label or even a drop of paint or marking pen.

Curt

 

[Red Raven]

Here is are the performance results of my rudder cavitation erosion epoxy repair. Overall I am pretty please how well it worked!

Before the repair.


After the epoxy repair but before anti-fouling paint.


After 6 months of use in salt water.


After Cleaning.


Some loss of epoxy and some additional erosion but overall good results. I plan to fill cavities again this year before next season. I should be able to get several additional years out of this rudder with this approach! Still no answer as to why I get this erosion on this particular boat. I am still convinced this is cavitation erosion and not galvanic or electrolytic corrosion.

Curt

 

[stwendl]

It may have been mentioned before but compare with other boats in same area and same usage. If there is a significant difference assume a casting with impure Metall und that gets washed out

 

[Red Raven]

It may have been mentioned before but compare with other boats in same area and same usage. If there is a significant difference assume a casting with impure Metal that gets washed out

Yeah, could be but hard to prove without a lot of expense. At this point I am working with what I have. I have a new rudder from the manufacturer but am saving it for after I have learned all I can from this one. I don’t want to burn up another one. I think it is interesting that the epoxy is more resilient than the original bronze.

 

[BB marine]

I have exact wear on my rudder but it has not pitted. I normally don't run above 3250 rpm. I know you have stated running at higher rpms for extended time. We are cruising a 650 mile trip and I have increased my cruising rpm to 3380 to 3400 running for 6 to 7 hours a day. I will post pics of the rudder and see if there is any significant change at the end of our trip. I do paint the rudder with epoxy paint and anti fouling each year. When I hauled the boat out this fall to tow to Knoxville all the paint was gone and the rudder was polished looking ( fresh water ). I truly believe it is cavitation.

 

[Scottfree]

I have a 2010 R29 And have owned the boat nearly 6 years. For the most part it's a wet boat in the Pacific. My annual routine is to pull the boat and redo zincs and bottom paint. I have drilled the rudder and added a 3 inch round zinc about 2/3 up from the bottom of the rudder. I use a 4 inch grinder with a wire brush and polish/clean both the rudder and the prop, install prop zinc and the added rudder zinc. I then mask the zincs and spray both the rudder and prop with a high zinc content prop spray. I spray multiple coats until the spray can is empty. I have had no pitting or wear or growth on either. Something else to check is the grounding wire connection on the shaft bearing at the adjusting nut. Always green corrosion. I disassemble, sand clean all surfaces and reassemble. I then spray a battery terminal corrosion protection on the assemble. Good for 2-3 years.


Good luck, Leon

 

[crowleykirk]

Check and make sure all your grounding it hooked up and not corroded.
Looks like serious corrosion to me. I don't see that being a cavitation issues unless the material used for the rudder is defective. Maybe look into a replacement?
The rudder needs and should have zincs. It looks like it is being attacked for sure. Where do you keep your boat?
If the marina is hot, usually due to live abroad folks, ( no offense) your zincs will corrode faster, possible electricity in water. Warm water also eats zincs faster.

 

[Chimo]

Hi Curt

As usual you have posted an interesting topic. Just for interest do you know the ratio of your reverse gear and the diameter and pitch of your prop?

 

[Red Raven]

Hi Curt

As usual you have posted an interesting topic. Just for interest do you know the ratio of your reverse gear and the diameter and pitch of your prop?

Hi Chimo,

Reverse gear is 2.03. Prop is 17x17. Standard for the R27 Classic.

Curt

 

[Red Raven]

Hi Scottfree and Crowleykirk,

Yes, galvanic corrosion was my original thought years ago. The bonding on the boat has been thoroughly checked. I have added a transom zinc to the boat that is bonded to the system including rudder and have used a zinc “fish” tied to the bonding system as well. I have added a galvanic isolator to deal with any hot marina issues. None of these improvements have had any impact on the rate of errosion on the rudder. I have purchased a silver-silver chloride reference which is the gold standard for determining effective galvanic protection. It measures -0.968 volts at the rudder (exactly the same as as all other underwater metal) which is in the recommended range. No change in reading whether connected to the dock or not. The prop and other underwater metal show no sign of corrosion. This leaves only cavitation errosion as the most likely suspect.

If the issue were galvanic or electrical it would be general across the whole rudder. The errosion pattern appears to be specific to the prop wash pattern. Cavitation errosion is a known issue in the industry though somewhat rare (you can look it up). There clearly, is something different, no matter how slight, in my setup though it is very difficult to determine what it is. The photos match up nearly exactly to the cavitation errosion photos found for the industry (see photo in link earlier in this thread). Just to make abosolutely sure it is not galvanic I will add a rudder zinc directly to the rudder this year.

This issue has been thoroughly reviewed by Fluid Motion, Marine Hardware (rudder manufacturer), Volvo, and ACME Propeller. No root cause has been proven. Fluid motion and Marine Hardware have acknowledged it is likely cavitation erosion and helped by providing a new rudder at a discount even though the boat is out of warranty (Thank You!) There is a European company that specializes in cavitation errosion epoxy products. They also confirmed this is classic cavitation erosion. I have chosen to try a similar epoxy method to see if I can protect from further errosion. This latest post was to publish the results of the epoxy repair. See thread details above if you are interested.

 

[BB marine]

Curt,
We pulled the boat on Saturday after a 3 week trip of close to 700 miles. We logged just over 60 hrs of run time @ average rpm of 3380. There was a small % of WOT and a few hours of 3600rpm. I normally don't run that high of RPM but we wanted to get some miles under the keel in a short period of time. I was interested to see if there was any cavitation burn similar to what you experienced with your boat. The answer is no. I do have the same areas of paint wear and the rudder is actually polished in those areas. The paint wear mirrors your rudder. I agree it is cavitation. I tend to think that the casting on your rudder may be on the marginal side for hardness. ( just opinion ). The epoxy filler you applied seems to be harder then manganese bronze material that the rudder casting is made of.

 

[Red Raven]

Hi Brian,

Thanks for posting your results. I agree it could be a weakness in the bronze as there doesn’t seem to be anything unique in my setup. I could answer that question for sure if I was willing to install the new rudder without any special treatment and run it for a season. However, I am inclined to keep the new rudder as a backup until the old is not serviceable. When I do install it I plan to give it an epoxy coating in that area as I don’t want to take the gamble that the problem is specific to the old rudder.

I don’t think the epoxy is actually “harder” than the bronze, just more forgiving and resistant to the type of percussion forces going on. Kind of like masking for sand blasting.

Curt

 

[bakerboat]

I'm seeing the same issue on our 2014 R27. No significant pitting yet so my plan is to place some fiberglass cloth (~5.0 oz.) bedded in some 'thickened' epoxy (West System G-Flex) over the offending areas and then AF paint. We'll see how that works out.