Prop burn on rudder?

H

Hydraulicjump

Well-known member
Joined
Feb 10, 2011
Messages
646
Fluid Motion Model
C-30 CB
Hull Identification Number
FMLT2911F415
Non-Fluid Motion Model
Necky Looksha VII, Liquidlogic Remix, Jackson 4Fun
Vessel Name
La Barka (2015)
After the Rendezvous (thank you Ranger Tugs!) we cruised for a bit in perfect PNW weather. I cannot over-hype how beautiful the San Juan Islands were this late summer/early fall, especially because the turoids (tourists) were gone. Sadly, we hauled the boat this past Thursday and towed it home to Northern California, putting it back in San Francisco Bay today.

Before putting it back in the Bay I replaced all the anodes (I use aluminum). The prop anode was deeply pitted and jagged. Unusually so, for some reason. After replacing the anodes I used 220 wet/dry sand paper to clean up the prop and the rudder, removing the encrusting bryozoan and barnacle markings and making everything shiny. This is when I noted pitting on the rudder that I had not seen before. On the starboard side of the rudder it is about a third of the way from the top. On the port, about a third of the way from the bottom. This erosion is not deep at this point, but it is the first that I have noticed it. If it continues at this pace I might have a problem.

A hypothesis for the brain trust. Could a deeply pitted prop anode generate cavitation bubbles that might damage the rudder? I have replaced the anode and cleaned everything up (the boat was very spry on the Bay today, with WOT at 4050; about 100 rpm higher than before cleaning things up and replacing the anode).

I'll haul the boat in a few months and check again, but wonder whether this is something to worry about or whether this is something that takes many years to manifest as a problem. That is, long after we have purchased a 29cb....

Jeff
 
I suspect electrolysis over cavitation.
Cavitation would occur at the prop blade tips and is unlikely on the rudder.
Pull the boat and shine the rudder and prop blades with 120 grit and spray them with cold galvanizing compound (Home Depot, Lowes, hardware, etc.) Multiple coats.
 
I don't think spraying galvanizing compound on a bronze rudder or prop is a good idea. They are quite dissimilar metals, and I imagine you would quite quickly be looking at some damage from the layers of galvanizing peeling off. Obviously the zinc will erode rapidly and slough off, as it is supposed to do ala zinc anodes attached to the rudder. I would hate to drop the transmission into reverse and suck a sheet of peeled-off galvanizing compound into the prop.

American Galvanizers Association lists galvanizing over bronze in seawater as "Zinc and galvanized steel may suffer severe additional corrosion and the contact should be avoided." http://www.galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-contact-with-other-metals
 
Perhaps - but perhaps like the old song, it ain't necessarily so.
http://www.wholesalemarine.com/pettit-z ... rosol.html
The cold galvanizing spray cans from the hardware store are the same product at 20% of their price :mrgreen:

There is CRC Soft Seal which I have no experience with. Word of mouth says that besides offering protection from corrosion, it repels barnacles, etc. Might be worth a trial.
And there is Volvo Penta Teflon Propeller Antifouling. My understanding is that it is expensive and the anti fouling layer does not adhere well to props.
And there is Prop Speed. Supposedly works. The word is that it will require the selling of your first born child into indentured servitude to pay for it.
And there is http://www.prop-shield.com/index.htm - or their Rigging Guard that might be good for rudders
And there is simply smearing a thin layer of grease on the metal before launch - the commercial fishermen are known for their frugality - though I suspect the slight sheen of oil on the water surrounding your boat at the dock may invite some questions.
And here is a bunch of varying opinions - though I do like the suggestion of Sex Wax (hmmm, maybe .. .. .. nyaa, she'd shoot me) :roll:
 
Fair enough. It does indeed look like that Petit product can be used on bronze. But I just did some quick searching and am unable to find a single "generic" gold galvanizing compound that lists bronze as an approved use. As a matter of fact the only one I could find that mentions anything other than "properly prepared steel" as the approved application is ZRC cold galvanizing compound, which mentions "Apply to properly prepared carbon steel, cast iron, hot-dip galvanized, and aluminum surfaces."

My suspicion is that the Petit product costs 5 times more because they are doing some proprietary magic with their epoxy base that results in better adhesion across a wider range of substrates.
 
Hi all,

After three seasons, I am now convinced that I am dealing with cavitation erosion (also referred to as prop burn) on the rudder of our R27. Here is a photo of the starboard side of the rudder.



We bought the boat only 6 months old in the fall of 2014. In the fall of 2015 we pulled the boat for the season and I noticed the start of the erosion, starboard side only. Unfortunately, the prop zinc was missing and photos from the 2014 haulout showed the prop zinc missing then as well (I believe unrelated but I was not sure at the time). My assumption was galvanic corrosion or electrolysis though I was not convinced as the pattern looked like it was related to the prop wash and was only on the starboard side. The errosion concerned me, a lot, and that winter I added a galvanic isolator, a transom zinc, and a zinc fish connected to the negative batt terminal. Fall 2016 showed signs of additional errosion but 2 months into that season I had lost the prop zinc again and replaced it (again unrelated but I couldn't be sure). I did not know how long it was missing so I still was not sure whether I was dealing with galvanic corrosion, electrolysis, or cavitation erosion. This season I was able to resolve the prop zinc issue. The prop zinc is still there after 6 months and I have plenty of transom zinc left as well as the fish (I'll make a separate post on this). They are all bonded together so should work in tandem. The cavitation erosion appears to continue.

Here is a photo of the port side of the rudder with no indication of erosion.



There are many examples of cavitation erosion on-line but mostly with big commercial ships. Here is just one example.



So...what do you all think? What would you do? I have added a bunch of zinc and don't think more would help though I could add a zinc directly to the rudder. For some reason Ranger Tugs doesn't recommend it. My dilemma is that while cavitation erosion is discussed on-line as a common issue for big ships I don't see much about it for small boats and there is very little about what to do about it. The only product I have found on-line is a glassflake reinforced coating called Ecoshield out of the U.K.

https://subind.net/coating-systems/product/18/ecoshield

I don't even know if it is an option on small boats like ours.

Curt
 
Have you considered perhaps a manufacturing defect from impure or contaminated material of the rudder
 
stwendl":1k3wjkfi said:
Have you considered perhaps a manufacturing defect from impure or contaminated material of the rudder
Click to expand...

No, I had not considered that. Not sure how to prove it and given the age of the boat it's well out of warranty anyway. I will be contacting the factory for advice as well though.

Thanks for the idea.
 
knotflying":o12v9yyl said:
I have been using cold Galvanized paint on all my running gear for 6 years now. No issues.
Click to expand...

Me too. The grey part of the rudder you see is cold galvanizing. Worked great to keep the barnacles off. The cavitation erosion however went right through it in that area!
 
I never noticed the prop cavitation last year. If I had it it was very minimal. This year I painted all the underwater hardware including the rudder. I applied an etching prime, Pettit metal primer and Vivid bottom paint to all metal hardware. When I pulled the boat for the season, 134hrs running time, and inspected the bottom paint to see how it held up I noticed the paint was gone ,bare metal on the top and bottom of the rudder. The cavitation wore the paint away and polished the metal. I didn't notice any pitting. I also believe it was on the Port side of the rudder. My rudder is slightly offset to the starboard for shaft removal. I thought possibly that is why it had some cavitation. The tunnel may have an effect too. I will take a couple of photos this week and post. My prop looks exactly like the example that Red Raven posted.
Brian Brown
26 Cutwater
PORT-A-GEE
 
Hi Brian,

Looking forward with interest to your photos. This is an odd (but interesting) one. Sounds like your propeller may have the opposite spin.

Curt
 
Curt, The erosion/cavitation may be on the starboard side of the rudder but my memory says Port side. (My memory tends to be wrong more often these days :shock: ) I remember looking at the rudder offset is to starboard. My rotation is right hand (std).The example photo that you posted is a left hand (opposite) with rudder offset to port and erosion/cavitation on starboard side. I'm working on the boat tomorrow and will post pics.
Brian Brown
Cutwater26
PORT-A-GEE
 
Hi Brian,

You are correct that the rudder offset is to the starboard and the propeller is a right hand rotation so our boats are the same. My mistake. My erosion/corrosion Is definitely on the starboard side which is counterintuitive.

Note however that on my boat the corrosion is most severe at the top of the rudder on the starboard side and while there is no severe erosion there is galvanizing removed on the bottom end of the rudder port side. This is consistent with the example photo in that the erosion is from a left hand propeller and on the bottom end of the starboard side of the rudder. Below is are another photo from the same example boat of the starboard rudder with a right handed starboard propeller with erosion at the top of the rudder and a rudder offset to starboard.



Curt
 
Curt,
I have the same pattern that you described. I noticed it on the Portside of the rudder and that stuck in my head.I can not see the pitting that you have but I don't have near the hours of run time that I assume you have. I am going to strip the paint off the rudder and apply the bearer coat epoxy that I used on my hull. I think it is a harder finish then the Pettit metal primer I used. I have only been in fresh water with the boat. Next year it will be used in salt also. Your erosion and pitting is in the same area as my mine. I don't have the pitting is that the difference of fresh and salt water or more hours of run time? I don't know. Bottom line find a way to prevent it.
Brian Brown
26 Cutwater
PORT-A-GEE


 
BB marine":fhg2feqt said:
Curt,
I have the same pattern that you described. I noticed it on the Portside of the rudder and that stuck in my head.I can not see the pitting that you have but I don't have near the hours of run time that I assume you have. I am going to strip the paint off the rudder and apply the bearer coat epoxy that I used on my hull. I think it is a harder finish then the Pettit metal primer I used. I have only been in fresh water with the boat. Next year it will be used in salt also. Your erosion and pitting is in the same area as my mine. I don't have the pitting is that the difference of fresh and salt water or more hours of run time? I don't know. Bottom line find a way to prevent it.
Click to expand...

Brian, I have 515 hours on the boat, all in salt water. We run a majority of the time at 3500rpm. That may also make a difference. What epoxy barrier coat do you use?

Curt
 
Pettit Protect High Build Epoxy Primer. I put three coats of this on the hull and then Vivid bottom paint. I used Pettit
6455/044 Metal Primer on all the under water metal along with Vivid. It held up very good. The High build primer is very hard and I think three coats and the bottom paint it may not wear thru on the rudder. I have a question, 3500 Rpm, close to 90%? Do you have a Yanmar? My boat runs great at 3500 (D3volvo) but I'm always reluctant to run it that high for any length of time. But I would like to. I run at 3200 but 3500 is the sweet spot same fuel economy has 3200 but 5mph faster.
Brian Brown
26 Cutwater
PORT-A-GEE
 
Yes, 3500rpm or near 90% nearly all the time. It really is a sweet spot for our D3-200, the noise is not bad at all and the boat rides up on the step well with good fuel mileage (2 mpg) and speed (15kts). It is a very comfortable ride. The Volvo guys at the seminar and my RT on the water captain/instructor said not only is it OK but preferable. Above 90% the noise becomes more of an issue and it is not recommended in the manual or by the Volvo guys at the seminars. The rudder, however, may not like it! :?
 
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