Is this galvanic corrosion?

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MYAG

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Just had the boat lifted for a survey prior to completing the sale. The only issue the surveyor picked up was the props suffering from a degree of "galvanic corrosion."

I was very surprised, the boat is 3 yrs old and last year there were no signs of any corrosion at all. I cleaned and polished the props myself last June, so I am sure of this. The circuitry is in good nick and the anodes were changed at the same time.

Do the pot marks in the pic look like they are caused by GC?
Can this happen in just 10 months?


I will be bringing the new boat to the same berth next year so any ideas on any extra steps I can take to prevent this happening (if it is GC) in the future would be helpful.

Cheers.
 
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Looks like GC however if these are Volvo Props and the boat is around the 2005 year there was a degree of bad props manufactured at that time due to a bad bronze mix. I had the props replaced on a 2005 boat on that basis FOC. Maybe too late to chase a claim now.

They all seem to do it to some degree and it dosn't look too bad to me. Could be easily fixed by prop speed for little cost.
 
I agree. It looks like GC to me as well. If it was cavitation, it would tend to be on the aft of the props and close to the hub.

Were the anodes you fitted worn? If not, something's up with the anode's wiring.
 
Bit of both ?

Its difficult to see from the photos.

From the photo the pitting at the root of the blade looks if its on the "Back" of the blade and this would be consistent with cavitation. How many hours have the props done?

In photo 1 the whole blade area looks a bit scabby and pink but difficult to see in a photo.

Has the prop nut if it is bronze and not an anode changed colour? It often goes pink first.

If the prop is scabby, scales or going pink it could be galvanic action.

What condition were the anodes in. Was the one nearer the port prop more or less worn than the starb? Have you done a continuity test?

If the anodes are less tahn 60% eroded away you are getting too little protection.
 
oceandrive said:
Looks like GC however if these are Volvo Props and the boat is around the 2005 year there was a degree of bad props manufactured at that time due to a bad bronze mix. I had the props replaced on a 2005 boat on that basis FOC. Maybe too late to chase a claim now.

They all seem to do it to some degree and it dosn't look too bad to me. Could be easily fixed by prop speed for little cost.
Click to expand...

Thanks, that is good to know bit my props are originals from CJR propulsion. I have now had them taken off and sent to Istanbul to be built up and balanced.
 
Bandit said:
Its difficult to see from the photos.

From the photo the pitting at the root of the blade looks if its on the "Back" of the blade and this would be consistent with cavitation. How many hours have the props done?

In photo 1 the whole blade area looks a bit scabby and pink but difficult to see in a photo.

Has the prop nut if it is bronze and not an anode changed colour? It often goes pink first.

If the prop is scabby, scales or going pink it could be galvanic action.

What condition were the anodes in. Was the one nearer the port prop more or less worn than the starb? Have you done a continuity test?

If the anodes are less tahn 60% eroded away you are getting too little protection.
Click to expand...


The pitting is actually on the leading face of the blade, although that may be difficult to see on the photo.

The blade securing nut seems fine and the anodes were worn about 50-60% on both sides equally which is the same consistent rate of wear over the past 3 years. The continuity test showed a very low resistance across the circuit, so that is fine too.

From what the posts have suggested, it seems that this is probably GC. If anyone has further suggestions on how I can improve on the standard protection that comes with most boats, (as this is clearly not enough) that would be great.

Cheers!
 
Bandit said:
Its difficult to see from the photos.

From the photo the pitting at the root of the blade looks if its on the "Back" of the blade and this would be consistent with cavitation. How many hours have the props done?

In photo 1 the whole blade area looks a bit scabby and pink but difficult to see in a photo.

Has the prop nut if it is bronze and not an anode changed colour? It often goes pink first.

If the prop is scabby, scales or going pink it could be galvanic action.

What condition were the anodes in. Was the one nearer the port prop more or less worn than the starb? Have you done a continuity test?

If the anodes are less tahn 60% eroded away you are getting too little protection.
Click to expand...

Props have done 260 hrs.
 
Which prop in Photo?

It looks to be the port prop in the photo, P dracket on left side and prop nut on right side with the contre blue keel line in the background?

Is it the port prop?
 
Bandit said:
It looks to be the port prop in the photo, P dracket on left side and prop nut on right side with the contre blue keel line in the background?

Is it the port prop?
Click to expand...

Yes, the port prop is the culprit, the stbd prop is fine.
 
MYAG said:
No rope cutters fitted. Why do you ask?
Click to expand...

I have seen similar cavitation marks on propellers where the rotating blade has not been correctly aligned with the 'leading' edge of the propeller blade, upsetting the flow of water at the blade root. It was on a high HP boat.
But thats just one possibility, I'm not claiming thats the only cause, of course.

My experience of galvanic corrosion is pitting over the whole area of the blades, and in particular at the tips.
 
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Had a similar issue on a 2004 boat. If they are not too bad, they can be welded up, pitched and balanced to look and work like new Cost is a fraction of replacement price. Hamble props who did the work, said they thought it was GC - these had only done 290hrs and there were no rope cutters.
 
photo

We are looking at the port prop.

The corrosion is at the root of the back of the blade. Ie the other face of the blade is the leading, front side.

Port prop, looking from behind the prop is turning anticlockwise.

I think it is root cavitation. It is not major problem at the moment , it needs filling, just an annoyance.

Speak to boat and propeller manuafacturer and get a spare pair of props for your stock.

Unless thay have a magic cure it will continue.

Cavitation is exacerbated by speed, rough weather.
 
Thanks Bandit, what you say does make sense and there is no other corrosion anywhere else in the vicinity, just in this localised spot. As you rightly point out, it is on the trailing face of the blade at the root.

I have asked the prop specialist to diagnose the reason for the pitting, should hear back tomorrow.
 
We used to have similar problems on hydrofoil propellers 35 knots service speed , we also ran them with two holes drilled through the blade to equalise pressure at root which helped but did not cure problem.

What speed are you running at ?
 
Roughly:

Choppy conditions 16-18 knots 20% of the time
Slow cruise 23-25 knots 30% of the time.
Fast cruise 28-30 knots 30% of the time.
Occassional full power, 34 knots, no more than 10 minutes at a time.
Remainder crawling in and out of marinas and anchorages.
 
galvanic corrosion

i'm not sure that this could be galvanic corrosion. galvanic corrosion need two different material in direct contact and usually the corrosion is located in the area of contact.. also the anode will give some protection from GC
from the photos is very difficult to define the causes of corrosion.. it will be necessary to have closer photos and an analysis of the part..
if the pits are very deep and narrows i'll exclude cavitation..

Vitt

corrosion protection
 
Prop can be electrically bonded to any number of other metals, through the shore cable. Over protection on adjacent boats or jetty piles can have an effect on this boat. It could be a boat to port that is causing a problem
 
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