Rudder erosion - common causes?

Assuming that electrolysis is ruled out, what would be the most common causes of leading edge rudder erosion in a typical inboard engine installation. In this case, both port/stbd rudders are offset a few inches outboard from shaft centerline and have significant clearance aft of propeller. Both prop and rudder are within cylindrical-shaped prop tunnels with less then 15% tip clearance. Any comments / suggestions would be most appreciated.

 

can you post a pic of rudder this may help

 

Pictures would help indeed.
Do you frequently run in shallow or sandy water? Have you observed similar erosion on the props? If not, are the rudders made of a weaker material than the props? Apart from good ole' corrosion, the first thing that comes to mind is that it's being bombarded by sand and gravel churned up by the props, but one would expect erosion on the props as well if this is true.

 

Rudder erosion - common causes

Turbulance is also a possible problem...though usually over a longish period. One solution might be to fix a steel leading edge (or wrap around sheath) to the rudder. But try not to alter the profile.

 

What are they made of?
Equally, what are the props, shafts, struts and rudder posts made of?

You may or may not have a problem with stray current, but the most likely explanation is dissimilar metals playing nasty buggers

 

This is likley to be cavitation damage. It can occure quite far back if its bad.

How far are the props from the rudder.

Are the prop edges clean and undamaged --At all.

It is quite likely the damage will favour the side of propellor rotation. Ie right had rotation will engourage cavitation damage slightly to the left of the rudders leading edge

 

Rudder erosion - probable causes - feed back support

Thank you all for your responses. Please see attached photos for review. The following is a spec of drive line equipment material that I believe to be correct-

Rudder: Cast manganese bronze
Shaft: Aqualoy 22 (WBM)
Strut: Manganese bronze
Prop: Nickel-aluminum bronze

While I do not have an accurate dimension between prop and rudder, the photos provide an approximation of this distance. There is a high probability that this vessel has traveled through shallow, sandy areas. Once again, your input is greatly appreciated.

 

This looks like a new build, very little antifoul and none on the prop nuts. I still think this is cavitation.

Why the 5 blader, what is your Hp length speed tonnage etc

 

Yep - I've got my doubts about 'sand-blasting': surely you'd see at least some sort of damage to the props as well if that were the cause.
It's an unsual one, that's for sure - it doesn't look like the sort of pitting that is typical of dissimilar metal / electrolysis corrosion. I don't have much experience with rudder cavitation based corrosion... Jack may be right with this one.
Another possibility is that when the rudders were cast it wasn't done properly - the metal can coolinto a sort of layered, sometimes crystaline, sort of thing.
Have you had the content of the rudders tested to see if they are in fact made the manganese bronze that they're supposed to be?

 

Corrosion? No way. Does normal cast manganese bronze usually have all those layers? Looks like someone got to them with a rotary grinder. How could any decent metal wear away like that? Seems impossible when the hull and props appear untouched. Something has to be fishy about those rudders.

Since they are history anyway, how about banging them with a hammer to see what happens.

 

I just don't see how cavitation bubbles could damage a rudder that far aft of the prop, not without some serious erosion on the propeller itself too. Sand/silt would damage the prop too. Electrolysis would destroy them evenly; if anything it would be worst on the sharp edges, and it would leave oxide residues.
In short, I don't have a clue what the hell happened to these rudders. My best guess at present is that when the bronze (if that's what it is) was originally formed, it was done in a manner so shoddy that the layers just flaked away.

 

take a wire brush to the rudders and see if it is deep enough and or if it penitraits the rudder matrial

 

I agree that something seems to be wrong with the rudder material. Are they bronze?

I thought they were fibreglass exposing layers of the build up.

There doesnt need to be prop damage from cavitation as it is the prop that is engouraging the cavitation.

I am concerend that the props do not have enough blade area for the HP of the boat ,especially if its heavy.

As I mentioned that this looks like a new build, It could be the wrong choice of props.

 

I notice a discoloration in the tunnal,,looks similar to the rudders exept not as pronounced,,,could be bad paint?