John_N
Well-Known Member
The bronze propellor on my Sadler 32 has acquired a pink tinge. Is this bad news and if so, how serious?
Prop deterioration?
- Thread starter John_N
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KRG
Well-Known Member
This is caused by dezinctification, have you got enough anodes? It can be serious if not rectified, it'll weaken it. Those 32's are great boats. I had a deep fin version.
Brad
Brad
paulrossall
Well-Known Member
Agree with BradC. If pink, prop might be past point of no return. I had same problem and have now fixed a suitable anode. I also found my rudder step (stainless steel) had suffered galvanic action. If you take the prop off and knock the edge with a spanner it should ring. If it gives a dull thud then you need a new prop. Cost should be less than £150. IMHO. Paul
Malish
Well-Known Member
Any metallurgists out there?
I've always had a bit of a difficulty with the argument that a pink tinge on the surface means that a bronze item is on its way out. After all one is only inspecting the surface layer not the interior - how far into the item has de-zincification penetrated?. The tap with a spanner test seems valid but again I would be interested to hear the opinion of a metallurgist.
I've always had a bit of a difficulty with the argument that a pink tinge on the surface means that a bronze item is on its way out. After all one is only inspecting the surface layer not the interior - how far into the item has de-zincification penetrated?. The tap with a spanner test seems valid but again I would be interested to hear the opinion of a metallurgist.
Metabarca
Well-Known Member
Well, let's put it this way: I had a new prop put on. When I set out over the horizon, one of the blades broke off. When I removed the prop, it was pink because of dezincification. The mechanic put on a new prop and a much bigger anode. I've since sold the boat...!
dickh
Well-Known Member
Sounds like dezintification, but if prop 'rings' when hit it should be OK for a while. Add a zinc anode to the propshaft, but make sure it can 'see' the prop - not too close to the 'P' bracket.
Wayward_Son
Well-Known Member
Why is my bronze propeller turning pink ?
The pink colouring is caused by galvanic corrosion / electrolisys which causes loss of zinc in the material. The pink colour is copper. The problem with the propeller is not repairable and as it becomes more advanced pieces will start to break of the blades, it should therefore be replaced.
You can minimise the risk and effect of this corrosion by doing the following:-
1) Check that all your boat electrics are of insulated return and not simply returned through the engine or running gear. This includes items on the engine such as oil pressure senders etc.
2) Provide and use an isolating switch in the main battery circuit.
3) Fittings such as sockets, bilge detection devices etc. must all be protected from water ingress.
4) Avoid overloading any one cable by looping several items back to it. This causes overheating and insulation breakdown.
5) Make sure cables are fixed correctly to avoid sagging and possible insulation fracture.
ALL THE ABOVE SHOULD REDUCE THE CHANCE OF CURRENT LEAKAGE.
That's the first part of the problem. Dissimilar metals in salt water create their own current so you must fit a zinc anode close to the propeller which is connected internally through the running gear to the propeller shaft in such a way that you can get a circuit between the anode and the prop. The corrosion will then take place on the anode rather than the propeller. The anode should be changed annually.
All the above is the best you can do without going to the expense of electronic cathodic protection which is another story.
The above information was gathered from Castlemarine
The pink colouring is caused by galvanic corrosion / electrolisys which causes loss of zinc in the material. The pink colour is copper. The problem with the propeller is not repairable and as it becomes more advanced pieces will start to break of the blades, it should therefore be replaced.
You can minimise the risk and effect of this corrosion by doing the following:-
1) Check that all your boat electrics are of insulated return and not simply returned through the engine or running gear. This includes items on the engine such as oil pressure senders etc.
2) Provide and use an isolating switch in the main battery circuit.
3) Fittings such as sockets, bilge detection devices etc. must all be protected from water ingress.
4) Avoid overloading any one cable by looping several items back to it. This causes overheating and insulation breakdown.
5) Make sure cables are fixed correctly to avoid sagging and possible insulation fracture.
ALL THE ABOVE SHOULD REDUCE THE CHANCE OF CURRENT LEAKAGE.
That's the first part of the problem. Dissimilar metals in salt water create their own current so you must fit a zinc anode close to the propeller which is connected internally through the running gear to the propeller shaft in such a way that you can get a circuit between the anode and the prop. The corrosion will then take place on the anode rather than the propeller. The anode should be changed annually.
All the above is the best you can do without going to the expense of electronic cathodic protection which is another story.
The above information was gathered from Castlemarine
John_N
Well-Known Member
Many thanks for all your informative comments. They confirm my understanding of the situation. However I am puzzled why this appears to be a recent development as I have made no relevant changes to the boat over recent years.
The boat is 23 years old but the prop is only 8. I changed the original 2 bladed folding for a 3 bladed fixed shortly after buying the boat. The prop does ring but not as clearly as a church bell. Ther is no sign of pitting or other damage. I thought I had enough anodic protection - a large anode on the iron keel, a pencil anode in the engine block, and a Mc duff anode on the shaft close to the prop. I inspect them annually and change as required (the engine anode sometimes requires an additional change mid-season). They all seem to decay fairly rapidly so I assume that they are doing their job.
Perhaps I am being over-anxious. As I restrict mysailing to coastal passages nowadays I think I'll just keep an eye on things and let nature take its course.
The boat is 23 years old but the prop is only 8. I changed the original 2 bladed folding for a 3 bladed fixed shortly after buying the boat. The prop does ring but not as clearly as a church bell. Ther is no sign of pitting or other damage. I thought I had enough anodic protection - a large anode on the iron keel, a pencil anode in the engine block, and a Mc duff anode on the shaft close to the prop. I inspect them annually and change as required (the engine anode sometimes requires an additional change mid-season). They all seem to decay fairly rapidly so I assume that they are doing their job.
Perhaps I am being over-anxious. As I restrict mysailing to coastal passages nowadays I think I'll just keep an eye on things and let nature take its course.
VicS
Well-Known Member
Although all Wayward_Son's advice is sound advice it is difficult to see how short comings in the internal wiring are causing the problem unless something pretty obscure is making the prop slightly negative compared with say the keel.
I would check that the shaft anode is secure and in good electrical contact with the shaft. Corrosion between the shaft and the anode could prevent that. Even then many boats have no shaft anode and do not suffer dezincification of the prop.
According to your profile you are based on the Dart but are you in a marina and if so do you have shorepower connected. If you do perhaps you should be looking at fitting a galvanic isolator.
A point in passing. You refer to your bronze propellor. Manganese bronze is the usual alloy for props and that is really a brass ie a copper-zinc alloy. Strictly bronzes are copper-tin alloys
I would check that the shaft anode is secure and in good electrical contact with the shaft. Corrosion between the shaft and the anode could prevent that. Even then many boats have no shaft anode and do not suffer dezincification of the prop.
According to your profile you are based on the Dart but are you in a marina and if so do you have shorepower connected. If you do perhaps you should be looking at fitting a galvanic isolator.
A point in passing. You refer to your bronze propellor. Manganese bronze is the usual alloy for props and that is really a brass ie a copper-zinc alloy. Strictly bronzes are copper-tin alloys
Wayward_Son
Well-Known Member
Any stray current running around on a boat will follow the path of least resistance and can create an electrolysis problem such as a poorly insulated splice on a bildge alarm feed. Poor bonding of through hulls can contribute to galvanic action. Yet it isn't uncommon for a boat to be neutral, where adding zincs can cause a problem. If there is a hot dock in your vicinity or welding taking place, you may notice problems with corrosion on your boat. Shallow moorages with close neighbors can be problematic as well.
John_N
Well-Known Member
I have a swinging mooring five miles up the River Dart so there is a fair amount of fresh water flowing past the boat. I don't know the details of the metalurgical properties of the propellor. It was made by Teignbridge Propellors and I assume it is their standard material. (Manganese bronze?)
VicS
Well-Known Member
No problem with dodgy shorepwer supplies then.
If you turn the battery isolator switch off when you leave the boat that should eliminate any strange effects caused by the internal wiring etc.
I wonder if the fresh water is significant. If it totally displaces the sea water it might. It may tend to layer! What is going through my mind is that I would imagine that the fresh water is coming from Dartmoor and may be on the acid side of neutral rather than alkaline like sea water. It's only a theory and will probably be rubbished very quickly!
In the absense of any other explanation I would check that the shaft anode is in good electrical contact with the shaft.
If you turn the battery isolator switch off when you leave the boat that should eliminate any strange effects caused by the internal wiring etc.
I wonder if the fresh water is significant. If it totally displaces the sea water it might. It may tend to layer! What is going through my mind is that I would imagine that the fresh water is coming from Dartmoor and may be on the acid side of neutral rather than alkaline like sea water. It's only a theory and will probably be rubbished very quickly!
In the absense of any other explanation I would check that the shaft anode is in good electrical contact with the shaft.
