cnh
Well-Known Member
I have a Betamarine saildrive, and I was wondering what material the protective anode is made from - zinc, or something more exotic?
Nicholas Hill
Nicholas Hill
Anode on saildrive.
- Thread starter cnh
- Start date
AIDY
Well-Known Member
Correct. Zinc - for a sea water boat
cnh
Well-Known Member
Thanks very much!
Habebty
Well-Known Member
Let me guess, you've been quoted £112.00 o.n.o for a new saildrive anode like I was last year?
If it is the Selva saildrive like the one attached to my BD722 20hp then I haven't been able to find anyone who casts that particular anode so had to buy the Beta spare.
Still, Beta were very apologetic about the price.
It would seem the chinese are buying all the bloody zinc as well as the copper.
/forums/images/graemlins/confused.gif
If it is the Selva saildrive like the one attached to my BD722 20hp then I haven't been able to find anyone who casts that particular anode so had to buy the Beta spare.
Still, Beta were very apologetic about the price.
It would seem the chinese are buying all the bloody zinc as well as the copper.
/forums/images/graemlins/confused.gif
cnh
Well-Known Member
No - there's a bit more to it than that.
The only thing on the previous prop which gripped the splines on the engine shaft was an aluminium insert - rather like a washer, but with cutouts to grip the splines. This failed, and one possible explanation put forward was that the the insert corroded due to electrolytic reaction. Hence my interest in anodes!
Nicholas Hill
The only thing on the previous prop which gripped the splines on the engine shaft was an aluminium insert - rather like a washer, but with cutouts to grip the splines. This failed, and one possible explanation put forward was that the the insert corroded due to electrolytic reaction. Hence my interest in anodes!
Nicholas Hill
boatmike
Well-Known Member
No! The Chinese are not buying all the zinc and copper. Both are extensively mined in China and they supply the rest of the world.
As far as anodes are concerned I just bought one for my Volvo saildrive from my local chandler. It's a Plastimo marketed product and cost £15.95. Made in China of course!
As far as anodes are concerned I just bought one for my Volvo saildrive from my local chandler. It's a Plastimo marketed product and cost £15.95. Made in China of course!
boatmike
Well-Known Member
One problem with saildrive anodes generally is that they often "fizz away" around the holding bolts and come loose. when loose they are not in good electrical contact with the saildrive and don't work very well. However, the propshaft is not connected very well to the casing either and that's why many makes of prop (like bruntons autoprop) have a seperate nose anode to protect the prop itself. The crappy duckbilled folding props don't however but on the Volvo all holding parts are at least stainless steel. Making what sounds like a spacer washer out of aluminium sounds like a pretty stupid idea....
cnh
Well-Known Member
It's not even a spacer washer. It's all that grips - or gripped - the splines.
A couple of pictures might help:
You can see the remains of the insert in the photo. Made of aluminium - and the only part which gripped the shaft.
Nicholas Hill
A couple of pictures might help:
You can see the remains of the insert in the photo. Made of aluminium - and the only part which gripped the shaft.
Nicholas Hill
boatmike
Well-Known Member
Hmm... doesn't look anything like a Volvo type... was it in fact a bronze prop or aluminium? Fixed or folding?
cnh
Well-Known Member
Bronze two bladed folding - made, if I remember correctly, by an Italian firm called ALPA.
I'm not convinced by the aluminium insert as the only means of transmitting the force from the shaft.
I'm not convinced by the aluminium insert as the only means of transmitting the force from the shaft.
VicS
Well-Known Member
[ QUOTE ]
Bronze two bladed folding
[/ QUOTE ] If you are saying that you have a bronze prop connected to an aluminium insert in the hub then your original assumption about about electrolytic corrosion was almost certainly correct. What is the hub itself made of.
I doubt if an anode will help much if at all.
It does not look as though there was enough metal there to transmit the drive any way.
Bronze two bladed folding
[/ QUOTE ] If you are saying that you have a bronze prop connected to an aluminium insert in the hub then your original assumption about about electrolytic corrosion was almost certainly correct. What is the hub itself made of.
I doubt if an anode will help much if at all.
It does not look as though there was enough metal there to transmit the drive any way.
VicS
Well-Known Member
[ QUOTE ]
Bronze two bladed folding
[/ QUOTE ] If you are saying that you have a bronze prop connected to an aluminium insert in the hub then your original assumption about about electrolytic corrosion was almost certainly correct.
I doubt if an anode will help much.
It does not look as though there was enough metal there to transmit the drive any way.
Bronze two bladed folding
[/ QUOTE ] If you are saying that you have a bronze prop connected to an aluminium insert in the hub then your original assumption about about electrolytic corrosion was almost certainly correct.
I doubt if an anode will help much.
It does not look as though there was enough metal there to transmit the drive any way.
Tranona
Well-Known Member
The anode on the saildrive housing is there to protect the housing, which is aluminium. It is not there to protect the prop. The shaft and prop are insulated from the housing and therefore its anode by the seals. The prop should have its own anode, particularly if it is made of a mix of metals. Many of the higher quality folding and feathering props for saildrives have their own anodes, and some waste very quickly depending on a number of factors including where the boat is moored, whether it is connected to shorepower etc.
I guess you can draw your own conclusions about how to solve the problem in the future. (I have a standard aluminium fixed prop on a Volvo saildrive and it is still there after 7 years!)
I guess you can draw your own conclusions about how to solve the problem in the future. (I have a standard aluminium fixed prop on a Volvo saildrive and it is still there after 7 years!)
cnh
Well-Known Member
Now that's a point -the shaft and prop are insulated from the housing? In that case, the anode would have no effect.
Useful to know. Thanks.
Useful to know. Thanks.
VicS
Well-Known Member
[ QUOTE ]
The anode on the saildrive housing is there to protect the housing, which is aluminium. It is not there to protect the prop.
[/ QUOTE ] Yes i realise that, at least i assumed there would be an anode on the housing. i also realise that the prop and shaft are insulated from the housing, if they weren't the anode on the housing would have an impossible job to do.
What surprises me is that the mechanical connection between the prop assembly and the splined shaft is only described as like a "washer". It is sandwiched between the hub, which looks as though it might be a copper alloy of some description, and the shaft which is stainless? monel? In any case there is a very large area of bronze in the form of the prop blades attached to it (unless they are also insulated from it) In this situation the small aluminium part is going to be at great risk and I doubted if even the provision of anode is going to be all that effective. Better than nothing I suppose .
What seems so wrong with this prop is the little aluminium bit in the centre which is going to be corroded away rapidly yet appears to be the only bit transmitting the drive from shaft to prop assembly. Why is it aluminium, why not also bronze?
If the boat is going to be connected to shore power then it goes almost without saying that a galvanic isolator should be fitted.
I am not sure i can draw my own conclusions about how to prevent this happening again, other than by replacing the prop assembly with one that is all aluminium, like yours, or all bronze or all stainless steel. Any little bit of aluminium in the centre is always going to be at risk.
The anode on the saildrive housing is there to protect the housing, which is aluminium. It is not there to protect the prop.
[/ QUOTE ] Yes i realise that, at least i assumed there would be an anode on the housing. i also realise that the prop and shaft are insulated from the housing, if they weren't the anode on the housing would have an impossible job to do.
What surprises me is that the mechanical connection between the prop assembly and the splined shaft is only described as like a "washer". It is sandwiched between the hub, which looks as though it might be a copper alloy of some description, and the shaft which is stainless? monel? In any case there is a very large area of bronze in the form of the prop blades attached to it (unless they are also insulated from it) In this situation the small aluminium part is going to be at great risk and I doubted if even the provision of anode is going to be all that effective. Better than nothing I suppose .
What seems so wrong with this prop is the little aluminium bit in the centre which is going to be corroded away rapidly yet appears to be the only bit transmitting the drive from shaft to prop assembly. Why is it aluminium, why not also bronze?
If the boat is going to be connected to shore power then it goes almost without saying that a galvanic isolator should be fitted.
I am not sure i can draw my own conclusions about how to prevent this happening again, other than by replacing the prop assembly with one that is all aluminium, like yours, or all bronze or all stainless steel. Any little bit of aluminium in the centre is always going to be at risk.
cnh
Well-Known Member
I'm grateful for your help, since this was the prop originally fitted to my boat. it failed, as you can see. I think the fitting is completely inadequate. Anyway, I'm having a meeting with various people on Monday to discuss this. Thanks for your input.
Nicholas
Nicholas
VicS
Well-Known Member
I hope you sort something out. Its not clear from what you have said if there was originally an anode on the prop. Clearly if there was it could not have been very large but if there was and it had wasted away and not been replaced you will be on a sticky wicket despite my feelings about the design of the thing. An anode made of aluminium/zinc/indium might be more effective than a zinc one (something I have only recently learned from Brendan I have to admit) but with all that bronze around its not going to last long.
Let us know the outcome won't you.
Let us know the outcome won't you.
boatmike
Well-Known Member
Call me a pedant but the shaft is not insulated from the housing by the shaft seals at all. In fact there are usually two opposing timken taper bearings on the shaft that were the gearbox not full of oil would transmit electricity OK. All that stops it is the film of oil on the bearings which is not a good electrical conductor so it's probably more true to say it's not well connected, only being insulated by a thin film of oil. Unfortunately for electrolysis to be a danger the current flow is measured in millivolts so this marginal insulation is enough to stop current flowing.
There is another point however, considering your prop. Some aluminium props have a seperate hub which is moulded into the outer component by rubber to give a certain amount of shock absorbing. Many outboard props are like this and outdrive leg props like on Sillette legs are too. In this case the prop itself is electrically insulated from the hub so some daft Itie engineer may have come to the conclusion that fitting an aluminium hub in a bronze prop is OK and used the inner component from an all alu prop or a plastic one. It's still terrible practice to rely on a lump of aluminium underwater not directly protected by an anode though. Many people say it's OK because it should be insulated by a liberal smear of waterproof grease on the splines so niether touching the shaft or prop, but they forget the connection with the nut and washer that holds it on. One way or another, having a bronze prop on a SS shaft, all components should be made of either one material or the other and the only way people get away with aluminium props or plastic ones with aluminium hubs is that outboards, and lifting outdrives are not or should not be left in the water when not in use.
There is another point however, considering your prop. Some aluminium props have a seperate hub which is moulded into the outer component by rubber to give a certain amount of shock absorbing. Many outboard props are like this and outdrive leg props like on Sillette legs are too. In this case the prop itself is electrically insulated from the hub so some daft Itie engineer may have come to the conclusion that fitting an aluminium hub in a bronze prop is OK and used the inner component from an all alu prop or a plastic one. It's still terrible practice to rely on a lump of aluminium underwater not directly protected by an anode though. Many people say it's OK because it should be insulated by a liberal smear of waterproof grease on the splines so niether touching the shaft or prop, but they forget the connection with the nut and washer that holds it on. One way or another, having a bronze prop on a SS shaft, all components should be made of either one material or the other and the only way people get away with aluminium props or plastic ones with aluminium hubs is that outboards, and lifting outdrives are not or should not be left in the water when not in use.
cnh
Well-Known Member
There was no anode on the prop - only on the leg of the saildrive.
However, the design does worry me - the alumiinium insert was about 1mm thick, and, as you say, was mounted in a bronze propeller and gripping a stainless steel shaft.
It's all very well saying 'check your anode at the beginning of the season', but the idea that your prop should suddenly fail if your anode wears out strikes me as barmy.
However, the design does worry me - the alumiinium insert was about 1mm thick, and, as you say, was mounted in a bronze propeller and gripping a stainless steel shaft.
It's all very well saying 'check your anode at the beginning of the season', but the idea that your prop should suddenly fail if your anode wears out strikes me as barmy.
Tranona
Well-Known Member
Maybe I was being a bit too subtle in my observations. Folding props on Saildrives have a a mixed history. The standard props have cushion drives to protect the rather fragile internals, particulalry on the earlier designs when they started whacking large horsepower through them. Remember the legs are essentially the same from 20 to 75hp. Some folding props have aluminium inserts which have the splines on them, then a rubber bush, then the bronze housing that holds the folding blade mechanism. If there is no anode on the prop you can get the result seen in the photos in the post. I have not seen a prop before that does not have a full length spline in the boss. As others have said it seems unreasonable for the thin washer you describe to transmit the power to the prop even when new.
If you look at the latest Volvo folding props they have an annular anode on the bronze housing, but even these can go in less than a season.
So, my suggestion is that whatever the outcome of your discussions with the supplier/manufacturer, use a different design of prop next time around.
If you look at the latest Volvo folding props they have an annular anode on the bronze housing, but even these can go in less than a season.
So, my suggestion is that whatever the outcome of your discussions with the supplier/manufacturer, use a different design of prop next time around.
