Electrolysis Problem Mystery - (aren't they all?) includes pics

Habebty

Habebty

Well-Known Member
Joined
16 May 2001
Messages
4,534
Location
Norfolk/Suffolk
Visit site
Had the boat lifted Wednesday, was a bit interesting in the gales especially as the folding prop seemed a bit iffy, but with the excellent help of the yard boys the lift was fine............until I saw why the prop seemed a bit iffy.

image2.jpeg


To give a bit more info, the boat/prop is 11 years old, started going a little pink last year, anode normally wastes by about 30% each year. The prop hub (Radice) is electrically isolated from the saildrive by a rubber shock absorbing bush and has no dedicated anode. Boat has been in the water 12 months.

The saildrive leg anode makes good contact with the leg and I assume this is how it is meant to be to protect the leg and not the prop. However it has not wasted anywhere near as much as normal? and the leg does show a little pitting.

What caused the prop to fizz away, the gear teeth have almost crumbled away so I was lucky to make it from my berth to the hoist?

The yard did blast/paint the hull last year and i spotted they had used the same copper antifouling on the leg (surely they should know this?)which I got them to blast off and repaint which they say they did and the leg is apparently ok, so ruled that one out.

I know everyone has their own theories on electrolysis, but any advice gratefully received before I send another prop to its fizzy doom.

thanks, and some more pics for your delectation.


image1.jpeg


image7.jpeg


image6.jpeg


image5.jpeg


image4.jpeg


image3.jpeg
 
Last edited:
No mystery. The prop is just made of poor quality material and dezincifies. It has done well you last so long. It really needs its own anode as the saildrive anode does not protect it. Suggest you replace it with a better quality prop such as a FlexoFold.
 
Would love to, but would probably mean changing the leg as well as the splined shaft (Selva) doesn't accept any other damn prop....I'm told?
 
Pete,
That's pretty shocking !
I cannot answer the main question but the painting of the saildrive leg with non-copper A/F is to prevent the erosion of the ally saildrive leg itself (which has not happened)
Will take a close interest in its replacement:)
 
Habebty said:
Would love to, but would probably mean changing the leg as well as the splined shaft (Selva) doesn't accept any other damn prop....I'm told?
Click to expand...

Not sure about that. Speak to Chris at Darglow Engineering. Pretty sure the Selva shaft is a standard spline but he will know. The Radice props are well known for this problem They might have now added an anode which will reduce the problem but the Flexofold is much better material and does not need an anode.
 
I think the Selva shaft is thirteen splines (as far as I can see from OP's photo) and that is the problem as most saildrive legs are 17 splines.

I have PMd OP with Darglow's response to me re a Flexofold
 
The anode you show appears to be electrically isolated from the prop and will therefore be protecting the drive unit to which it is attached. The only protection it will provide to the prop will be from its proximity. I would suspect that there is a breakdown in the rubber isolating bush and the prop is working as an anode to protect the saildrive propshaft!

If you are at Foxs I can show you how to check the system with a multimeter.
 
'
Sinbad1951 said:
The anode you show appears to be electrically isolated from the prop and will therefore be protecting the drive unit to which it is attached. The only protection it will provide to the prop will be from its proximity. I would suspect that there is a breakdown in the rubber isolating bush and the prop is working as an anode to protect the saildrive propshaft!

If you are at Foxs I can show you how to check the system with a multimeter.
Click to expand...

Thanks,

Is that a resistance check between the prop and the Leg/anode?

And in answer to Tranona, it is a 13 spline shaft, so probable custom machining of adapter bush needed for another make prop.
I will be investigating alternative props further though - thanks
 
Last edited:
Yes a resistance check. Multimeter set on ohms. It would appear that the problem is a lack of anode on the prop. I may be possible to have a split anode machined to fit your new prop. Try Latham Engineering. (Mr Google) and tell them what you need.
 
Sinbad1951 said:
The anode you show appears to be electrically isolated from the prop and will therefore be protecting the drive unit to which it is attached. The only protection it will provide to the prop will be from its proximity.
Click to expand...

If the anode is electrically isolated from the prop can it provide any protection at all to the prop, irrespective of its proximity?

Just asking to aid my understanding.

Richard
 
In general terms it would if the prop was also electrically isolated but as the prop is attached to a stainless shaft it's the difference in potential between those two connected items which will be causing the prop to erode. The only sensible solution here is to have prop anode attached.
 
Sinbad1951 said:
In general terms it would if the prop was also electrically isolated but as the prop is attached to a stainless shaft it's the difference in potential between those two connected items which will be causing the prop to erode. The only sensible solution here is to have prop anode attached.
Click to expand...

I was under the impression that the prop was completely isolated by means of the rubber bush between the spline and the hub?
 
Sinbad1951 said:
In general terms it would if the prop was also electrically isolated but as the prop is attached to a stainless shaft it's the difference in potential between those two connected items which will be causing the prop to erode. The only sensible solution here is to have prop anode attached.
Click to expand...

OP stated in his first post that the prop is electrically isolated from the shaft. There is no mystery here, as Tranona has said it's dezincification, due to selection of brass or maybe manganese bronze, effectively the same thing. An anode will most definitely help but probably not delay the problem indefinitely.
 
vyv_cox said:
OP stated in his first post that the prop is electrically isolated from the shaft. There is no mystery here, as Tranona has said it's dezincification, due to selection of brass or maybe manganese bronze, effectively the same thing. An anode will most definitely help but probably not delay the problem indefinitely.
Click to expand...

The similar prop that Radice make for Volvo has an anode following their early experience with dezincification. Not easy to retrofit though as it has 2 segments set in machined grooves in the hub.
 
vyv_cox said:
OP stated in his first post that the prop is electrically isolated from the shaft. There is no mystery here, as Tranona has said it's dezincification, due to selection of brass or maybe manganese bronze, effectively the same thing. An anode will most definitely help but probably not delay the problem indefinitely.
Click to expand...

If you look at similar threads you will see that a breakdown of the insulating bush in Radice props is not unusual. This would prevent it being effectively isolated from the stainless shaft and would accelerate any dezincification. Easily checked with a multimeter.
If the OP is restricted to replacing the prop like for like, I would recommend the tapping of the new props central hub on two opposing sides and fitting equal weight teardrop anodes. As Vyv says, it wont make up for poor quality metals in the prop but will considerable delay electrolysis.
 
Sinbad1951 said:
If you look at similar threads you will see that a breakdown of the insulating bush in Radice props is not unusual. This would prevent it being effectively isolated from the stainless shaft and would accelerate any dezincification. Easily checked with a multimeter.
If the OP is restricted to replacing the prop like for like, I would recommend the tapping of the new props central hub on two opposing sides and fitting equal weight teardrop anodes. As Vyv says, it wont make up for poor quality metals in the prop but will considerable delay electrolysis.
Click to expand...

The Volvo type anodes would be relatively easy to fit if one is doing some machining and tapping. They look like keys used in keyways with a central screw to fasten. The machined groove would be flat and the curved side is the same curve as the hub. Illustration on the Keyparts site under saildrives.
 
Tranona said:
No mystery. The prop is just made of poor quality material and dezincifies. It has done well you last so long. It really needs its own anode as the saildrive anode does not protect it. Suggest you replace it with a better quality prop such as a FlexoFold.
Click to expand...

I have the same propeller which I fitted about 19 years ago so I have been really lucky. I cleaned it down to the bare metal this winter and didn't notice anything amiss. Maybe it helps being on a swinging mooring and not in a marina?
 
I had a couple of Goris go the same way on a Yanmar saildrive, took about 3 years each. Our discussions with the agent failed to resolve what the cause was, the agent maintained the isolation meant that anodes on the prop were unnecessary and it was due to other causes, the Gori is aluminium bronze and in contact with nothing that could have caused electrolysis. I thought stray currents were to blame, AC currents can cause electrolysis by the way.
 
You must log in or register to reply here.

Share this page

Top