Second hull anode on saildrive boat?

Cantata

Cantata

Well-Known Member
Joined
1 Aug 2003
Messages
4,943
Location
Swale/Medway
Visit site
I have a boat with a Volvo 2040 and saildrive. I assume that as with the other 20n0 series engines, the leg is electrically isolated from the block.
The block is connected to a hull anode along with other odds and ends.
I'm fitting an Autoprop, replacing the original Volvo aluminium fixed prop. I know from experience that fitting a bronze (or similar) prop to a saildrive increases the demands on the leg anode, even if the prop has its own anode as with the Bruntons.
Bruntons now suggest that in these circs, you fit a second hull anode, wired only to the saildrive leg, to help out. Wondering if anyone has done this and if there are any snags, or indeed if peeps have found it worthwhile?
 
It's a good idea in theory but I have not heard of it in practice.

Another option, which I think Bruntons seem to have some problems with, is to paint the propeller. I have posted many times, including photos, of success with Velox antifouling paint on my Bruntons prop. The second bonus has been that anode loss has been reduced by a factor of more than two. Previously, with a bare prop, its anode would be gone in one season and the shaft anode in two. Since painting five seasons ago I have renewed the prop anode twice and the shaft anode only once, last season, although I suspect it might well have done a couple more years. The prop is the cathode in the cell, so reducing its surface area will help the anodes.
 
vyv_cox said:
It's a good idea in theory but I have not heard of it in practice.

.................................................. The prop is the cathode in the cell, so reducing its surface area will help the anodes.
Click to expand...

is the prop not insulated from the shaft ?
 
There is no reason why you should have an anode connected to the engine - there is nothing to protect. The prop will have no influence on the leg anode as it is isolated electrically from the leg and protected by its own anode, the life of which you can extend as Vyv suggests.

So, there is no need to do anything different with the leg anode - the split 130 leg anode gives a long life (3-4 years in my case) and is easy to change.

What are the other "odds and ends" connected to the hull anode? Cannot think of anything, except perhaps the rudder tube, but that is nothing to do with the engine. There is no need for an anode on anything else.
 
vyv_cox said:
I thought that the Volvo prop has a rubber sleeve but the Bruntons, and others, fit in the conventional way onto the metal shaft. Is this not correct?
Click to expand...

Almost certain it will have an isolating bush. It will not have any effect on the drive housing as there is also a delrin washer behind the prop nut and no path from the shaft to the housing. That is why it needs its own anode whereas an aluminium prop does not as it is just aluminium and not in contact with anything else.
 
pvb said:
I'm not sure that the Brunton Autoprop is electrically isolated.
Click to expand...

Mm. I have no specific knowledge of the Brunton, but I thought that the Volvo 3-blade folder, which certainly is isolated, was relatively unusual in being so.

Pete
 
prv said:
Mm. I have no specific knowledge of the Brunton, but I thought that the Volvo 3-blade folder, which certainly is isolated, was relatively unusual in being so.
Click to expand...

Reckon that's right. If you Google brunton autoprop galvanic you'll find some horror stories. Perhaps this is why Brunton suggest an additional hull anode.
 
So, Volvo, Flexofold etc go to great trouble to fit an anode to their bronze props but Brunton do not bother? But they suggest an anode connected to the engine may help protect their prop? The anodes on our VP 3 blade folder had a very short lifespan which was a pain but a lot better than no protection, the relatively rapid degradation even with a hanging anode supplementing the leg anode suggests that they were needed.
Perhaps a Saildrive/ Brunton combination is not the best choice?
 
Cantata said:
<snip>
Bruntons now suggest that in these circs, you fit a second hull anode, wired only to the saildrive leg, to help out. Wondering if anyone has done this and if there are any snags, or indeed if peeps have found it worthwhile?
Click to expand...

If there is a direct electrical path to the Brunton prop, that was no there with the original prop, the workload of the leg anode is obviously increased. The prop anode will help with this but i cannot see how it would hurt to fit a hull anode connected to the leg.

That said, Vyv seems to have cured the issue by painting the prop, which has the added advantage of reducing fouling, so that seems like a good choice to me.
 
PaulRainbow said:
OP states in post #1 that it has an anode, but Bruntons suggest a hull anode too.
Click to expand...

Fail to see how a hull anode will help as it cannot be connected to a rotating prop, nor the shaft that it is on. That is the whole point of keeping the prop isolated from the housing. The Flexofold, Volvo and Featherstream props for saildrives are all bushed just like the standard aluminium prop, and the folders/featherers mostly have their own anodes.
 
I thought that all props , at least those made of highly cathodic alloys such as Nibral, intended for use on saildrives had insulating hubs. Apparently this is not always the case.

An uninsulated prop puts the entire sail drive leg at risk of galvanic corrosion with only the ring anode to protect it. This I imagine is why Bruntons suggest a hull anode bonded directly to the sail drive. Because an anode only protects what it "can see" I'd suggest fitting two, one either side.

This, I emphasise, is to protect the sail drive leg from galvanic corrosion caused by the prop. Not for protection of the prop.

Folding props need their own anodes because they are usually constructed from a mix of incompatible alloys.
 
Last edited:
Tranona said:
Fail to see how a hull anode will help as it cannot be connected to a rotating prop, nor the shaft that it is on. That is the whole point of keeping the prop isolated from the housing. The Flexofold, Volvo and Featherstream props for saildrives are all bushed just like the standard aluminium prop, and the folders/featherers mostly have their own anodes.
Click to expand...

Not having detailed knowledge of saildrives I was under the impression that in the case of Volvos the complete leg, including the shaft and gears, and of course the prop, was electrically isolated from the rest of the boat. I cannot imagine how a shaft could be isolated from the casing. In that case connecting an anode to the hull with a wire back to the casing would be helping to protect the prop.
 
Tranona said:
Fail to see how a hull anode will help as it cannot be connected to a rotating prop, nor the shaft that it is on. That is the whole point of keeping the prop isolated from the housing. The Flexofold, Volvo and Featherstream props for saildrives are all bushed just like the standard aluminium prop, and the folders/featherers mostly have their own anodes.
Click to expand...

It's been stated that the Brunton prop is not bushed, so not isolated from the shaft.
 
vyv_cox said:
Not having detailed knowledge of saildrives I was under the impression that in the case of Volvos the complete leg, including the shaft and gears, and of course the prop, was electrically isolated from the rest of the boat. I cannot imagine how a shaft could be isolated from the casing. In that case connecting an anode to the hull with a wire back to the casing would be helping to protect the prop.
Click to expand...
Yes Vyv. In most modern VP setups the whole SD is isolated from the engine. The MD 22 is the notable exception.

The prop shaft is not isolated from the SD casing.

The hull anode suggested, I suppose, offers additional protection to the whole lot but i would have thought that protection of the SD casing, being the most vulnerable, was the most important to protect.
 
I have a Brunton prop on a Volvo saildrive.
The prop is isolated from the saildrive shaft by a rubber bush.
It is isolated from the rope cutter by PTFE washers.

I have fitted an extra anode to the gearbox, situated in full direct and close view of the prop and it is very effective.

I am now trying a/f on the prop.

Bruntons concern is that is the prop edges get the A/F worn off, they could erode.

Tony
 
You must log in or register to reply here.

Share this page

Top