Unexpected prop anode wasted

[chris-s]

Whilst taking a swim overboard yesterday we noticed that there was as barely any of the conical prop anode left, just the bolt and a tiny piece of anode visible.

This late-eighties Beneteau is a new-to-us boat this year and the first one we’ve had where anodes have been fitted to anything other than an outboard so I’m not entirely sure what to expect or check.

A new anode was fitted this year, along with a new prop (no idea what happened to the old one but the previous owner had all this arranged), and the boat was splashed mid-May this year. Has been kept on a swing mooring in the Fal and not connected to shore power whilst afloat. I’ve no idea what metal the anode was made from but pretty safe to assume it was zinc rather than aluminium.

There is no length of shaft exposed for a shaft anode. The boat has a raw water cooled Volvo 2003 which had a new engine anode fitted in June.

Finally, there is also a large hull anode that was also new this season and from a glance yesterday appeared untouched with a light covering of growth.

Is there anything I should check that might cause such quick corrosion?

The boat is coming out for the winter on 2nd October. I’m not sure how, but should I try to fit a new anode now?

 

[Plum]

Whilst taking a swim overboard yesterday we noticed that there was as barely any of the conical prop anode left, just the bolt and a tiny piece of anode visible.

This late-eighties Beneteau is a new-to-us boat this year and the first one we’ve had where anodes have been fitted to anything other than an outboard so I’m not entirely sure what to expect or check.

A new anode was fitted this year, along with a new prop (no idea what happened to the old one but the previous owner had all this arranged), and the boat was splashed mid-May this year. Has been kept on a swing mooring in the Fal and not connected to shore power whilst afloat. I’ve no idea what metal the anode was made from but pretty safe to assume it was zinc rather than aluminium.

There is no length of shaft exposed for a shaft anode. The boat has a raw water cooled Volvo 2003 which had a new engine anode fitted in June.

Finally, there is also a large hull anode that was also new this season and from a glance yesterday appeared untouched with a light covering of growth.

Is there anything I should check that might cause such quick corrosion?

The boat is coming out for the winter on 2nd October. I’m not sure how, but should I try to fit a new anode now?

My Beneteau prop anode only lasts 3 or 4 months. I periodically dry out on the scrubbing posts because the prop usually needs cleaning too so I fit a new anode at the same time. When you fit the new anode, fill the recess (counterbore) that houses the screw head with mastic (any type) as this stops the anode from wasting away around the bolt head which then leads to the anode coming loose and wasting from the inside as well as the outside. As a backup, I also have a hull mounted anode with a bonding cable linking it internally to the prop.

for now, check the electrical resistance between the hull anode studs inside the boat to the prop shaft inside the boat. Ideally below 1 ohm. if not, then you can fix this while you are still afloat.

www.solocoastalsailing.co.uk

 

[Poey50]

My neighbour in the boatyard was shocked on haul-out to discover his shaft anode had gone and deep pits in his prop blades. A check with a multimeter showed current from the prop to earth. Even a very small current sets up electrolytic corrosion. For a very rapid loss of anode that should be on the list of suspects.

 

[Tranona]

What sort of propeller is it? A standard fixed blade has an anode on the prop nut and that should last a year. Similarly a folding or feathering prop has an anode on it but these have variable life.

The hull anode should be connected to the prop shaft via the gearbox with a connection across the flexible coupling if there is one. The fact that it is not wasting suggests it is not connected. It is there to "share2 with the prop anode, although it will be most effective when the prop anode is gone. However you should be able to see some wasting.

There is no relationship between the engine and its anode and the propeller. The engine anode is to protect the internals of the engine cooling system which is completely different seawater from where the prop is. Galvanic action occurs between 2 metals in contact in the same seawater.

 

[VicS]

Whilst taking a swim overboard yesterday we noticed that there was as barely any of the conical prop anode left, just the bolt and a tiny piece of anode visible.

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Finally, there is also a large hull anode that was also new this season and from a glance yesterday appeared untouched with a light covering of growth.

As Tranona says the fact that the hull anode is not wasting suggests that there is no electrical connection between it and the prop.
When the boat is out of the water check the electrical continuity between the two. You should be looking for a resistance of no more than 0.2 ohms.

(The anode should be reasonably close to the prop and "able to see" the prop without affecting the water flow to the prop.

Useful video here:

 

[chris-s]

Thanks for the replies.

Yes, it is a fixed-three bladed prop, tho that might change for next season.

I will go and check the connectivity between hull anode and prop shaft, also check the condition of the wiring between the two and map out what bonding is in place. So far as distance of hull anode from prop, from recollection I reckon it is more like 2m, is that too far?

The other thing I noticed from the video that Vyv posted is that the setup shown has a collar anode, I don't have one of those. There isn't much of a gap to fit one, say 20mm and that might impede water flow out of the cutlass bearing, so maybe it is not expected to be present in this setup.

 

[Plum]

Thanks for the replies.

Yes, it is a fixed-three bladed prop, tho that might change for next season.

I will go and check the connectivity between hull anode and prop shaft, also check the condition of the wiring between the two and map out what bonding is in place. So far as distance of hull anode from prop, from recollection I reckon it is more like 2m, is that too far?

The other thing I noticed from the video that Vyv posted is that the setup shown has a collar anode, I don't have one of those. There isn't much of a gap to fit one, say 20mm and that might impede water flow out of the cutlass bearing, so maybe it is not expected to be present in this setup.

My hull anode is 1.2m from the prop and I emailed MG Duff. They said the closer the better but 1.2m was ok. Suggest you ask them if 2m is ok.

Recommend you do not interfere with that 20mm gap.

Www.solocoastalsailing.co.uk

 

[Tranona]

Thanks for the replies.

Yes, it is a fixed-three bladed prop, tho that might change for next season.

I will go and check the connectivity between hull anode and prop shaft, also check the condition of the wiring between the two and map out what bonding is in place. So far as distance of hull anode from prop, from recollection I reckon it is more like 2m, is that too far?

The other thing I noticed from the video that Vyv posted is that the setup shown has a collar anode, I don't have one of those. There isn't much of a gap to fit one, say 20mm and that might impede water flow out of the cutlass bearing, so maybe it is not expected to be present in this setup.

Closer would be better, particularly as it is secondary protection, but that might not be physically possible. Shaft anodes are primarily used where the shaft is on a P bracket and there is exposed shaft forward of the bracket. You are right the 20mm gap is essential.

 

[chris-s]

So, new anode fitted to the prop and I checked the wiring of the hull anode. It is connected to a keel bolt, nothing else. And none of the other keel bolts are connected to anything else!
I didn’t have enough wire onboard to connect it to the engine tonight.

 

[vyv_cox]

So, new anode fitted to the prop and I checked the wiring of the hull anode. It is connected to a keel bolt, nothing else. And none of the other keel bolts are connected to anything else!
I didn’t have enough wire onboard to connect it to the engine tonight.

No reason to connect an anode to the keel. Its corrosion rate is so low as to be insignificant and a small anode will do nothing to reduce it. A paint film is far more effective.

 

[Tranona]

So, new anode fitted to the prop and I checked the wiring of the hull anode. It is connected to a keel bolt, nothing else. And none of the other keel bolts are connected to anything else!
I didn’t have enough wire onboard to connect it to the engine tonight.

That perhaps explains why it is so far from the propeller. Typically if it was intended for the prop it would be mounted close to the shaft log

 

[MattK1969]

The Hull anode may not be an anode if it is not wasting away. It may be a grounding plate which boats from the 80's used to have. We have one and I cannot work out what it does, I think they are pretty useless.