Will hung anodes work to protect a saildrive prop?

[Swagman]

Can anyone give sound advice on this?
I appreciate the hanging anodes which are connected back by wire straight to a shaft should give protection to a prop.
But I've been told there is no value in hanging an anode and attaching it to a saildrive leg - as the metal to metal contact in the leg may not provide the required circuit for the anode to do its work.
Despite this view I did attach one on my last yacht with a saildrive, and the anode did appear to sacrifice itself over a period.
I've a new yacht with a saildrive and wish to know if I'm wasting my time by using one.
Can anyone please clarify this for me?
Cheers
JOHN

 

[snowleopard]

If there is good electrical contact, the anode should work. You can test by connecting a multimeter between the leg and the anode (when ashore of course).

The fact that an anode dissolves doesn't necessarily mean it is protecting the leg: it could just be setting up a cell between the anode and the wire it's hanging from. Also stray currents in a marina can cause a lot of electrolysis.

 

[TigaWave]

The sail drive has an anode between prop and leg or should have. Its my uderstanding that with two dissimilar metals in this case ss and Al the anode should be close to where the reaction/circuit will take place.

Hanging one would give additional protection if there was boat to boat electrolysis in the marina, thus reducing the ammount your saildrive anode depletes, but you really should have one on the leg itself.

 

[Swagman]

Yes, there are two anodes already fixed underwater - one twix leg and prop boss - and one on the end of the prop boss itself. Both appear to have had a good 'hammering' when viewed underwater - despite being only 9 months old.
The intent on adding a third suspended anode - connected back to the saildrive / gearbox by cable and hung over the side of the yacht - is to compliment those already in place and hopefully extend their life until I lift the boat later this year.
The issue I sought clarification on was whether a wire hung anode - attached firmly to the saildrive leg housing inside the boat and hung over the side alongside the prop suspended in the water - would work ok.
The view given by another is that there is little if any direct metal contact from my connection point to the prop itself (from the leg casing, through bushes / bearings / gears / shaft etc) - so his view was such an anode is unlikely to be any use.
I was hoping someone could perhaps assure me he was wrong.......or else it looks like I'll have to slip the boat earlier than planned
Cheers
JOHN

 

[TigaWave]

I would have thought it would help, but the majority of loss will still be from the anode between the Al and stainless shortest route less resistance etc.

your hanging anode would be part of the circuit as the anode ring on a sail drive is bolted to the casing which is the same piece of metal as that inside the boat.

I personaly think you're taking a risk, can't you just lean her against a wall for an afternoon and change them? that woulkd seem the easiest solution.

 

[DaveS]

If you're happy to get at the prop underwater, I don't see why you can't temporarily connect a length of insulated wire to the prop using a croc clip, take the other end on board, then connect a ohmmeter between it and the point(s) you were thinking of connecting your supplementary anode to. Measure with the meter both ways round, to make sure that galvanic action isn't confusing things. If you measure a low resistance (3 ohms or less, under 1 ohm ideally) in both directions then you could safely assume good metallic contact and there should be no reason why the new anode shouldn't work if connected to that point.

I don't think that testing as above will be significantly affected by the boat sitting in salt water: while it is true that the water will provide a parallel conductive path this should be of much higher resistance than the value you're looking for. You can confirm this by measuring (with the same meter) the resistance between two pieces of insulated wire with their ends dipped in the sea a few inches apart.

 

[davidwf]

Did this with one of my boats and it really helped slow down errosion on the sildrives anode. To dit you have to connect it to a bolt on the gearbox not the engine itself (assuming its a Volvo) because the saildrive is electrically isolated from the engine block.

On the next boat I fitted a whopping great pear shapped anode next to the leg on the hull and connected it in the same way to the gearbox.

I connected both of them to the bolt holding the gear change bracket in position.

 

[Swagman]

Thanks guys for all the advice......
JOHN