Hi, I recently hauled our 2011 Hanse 355 after 12 month in the water. The anodes were new 12 months earlier and as she came out of the water it was obvious we had a serious problem. The sail drive had catastrophic corrosion and a hole in the leg you could put your hand through.

The cause is not clear, the shaft anode was largely intact and not significantly eroded. The prop was unaffected, I have heard that in the last 12 months as many as 30 boats here in aussie have suffered similar fate and I would love to connect with other Volvo sail drive owners who have had challenges like this..

What are those S shaped things I see jumping out the till here??

What you been moored next too

Jolene said..

SailingGuy said..
Hi, I recently hauled our 2011 Hanse 355 after 12 month in the water. The anodes were new 12 months earlier and as she came out of the water it was obvious we had a serious problem. The sail drive had catastrophic corrosion and a hole in the leg you could put your hand through.

The cause is not clear, the shaft anode was largely intact and not significantly eroded. The prop was unaffected, I have heard that in the last 12 months as many as 30 boats here in aussie have suffered similar fate and I would love to connect with other Volvo sail drive owners who have had challenges like this..

I have seen this a few times before and if I recall correctly it stems from having a poor fitting or an incorrectly fitted anode not making good contact with the leg.
I can't remember if it was a fault with the fitting dimensions of the anode or just inexperience in making sure it is correctly fitted. Either way, the anode was not doing its job and the leg was dissolving whilst the anode was unaffected.

This is what Joelene is referring to. In seawater two dissimilar metals that are electrically connected will act as a battery, where current will flow between the two.
This means that one metal will be losing material, or metal ions, to the seawater. Which one loses material is determined by the metals anodic index. To stop the damage to the expensive bits, you introduce a third sacrificial metal, the anode.
If you are protecting brass with zinc, there is a voltage difference of approx 1.4V. The zinc has the lowest anodic index, it will lose material, not the brass.
If you protect aluminium with zinc, there is a voltage difference of approx 0.3V. The zinc is the lowest anodic index, it will lose material, not the aluminium.
If you tried to protect brass with gold for example, this won't work, the brass will lose material and not the gold.

So, as Joelene said, if you do not have a good electrical contact between the zinc and the aluminium saildrive, the zinc is not losing material, something else will. The bigger the voltage difference, the quicker the material will be lost. This is why your prop (bronze) anode will decay faster than your saildrive (alloy) anode.

If the cause is not external to the boat (a dodgy neighbour in a marina) then the aluminium in your saildrive is unfortunately the metal with the lowest anodic index in the circuit and is losing material....ergo it's going to be toast.

Example Case Study: You don't have any anodes on your saildrive or prop and neither have any waterproof coating. The bronze and aluminium are electrically connected and immersed in seawater. Which metal is going to corrode first?

Edit: Whilst I am using the prop and the leg as an example, my Volvo saildrive had electrical isolation between the prop and the leg, (hence the two anodes), I am not sure if this is the same for all Volvo legs. Metals need to be connected electrically to cause galvanic corrosion, so use a multimeter to check for high resistance (good) if you are not sure.

shaggybaxter said..

SailingGuy said..
Hi, I recently hauled our 2011 Hanse 355 after 12 month in the water. The anodes were new 12 months earlier and as she came out of the water it was obvious we had a serious problem. The sail drive had catastrophic corrosion and a hole in the leg you could put your hand through.

The cause is not clear, the shaft anode was largely intact and not significantly eroded. The prop was unaffected, I have heard that in the last 12 months as many as 30 boats here in aussie have suffered similar fate and I would love to connect with other Volvo sail drive owners who have had challenges like this..

HI SG,
I hadn't heard anything, I had the boat on the hard in January and there was not a hint of corrosion (2015 DI-30) .
Is it in salt water? In a marina or private mooring? Kinda reeks of a short, sounds like a lot of current flowing to earth.
Edit: was the saildrive anode eaten away ? And the prop anode unaffected?

Thanks, sail drive anode only minimal reduction for 12 months in the water, there is no anodes on my prop so just the volvo split anode on the shaft.

shaggybaxter said..

Jolene said..

SailingGuy said..
Hi, I recently hauled our 2011 Hanse 355 after 12 month in the water. The anodes were new 12 months earlier and as she came out of the water it was obvious we had a serious problem. The sail drive had catastrophic corrosion and a hole in the leg you could put your hand through.

The cause is not clear, the shaft anode was largely intact and not significantly eroded. The prop was unaffected, I have heard that in the last 12 months as many as 30 boats here in aussie have suffered similar fate and I would love to connect with other Volvo sail drive owners who have had challenges like this..

I have seen this a few times before and if I recall correctly it stems from having a poor fitting or an incorrectly fitted anode not making good contact with the leg.
I can't remember if it was a fault with the fitting dimensions of the anode or just inexperience in making sure it is correctly fitted. Either way, the anode was not doing its job and the leg was dissolving whilst the anode was unaffected.

This is what Joelene is referring to. In seawater two dissimilar metals that are electrically connected will act as a battery, where current will flow between the two.
This means that one metal will be losing material, or metal ions, to the seawater. Which one loses material is determined by the metals anodic index. To stop the damage to the expensive bits, you introduce a third sacrificial metal, the anode.
If you are protecting brass with zinc, there is a voltage difference of approx 1.4V. The zinc has the lowest anodic index, it will lose material, not the brass.
If you protect aluminium with zinc, there is a voltage difference of approx 0.3V. The zinc is the lowest anodic index, it will lose material, not the aluminium.
If you tried to protect brass with gold for example, this won't work, the brass will lose material and not the gold.

So, as Joelene said, if you do not have a good electrical contact between the zinc and the aluminium saildrive, the zinc is not losing material, something else will. The bigger the voltage difference, the quicker the material will be lost. This is why your prop (bronze) anode will decay faster than your saildrive (alloy) anode.

If the cause is not external to the boat (a dodgy neighbour in a marina) then the aluminium in your saildrive is unfortunately the metal with the lowest anodic index in the circuit and is losing material....ergo it's going to be toast.

Example Case Study: You don't have any anodes on your saildrive or prop and neither have any waterproof coating. The bronze and aluminium are electrically connected and immersed in seawater. Which metal is going to corrode first?

Edit: Whilst I am using the prop and the leg as an example, my Volvo saildrive had electrical isolation between the prop and the leg, (hence the two anodes), I am not sure if this is the same for all Volvo legs. Metals need to be connected electrically to cause galvanic corrosion, so use a multimeter to check for high resistance (good) if you are not sure.

Really interesting, I just purchased a silver reference anode so I can keep a close eye on the bond system voltage and this table makes lots so sense so much appreciated.

What antifouling paint was on the saildrive?

Whatever you say on saildrive legs but I certainly would not use copper rich antifoul on an aluminium hull.

That is the one big drawback with aluminium yachts. Very regular antifouling with fairly ineffective paint.