dangling an anode

[tckearney]

Dangling an anode.  Can anybody advise if there is any benefit of dangling  a zinc anode over the side while in the marina.   I only come out the water  every three years or so and the saildrive anode is always just about holding in there by then.  Would it help prolong the life if I dangled an anode bonded to the mast shroud,  which is bonded to the keel. 

[Symphony]

No use whatsoever as neither the shroud nor the keel is directly connected to the saildrive housing if that is what you are trying to protect. To be effective an anode has to be electrically connected to the metal you are trying to protect and physically close in the electrolyte (seawater). That is why the anode is physically bolted to the housing with the bolts providing the electrical connection.

So any additional anode must first be bonded to the housing and second close  to it. You can achieve the first by something like a bonding wire from the gearbox housing to a stud through the deck and connecting the hanging anode to that when needed. Achieving the second condition is very difficult given the saildrive housing is in the middle of the boat and the hanging anode will be at least half a beam distance away. In addition the anode closest to the object being protected will always erode first. Therefore your hanging anode will only come into effect once the ring anode is completely depleted.

It is possible that a hull anode sited directly behind the saildrive (and bonded internally to it) will be more effective, but again only once the ring anode is depleted.

The only way of increasing the life of the anode is increasing the volume attached directly to the housing and this is physically impossible to do on a 120 drive. The later 130/150 saildrives have an anode over twice as big and have a corresponding longer life. Indeed based on my experience of 7 years with 130 drives I estimate the usable anode life of over 10 years.

[tckearney]

Thanks Symphony  very helpful indeed.    I changed the prop to a bronze folding prop about four years ago and its only since then I had this problem. The original aluminium prop which I understand is insulated by the vibration bush from the saildrive,  never caused any wearing of the anode.

[Symphony]

The bronze prop should not have any effect. Most have anodes of their own. Which prop do you have?

[Salty]

Thanks Symphony  very helpful indeed.    I changed the prop to a bronze folding prop about four years ago and its only since then I had this problem. The original aluminium prop which I understand is insulated by the vibration bush from the saildrive,  never caused any wearing of the anode.

I also did the same on my B36(2002), and have noticed that like you the anode gets eroded more quickly than was the case with the original aluminium propeller. The folding bronze propeller is one marked “Volvo,” but which has no separate replaceable anode of its own. Having said that, there appears to be an area around the central bushing at the forward end of the prop which has in the past had some of the metal eaten  away. The metal colour and appearance suggests it just might have been some kind of anode, but I have no idea if it can be removed, let alone be replaced. I have not seen any advertisement offering a replacement either in the Volvo saildrive catalogue, or in those from chandlerys or anode manufacturers. That area on my prop does not appear to have lost any metal during my use over the last four years or so, and I do fit a new standard anode for seawater use each year. I still have and carry onboard the original aluminium prop, and with a number of drying out areas near to where my boat is moored I can and have used one to dry out where I changed the original aluminium, but on that occasion “singing propellor,” for the folder.

[tckearney]

Hi Symphony and Salty, I have a Brunton, which does have its own anode, that goes as well.  As mentioned with my original prop, (that I may replace the brunton with), the anode lasted many many years.

[Symphony]

Older Volvo folding props have a reputation for corrosion owing to the alloy used for the hub and blades. It usually manifests itself in pitting on the gear teeth and wear in the bores where the pivot pins run. There can also be localised pitting in the hub and on blades. The latter is nothing to do with interaction between the hub and the stainless pins but electrolysis between the constituent metals of the ally, much as you get with low grade through hulls. Volvo seem to have changed their alloy as more recent props don't seem to suffer the same problems.

My FlexoFold 2 blade has no anodes at all (although later ones do have small buttons). I hauled out last week and still no signs of corrosion after 8 years in the water. Also the saildrive anode fitted new last year is barely pitted. I painted the blades last year with white Velox and not only no fouling at all but paint still firmly attached. Will just abrade and apply another coat as per their recommendations. So far very pleased.