Should I escape my electrical bondage?

[kittan]

I have had the engine out of the boat for repairs and as the time comes to put it back in I am reflecting on the anode bonding system, which has always been 'as is' since buying the boat ten years ago. While I have good access to the anode studs etc I am just looking for a second opinion on whether I should keep on the same course.

So what is in the electro-galvanic mix? The hull is mahogany planking with copper fastenings, iron keel, iron straps on the wooden rudder, stainless steel prop shaft, bronze propeller, bronze stern tube, bronze skin fittings. There is a flex drive between the gearbox and prop shaft so the shaft and prop are electrically isolated. There are two anodes on the rudder, and one anode on the hull, which is bonded to the stern tube, and also bonded to the engine bed which is a common 12VDC negative connection point. The engine alternator's -ve path is through the engine block, so it is also connected to the engine bed via the battery negative (although it is on flex mounts so would otherwise be electrically isolated). I don't have an AC system or shorepower connection.

There isn't space on the shaft for a shaft anode, but there is line of sight from the prop to the two rudder anodes.

I cannot see the point of the bonding connection between the hull anode and the engine bed, as the engine bed isn't electrically connected to anything in the water. If I made a connection across the shaft coupling it would connect the shaft and propeller to the anode, but it would also connect them to the battery negative circuit which I have read is something to avoid. I think the only way I can avoid this would be installing brushes on the shaft.

As the stern tube is bronze, and painted every season, it isn't obvious that it even needs anode protection (and the associated risk of timber damage).

My planned change is to remove the connection between the hull anode and the engine bed, on the basis that it isn't protecting anything but it is creating a path from battery negative to the stern tube. The more radical option is to remove the hull anode. as it has caused some localised damage to the planking (which I have repaired). Would appreciate thoughts on both options as well as any others- although I only have 48 hours to decide!

 

[kittan]

If anyone can tell me the dimensions of the McDuff EE1 electro-eliminator that would also be helpful as I can put easily put a mounting bracket for it in while the engine is out. There don't seem to be any technical drawings here and I need the distance between the mounting post and brushes: mgduff.co.uk/products/EE1.

 

[Tranona]

Get rid of the lot including the ones on the rudder. You only need one for the propeller and shaft (and maybe not even that if it is an old style proper bronze propeller). Nothing needed for the stern tube - you are right it will likely lead to damage to the timber. Definitely nothing connected to the engine or beds, nor skin fittings.

For the propeller there are 2 choices. Simplest is a prop nut mounted anode, although these are rather small so may not be enough to last a season if your propeller is the common manganese bronze. Second as you suggest is a hull anode as close to the prop as possible bonded to the shaft using an electro eliminator. I have just had one fitted to my Golden Hind as I have a Featherstream which can get through an anode in less than a year. It is attached to one of the stern tube mounting studs with a modified cranked rod from the basic kit. Photos show the mounting inside and the external positioning of the anode. Would have liked it further aft but a fuel tank is in the way!

Hope this helps

 

[DoubleEnder]

My old boat was of very similar construction and materials. Over the years I got rid of all anodes except the one in the engine and one on the prop nut. I’m not sure the latter was much use, but I had a bad experience one winter in a marina, where the bronze prop nut suffered a large amount of corrosion. I never used that marina again so hard to know. But no anodes on the hull or the rudder and no problem.

 

[kittan]

Thank you both for helpful experience/ suggestions. I am inclined towards the electo-elminator/ shaft brushes route for the shaft/ prop protection, thanks for the photo Tranona. Is the rationale for removing the anodes on the iron strapping on the rudder that it is already pretty corrosion resistant? And likewise for the stern tube as it is bronze (I have a traditional stuffing box so sea water does pass through it)? As the prop is bronze I'm not clear why I would protect the prop but not the stern tube (much smaller surface area maybe?). Double Ender good to know I will likely be ok without any anodes, given the materials on my boat, but I think I am going to try to reduce rather than let go completely.... yet!

 

[Tranona]

It is not the material itself that is the problem but when 2 dissimilar metals are in contact in seawater. Prop/shaft are the obvious as the prop is lower potential than a stainless shaft so the galvanic action will deplete the zinc in the "brass" of the prop. introduce an anode and that depletes first. a bronze stern tube does not normally have any other metal connected to it in seawater unless possibly the outer housing has stainless fastenings, but even then they are usually set in sealant so not in electrical contact. Iron or steel rudder straps do not need anodes normally for the same reasons. If they corrode it is straightforward rust.

 

[kittan]

Thanks Tranona for the clear explanation. I thought anodes were used to protect against conventional corrosion as well as galvanic corrosion.

I've attached photos of the initial fit of the electro-eliminator bracket and then the final one. I will profile the brushes with a bit of sandpaper but the fit is pretty good. I made up the bracket mount with some stainless plate and nuts which I tacked together. I needed to fit the bracket before the engine went back in which is why I made up the model EE1 with some lollypop sticks and epoxy. The clamp for the actual EE1 is designed to go on a 10mm shaft so I used 8mm threaded rod and two layers of heat shrink tubing which gave a good fit and is insulated from the metal work around the stern tube. I still need to fit the bonding wire for it as the terminal on it is only 4mm diameter and the previous cable that had been on the stern tube was too big. Many thanks to Marine Superstore for measuring up the unit for me so I could make the model.





To isolate the engine bed from the 12V system (which the hull anode was previously bonded to) I used a Victron 250A DC bus with 8mm studs. There are cheaper ones on Ebay but the Victron one is stainless flat bar and studs.

Interestingly I spoke to MG Duff who recommended bonding the hull anode to the prop shaft via engine/gearbox as well as the electro-eliminator, even though this meant tying the hull anode circuit into the 12V electrics. I'm going to test the 'hull' (actually prop shaft) potential once the boat is back in the water to see if this is necessary but I doubt it will be and would rather avoid the further complexity.

 

[Tranona]

Looks very neat. Bonding to the engine/gearbox is very common as it can provide a good electrical connection to the shaft. However the brushes are usually sufficient. There is no problem with an external anode connected to the engine - it doesnot do anything in relation to the engine electrics - just a route to the shaft.

There is a lot of misunderstanding about galvanic action and corrosion in single metals or alloys that contain metals with different potentials. Cast iron keels are an example. They rust and people put anodes on thinking that will stop it. It won't but the anode might deplete because cast iron of the type that is used for keels contains impurities (other metals) and there can be local galvanic cells formed resulting in local pitting. an anode might reduce this, but won't stop the rust of the iron exposed to water.