Earth Bonding to Zinc Anode - Advice Required

[Deleted member 36384]

Hello,

I fitted a new propellor as the old one had dezinced and sounded dull. The 10 year insurance survey commented on that and when the surveyor looked at the propellor with me present, a small section of the tip broke off exposing pink metal and it rang dull. Now my new propellor, 2 years old has similar markings, they look like barnacle marks, and the ring is now dull.

Clearly there is something wrong. Anodes on my boat deplete very slowly, new 2 years ago, has almost zero depletion and is in direct line of sight with the propellor; no space for a shaft anode.

The boat is fitted with a Halyard Marine CV joint / thrust bearing which I believe may isolate the propellor from the engine. I have looked around my engine and there is no ground cable to the anode from the engine block. The anode is wired to both the stern shaft greaser and rudder shaft greaser, but that is it. I have 240V fitted with a galvanic isolator. The only new thing that has been installed is the Sterling Pro Charge Ultra (PCU). The PCU 240V is wired with the earth as expected in the 3 mains plug but there is an earth screw on the case, that has nothing attached to it. It is point 2 below from the installation manual.

Grounding / Earthing
Ground / bonding / earthing points:
1) The earth wire (AC input, the ground).
2) The chassis / bonding ground (going to a vehicle body / boats bonding system).
3) The DC negative.
In most installations all these will end up at the same point. The AC power source should be connect to the boat/vehicle chassis (for safety). The chassis earth will also go there and the DC negative should also go there. In effect, bonding the total system together ensuring any fault to the chassis will blow a fuse. This could vary for steel/aluminium boats.

My query: -

1. Should the engine block be connected to the anode? The starter battery negative, via the starter motor is connected to the block.
2. Should the domestic battery negative be connected to the engine block, if I check and find that it is not, likely is via a big box where all the heavy black cables are connected to a common negative bus? The domestic and engine batteries are linked via an isolator switch via the red + cables, so I assume that the domestic batteries negative is common via the bus. Engine sits on slim line mounts with rubber cushions but studs that connect the steel bearers to the engine. The bearers are isolated, sitting on GRP.
3. When I check the galvanic isolator, should I expect to see a cable to the zinc anode on the hull, or to the negative bus?
4. I have an electro isolator for rotating shafts. I am going to fit this onto the shaft between the Halyard CV coupling and propellor and connect that to the zinc anode. Should I also try and bridge the CV / Thrust bearing coupling so that both shafts halves are connected?

I have been reading up on this in Calder's tome and he shows the negative bus connected to the zinc anode but not the engine block, so I am not sure about that. He also shows the electro eliminator connected to the zinc anode, similarly the galvanic isolator but it is not clear how that should be wired to me. The 240V was installed professionally before my ownership, and looks to be all good and proper with circuit breakers and RCD units. The galvanic isolator sits to the side of the consumer unit with an earth cable each side. I believe shore side of the galvanic isolator is connected to the shore power socket negative, and the consumer unit side of the galvanic isolator is connected to the consumer unit earth. I have not opened these up to look. I don't know if the galvanic isolator has a connection to the boats negative bus, but Calder suggests it should be.

One observation: all my seacocks are Blakes from about 1974, not connected to the zinc anode and they are sound, with the surveyor scraping back and inspecting at the last 10 year survey.

Any advice appreciated.

Thanks, BlowingOldBoots

 

[Tranona]

No, the anode should not necessarily be connected to the engine, although this is often done using the gearbox as a path to the shaft and propeller. However as you correctly point out the Aquadrive probably isolates the shaft from the gearbox.

To protect your prop from galvanic action with the stainless shaft you need an anode connected to either. To use the shaft you may be able to use the MG Duff electro eliminator which is on p23 of their catalogue in the link attached. Not always easy to fit though depending on your installation. A shaft anode is probably out of the question because the type of stern gear you have does not leave enough exposed shaft, but you may be able to fit an anode on a special prop nut. The problem you have probably stems from a poor quality alloy commonly used in propellers, so when replacing the prop you might talk to specialists like Hamble Propellers or Teignbridge Engineering about having a prop cast in a better alloy. Alternatively fit a feathering prop as usually they are made of better alloys, but because they use stainless in the gears, have their own anodes.

Don't think the galvanic action between the prop and the shaft is anything to do with the shorepower system. Blakes seacocks (nor any other seacocks) need anodes in a GRP boat.

Lots of useful information on the subject here
mgduff.co.uk/downloads/Leisure%20Catalogue.pdf

 

[JumbleDuck]

Can you measure the resistance between the anode and the propeller? I suspect that a stern shaft greaser is not a terribly good electrical connection.

 

[john_morris_uk]

The anode needs both to be in close proximity to the propeller and electrically connected with a low resistance connection.
As others have said, measure the resistance (it should be down in the fractions of an ohm to one or two ohms max) and if it’s high fit one of these on the shaft. MG Duff Electro Eliminator for Shafts up to 50mm
Other suppliers available.

(obviously you then connect it to the anode!)

 

[TernVI]

Don't worry about the internal stuff like the engine, the anode needs to be solidly connected to the prop and physically close to it.
Ideally a shaft anode a few inches from the prop, or a prop nut anode.
The less other crap connected to the prop's anode, the better for the prop IMHO.
If you can fit a second anode and wire the galvanic isolator to that, the prop might stand a chance.

 

[PaulRainbow]

Don't worry about the internal stuff like the engine, the anode needs to be solidly connected to the prop and physically close to it.
Ideally a shaft anode a few inches from the prop, or a prop nut anode.
The less other crap connected to the prop's anode, the better for the prop IMHO.
If you can fit a second anode and wire the galvanic isolator to that, the prop might stand a chance.

Sorry, but the galvanic isolator needs to be fitted to the shore [ower earth, before it connects to anything else.

 

[Deleted member 36384]

Thanks everyone for the advice. First job is to fit the electro eliminator, which is on the boat and wire to the anode. This winter I'll look at fitting a shaft nut anode weld on stud and nut anode. Something as below: -

Tecnoseal Weld On Zinc Plated Stud 10mm
50mm anode for hexagonal propeller nut in zinc 8033837061731 | eBay

Pretty sure the galvanic isolator will be wired correctly. Your comments do suggest that the whole issue is with the lack of anode connectivity to the prop shaft. The CV joint was fitted 11 years by a yard in first year of ownership.

As always, thanks for your advice.

 

[PaulRainbow]

No, the anode should not necessarily be connected to the engine, although this is often done using the gearbox as a path to the shaft and propeller. However as you correctly point out the Aquadrive probably isolates the shaft from the gearbox.

To protect your prop from galvanic action with the stainless shaft you need an anode connected to either. To use the shaft you may be able to use the MG Duff electro eliminator which is on p23 of their catalogue in the link attached. Not always easy to fit though depending on your installation. A shaft anode is probably out of the question because the type of stern gear you have does not leave enough exposed shaft, but you may be able to fit an anode on a special prop nut. The problem you have probably stems from a poor quality alloy commonly used in propellers, so when replacing the prop you might talk to specialists like Hamble Propellers or Teignbridge Engineering about having a prop cast in a better alloy. Alternatively fit a feathering prop as usually they are made of better alloys, but because they use stainless in the gears, have their own anodes.

Don't think the galvanic action between the prop and the shaft is anything to do with the shorepower system. Blakes seacocks (nor any other seacocks) need anodes in a GRP boat.

Lots of useful information on the subject here
mgduff.co.uk/downloads/Leisure%20Catalogue.pdf

• The incoming shore power Earth connects to one end of the GI and a cable goes from the other end to the Earth busbar in the consumer unit (not the neutral).
• You should have a cable from the Earth busbar to the anode (in the absence of a steel hull or a ground plane).
• There is no need to connect the DC negative to the node.
• You will need to make sure the anode has goo electrical conductivity with the shaft/prop. This could be the electro eliminator already mentioned, or if there is room, it's accessible and you have a good memory, you could have a flying lead from the anode to the shaft with a clip/clamp to connect it. Must remember to fit and remove it as appropriate.

 

[PaulRainbow]

Thanks everyone for the advice. First job is to fit the electro eliminator, which is on the boat and wire to the anode. This winter I'll look at fitting a shaft nut anode weld on stud and nut anode. Something as below: -

Tecnoseal Weld On Zinc Plated Stud 10mm
50mm anode for hexagonal propeller nut in zinc 8033837061731 | eBay

Pretty sure the galvanic isolator will be wired correctly. Your comments do suggest that the whole issue is with the lack of anode connectivity to the prop shaft. The CV joint was fitted 11 years by a yard in first year of ownership.

As always, thanks for your advice.

If you fit the electro eliminator i doubt you'll need anything else. In my experience, the nut anode will vanish relatively quickly and you'll be relying on the hull anode anyway.

 

[Tranona]

• You will need to make sure the anode has goo electrical conductivity with the shaft/prop. This could be the electro eliminator already mentioned, or if there is room, it's accessible and you have a good memory, you could have a flying lead from the anode to the shaft with a clip/clamp to connect it. Must remember to fit and remove it as appropriate.

Given that most boats spend their life stationary in the water, most galvanic action occurs when the boat is not moving. So a hanging anode clipped to the output coupling of the Aquadrive could work well - provided you remember to unclip it when on the move. Anodes on the prop will go first because they are closest. This is one of the problems with saildrive housings - even if you add another anode to the housing, either a hull anode or a hanging anode it only comes into play when the ring anode is depleted. Useful if you are going long periods without being able to change the main anode, but does rely on a good connection and the extra anode being close to the drive housing.