Grounding to reduce electrolysis

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SimonD

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The ring anodes on my VP 290 DP-E drives are eroding rather too quickly. The last pair disappeared completely in nine months resulting in some pitting on the stainless steel duo props. I replaced the anodes five weeks ago and have not left the boat with the mains cable connected. I had a look at them yesterday and they are already showing signs of corrosion. Given this high rate of erosion, I suspect there is some electrolytic corrosion at work.

I've tested the galvanic isolator and it seems fine. Next step is to check for stray current in the bilge. The bilge pump switch doesn't work and I suspect it's failed partly closed allowing some current to pass (reed switches can do this). I'll also check the grounding of the legs. I assume that if the legs have good continuity to ground, this effectively stops electrolysis. This gives rise to my question: assuming the legs are properly grounded to the battery negative, does it help prevent electrolytic corrosion if the shorepower lead is connected, i.e. is it better to have the boat's ground connected to the marina supply earth?
 
The fault doesn’t neccessarily have to be on your boat, it can be a nearby boat causing the problem. We had a Princes V 42 that had such severe corrosion on the sterndrive mounts that both brackets had to be replaced. The fault was in the boat next to him which had bad wiring and was leaking current into the water from the shorepower, which then went straight to the nearest piling and the V42 was in its path. We had a specialist take measurements around the boat and found where the problem lay.
 
The GI only stops galvanic current flow, not electrolytic flow from your own boats systems.

I learnt from Nigel Calder the best way to seek an internal electrolytic leak from your own DC system is to turn all services off except those left on when the boat is out of normal service, e.g. the bilge pumps.

Then disconnect the +VE cable at the battery, then measure the DC voltage between the battery +VE terminal and lead just removed. Any reading above 0v means there is a leakage. Number 1 culprit will be the pump flappy switch/es shorting to salty bilge water.

Another possible cause is if your boat is forming a pathway to a stray current along your bonding circuit. The bonding circuit will likely be a lower resistance than the seawater surrounding the boat, so this can encourage stray currents to hop a lift through your bonding, and they thank you by stealing ionised material at the anodic exit point.

I have some excellent books on galvanic and electrolytic corrosion (they are similar effects but not the same thing). PM me if you would like the links.
 
I've now checked the earth to the port engine (the starboard is impossible to reach) and it's OK. I then disconnected the battery as suggested, but don't quite understand the theory of measuring the voltage. I did it anyway and got full battery voltage. I then switch to amps and, with the batteries switched off, got a current from the battery of 0.230mA. Tripping all the circuit breakers didn't have any effect. With the batteries on, the current draw rose to 197mA. Most of this was drawn by the VHF and the rest by the radio.

So, I can only think that there is a small stray current drain from somewhere 'upstream' of the circuit breakers. I'll have a look at the boats wiring diagram for clues. Meanwhile, I've taken the precaution of hanging a couple of anodes of the back of the boat as near to the props as possible. It will be interesting to see if these erode.
 
SimonD said:
I've now checked the earth to the port engine (the starboard is impossible to reach) and it's OK. I then disconnected the battery as suggested, but don't quite understand the theory of measuring the voltage. I did it anyway and got full battery voltage. I then switch to amps and, with the batteries switched off, got a current from the battery of 0.230mA. Tripping all the circuit breakers didn't have any effect. With the batteries on, the current draw rose to 197mA. Most of this was drawn by the VHF and the rest by the radio.

So, I can only think that there is a small stray current drain from somewhere 'upstream' of the circuit breakers. I'll have a look at the boats wiring diagram for clues. Meanwhile, I've taken the precaution of hanging a couple of anodes of the back of the boat as near to the props as possible. It will be interesting to see if these erode.
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I doubt your hanging anodes will help at all as you have found a definite DC leakage current. You now need to trace it by disconnecting everything and then re-connect one circuit at a time until you find the culprit. To reduce the hassle start with the bilge pump float switches, but do not rule out the fresh water pump and control switch.

Also for the hanging anodes to work they must be connected to the bonding circuit.
 
Check that you have electrical isolation between the prop hub and the blades, the ss steel in the props should be isolated with plastic from the rest of the drives. If they are not the ring anodes will go very fast. There is a metal ring at the prop hub that very easily breaks the intended isolation.
 
According to the boat's wiring diagram, the domestic battery bank is connected directly (i.e. before the isolation switch) to the battery charger and split charge relay. It is also connected through circuit breakers to both bilge pumps and the shower sump pump.

I can't imagine that the battery charger would take any current when not connected. It's a relatively new Sterling charger. The bilge and shower pumps have circuit breakers in the main circuit breaker box. I tripped these with no result. However, there appears to be a second set of breakers which do not appear on the circuit diagram, but are located on a separate panel next to the battery isolation switches. They're a type I'm not familiar with being a white button under a clear silicone membrane. Annoyingly, they cannot be tripped manually. I'll have to short the supply wire to trip them and see what happens.

Thanks for the tip about the isolation. I'll certainly check that when I'm next lifted out. However, I'm a bit hazy on this issue. If the props are not isolated from the hub (and therefore the legs I assume) why does this erode the ring anodes? Come to that, why do these anodes usually erode more quickly than the other anodes?
 
Its the same rationale your anode needs to have a good electrical connection to the leg in order to work as a sacrificial anode but in reverse so to say. Generally the ring anodes are small/don't have a lot of mass in relation to what they are protecting so will erode "quicker."
 
I was chasing the same issue for years and in the end it was the ss prop hub isolation that had failed. Once sorted the ring anode lasted ok for maybe 6-8 mths vs 3-4 mths before but later I changed to alu props and the ring anode was still good after 9-12 mths and everything else equal (same place, shore power connected and the same neighboring boats). I did add additional anodes to the leg before I figured it out and it helped.
 
Thanks again; that's helpful. Reading various sources, I see that some people recommend aluminium anodes rather than zinc. I would have thought we're in salt water in Weymouth marina, but the harbour is fed by the river Wey (of course!). There might, therefore, be some dilution of the salt content. I wonder if I should switch to aluminium anodes (after checking the isolation of the props as you say. What do you think?
 
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