Saildrive anode and corrosion

[nigelmercier]

Until this year we have changed the anode on our Volvo Penta 120S saildrive every year, when we lifted out. It was never more than 50% gone. This year we left it for about 18 months, thinking we would be OK, but we have discovered the anode is virtually all gone, and the saildrive leg and the propeller are corroded.

We should have checked it, but I'm trying to rationalise what went wrong, and have come up with this list, in no particular order:

Now in the Atlantic since May, not the Mediterranean.
We now often leave shore power connected, but we have a GI.
A recently added battery charger was found to connect boat mains earth and battery 0V (but see above).
The last anode was not a Volvo Penta part, certainly lighter and possibly inferior.

Any thoughts?

 

[minkysailing]

Nigel,

I don't think you should over-analyse too much, yes the change in location could be a factor, yes the anode fitted may have be inferior to the original VP fitted, it may be the new marina you are in with vessel in close proximity that are affecting your vessel's anodes. With a GI fitted I do not think the recent battery charger install or shore power connection should be a problem. I suspect you will need to closely monitor the vessel with new anodes fitted and possibly fit a hull anode if one os not already fitted.

 

[VicS]

How does the salinity of the Med compare with the Atlantic .... Presumably no less but may be worth looking up somewhere.
Atlantic presumably colder water so that should not be an explanation.

Leaving shore power connected may be a problem. Need perhaps to check the operation of the GI.

leaving anything switched on that could be putting a current into the earth circuit and causing the GI to be conducting. Modern switch-mode battery charger for example.

Puzzled by your remarks about the charger connecting DC 0V to earth. I'd have expected the DC output to be isolated .

I'd expect the shorepower earth to be bonded to the DC negative (0V if you prefer). except where the boat has an isolated DC system... which you may have ??

BUT is the saildrive leg not isolated from the engine block anyway? Later ones are... not sure about the 120S.


Mass of the anode is the important factor re expected life. If its lighter due to there being less zinc then its not going to last so long!

If it was lighter but physically the same size any chance that it was aluminium or even magnesium..

Aluminium would be OK and could be a lot lighter because not only is the RAM of Al a lot less than that of Zn but its also trivalent while zinc is divalent

magnesium would be far to reactive in salt water and be lost very quickly.


Just thoughts...

 

[ytd]

we have a 120s saildrive and the collar anode has corroded more every year. The older the boat the more corrosion. No idea why. Our anode used to corrode only 25%. A couple of years ago this had increased to 50% and last year 75%. When I questioned the VP mechanic about this he put it down to different antifoul. With the saildrive leg touching the antifouled fairing skirt maybe there was increased conductivity. The 3 small anodes on the folding prop have always corroded 75% pa.

 

[DaveS]

we have a 120s saildrive and the collar anode has corroded more every year. The older the boat the more corrosion. No idea why. Our anode used to corrode only 25%. A couple of years ago this had increased to 50% and last year 75%. When I questioned the VP mechanic about this he put it down to different antifoul. With the saildrive leg touching the antifouled fairing skirt maybe there was increased conductivity. The 3 small anodes on the folding prop have always corroded 75% pa.

I read somewhere that it's worth painting the rubber hull fairing with the same hard antifouling as the leg. Since starting to do this I've been consistently getting two seasons out of the anode before it gets down to c. 50% - 60% left.

I've currently got my leg on the bench, changing the seal (a pointless, guilt driven exercise - the old one looks as good as when I fitted it - as does its predecessor) and I've taken the opportunity to apply hard antifouling to the water passageways. Hopefully this will dissuade things like the small mussel I found jammed into the base of the sea cock and half blocking it.

 

[garethmking]

Saildrive anodes

I have owned a boat with the 110S drive for 30 years and been in the med, carib and pacific. I have changed the anode annually but some times stretched to 18months. The rate of depletion of the anode seemed to vary with the waters and warmer water was worse. Also stray currents in marinas with steel piling is to be avoided. Barnacle growth eventually blocked the water intake and I was obliged to put some muriatic acid down the leg to clear it. Had no option. Leg has been fine with no water appearing in the oil and no external corrosion but the possiblity of corrosion inside the leg is a concern. Some boat builders are now sealing the water intake in the leg and using a conventional skin fitting for the raw water inlet. I never experienced any problem with the seal and I swear by the Sdrive compared with the conventional stern tube. All the wiring on the boat is double poled ie the engine block is not used for the -ve return. Most alternators are not double poled and thus will create a voltage differential in the engine block. Cheers, gareth

 

[haydude]

How does the salinity of the Med compare with the Atlantic .... Presumably no less but may be worth looking up somewhere.
Atlantic presumably colder water so that should not be an explanation.

The average salinity of the Mediterranean Sea is higher than the Atlantic, and increases going towards the Eastern Mediterranean. This is due to the evaporation that removes a greater amount of water than what can be replaced by rain and rivers that on their end bring more salt and other elements taken on their path from mountains' rocks and soil.

Someone who have had a swim in the crystal clear waters of Greece will have felt the skin hitching under the sun and seen white marks which is the effect of salt drying on the skin. The same does not happen if we have a swim in a Tropical Ocean Island.

 

[haydude]

BUT is the saildrive leg not isolated from the engine block anyway? Later ones are... not sure about the 120S.

Aluminium would be OK and could be a lot lighter because not only is the RAM of Al a lot less than that of Zn but its also trivalent while zinc is divalent

magnesium would be far to reactive in salt water and be lost very quickly.

I understand that Nigel has a Bavaria and the 120S is NOT connected to negative or ground by the manufacturer.

Since the saildrive leg is Aluminium, I believe that the Anode should NOT be of the same metal. In salt water ZINC is just fine.

Nigel,

Is your propeller Bronze or Aluminium? If the former, is it electrically isolated from shaft and saildrive? It MUST be or it will use the saildrive as its anode once it has finished its own zinc anodes.

VP folding propellers have the hub electrically isolated. Some other manufacturers might overlook that aspect and do not do it putting at risk the saildrive.

Mains Earth and battery negative connected is NOT recommended, if you cannot modify the battery charger replace it with one that isolates the mains from 12V completely.

 

[VicS]

Since the saildrive leg is Aluminium, I believe that the Anode should NOT be of the same metal. In salt water ZINC is just fine

Aluminium anodes are not pure aluminium. They are an alloy containing some zinc and crucially a very small percetage of indium.
They have an electrode potential that is in fact slightly more negative than zinc anodes

 

[Mrnotming]

Good posts,
just to add that I have used up two Volvo props so far,(first one in right holy state) as as soon as the epoxy paint is breached, the prop becomes as attractive as the real anode as regards material loss.There is a rubber bushing and a plastic washer,latter which must be present.eBay provides fresh supplies from time to time as dealers or individuals swop to the bronze or other types of folders.I stick to the two blade fixed alloy prop as supplied.
The 120SE leg & gear unit is isolated at the bell housing, and must remain so.
I avoid as an OP says, log term berths near steel pilings as anode loss seems dramatic in these locations.
I keep the boat ashore last good few years near Marseille, launching as needed.
Otherwise, if afloat mostly, I would have to fit that box of diodes thingy,(galvanic isolator) which is not difficult really,but I'm not sure if it would solve the problem of an adjacent boat having current leakage?
The S drives are quiet running as regards vibration etc, but the donut replacement is best left to an expert unless you have dabbled in mechanics via bikes and cars as a youth! The work might be done in one day but I seem to need a bit of the next day as well!

 

[VicS]

Otherwise, if afloat mostly, I would have to fit that box of diodes thingy,(galvanic isolator) which is not difficult really,but I'm not sure if it would solve the problem of an adjacent boat having current leakage?

A galvanic isolator should protect you from galvanic corrosion eg resulting from steel piles etc.

It wont however protect you from electrolysis caused by defective 12 volt installations on your own or another boat.

 

[nigelmercier]

Thanks for the replies. I've had a further thought: as you may remember, my immersion heater has been faulty for some months. If this was left switched on, I imagine that a small stray current (less than 30mA) could have flowed into the boat earth, which was connected to 0V via the non-isolated charger. Or perhaps even a neutral fault which could have produced current to earth even if the heater was not powered.

I'm 99% sure that the depleted anode was zinc, I certainly ordered zinc. My weight comparisons were made with new anodes from the same supplier.

I'm going to contact the supplier of the new 24V charger and complain about the 0V being earth bonded, but I still think the GI should have removed this risk. I have tested it with 12V and a series bulb, it measures about 1.4V in each direction as expected.

Although the engine has the requisite stop/start relays that imply it is isolated from the 0V, I'm pretty sure that it never has been, the alternator that was fitted for the last 7 years was grounded for a start. I removed the saildrive a couple of years ago, I didn't notice any form of isolation. Perhaps this is the problem, what does the isolation look like?

 

[VicS]

Thanks for the replies. I've had a further thought: as you may remember, my immersion heater has been faulty for some months. If this was left switched on, I imagine that a small stray current (less than 30mA) could have flowed into the boat earth, which was connected to 0V via the non-isolated charger. Or perhaps even a neutral fault which could have produced current to earth even if the heater was not powered.

I'm 99% sure that the depleted anode was zinc, I certainly ordered zinc. My weight comparisons were made with new anodes from the same supplier.

I'm going to contact the supplier of the new 24V charger and complain about the 0V being earth bonded, but I still think the GI should have removed this risk. I have tested it with 12V and a series bulb, it measures about 1.4V in each direction as expected.

Although the engine has the requisite stop/start relays that imply it is isolated from the 0V, I'm pretty sure that it never has been, the alternator that was fitted for the last 7 years was grounded for a start. I removed the saildrive a couple of years ago, I didn't notice any form of isolation. Perhaps this is the problem, what does the isolation look like?

If you had a small leakage to earth ( too small to trip the RCD) it would, because it was from a high voltage source, have "swtiched the GI on" so that it would then not have protected you from the "galvanic" effects that it otherwise should do.

The same can happen with modern 240 volt equipment that dumps RF into the earth. There is something about this in one of the Tech Info pages on the Samartgauge website

If you are measuring 1.4 volts across the GI that is worrying .. thats the level at which it will conduct. Maybe thats not quite what you mean though.

If the engine is supposed to be an isolated type the alternator will have an insulated negative terminal and the temperature and oil pressure sensors will be the two wire type. However because the glowplugs and starter probably use the engine block for the negative there will be , in addition to the starter and glowplug relays, an earthing relay ( a heavy solenoid in fact) that connects the block to battery negative during t preheating and cranking.

If the sail drive is isolated then there is, IIRC, an insulating joint and insulating spacers around the fixing bolts.

 

[grumpygit]

If you had a small leakage to earth ( too small to trip the RCD) it would, because it was from a high voltage source, have "swtiched the GI on" so that it would then not have protected you from the "galvanic" effects that it otherwise should do.

The same can happen with modern 240 volt equipment that dumps RF into the earth. There is something about this in one of the Tech Info pages on the Samartgauge website

If you are measuring 1.4 volts across the GI that is worrying .. thats the level at which it will conduct. Maybe thats not quite what you mean though.

If the engine is supposed to be an isolated type the alternator will have an insulated negative terminal and the temperature and oil pressure sensors will be the two wire type. However because the glowplugs and starter probably use the engine block for the negative there will be , in addition to the starter and glowplug relays, an earthing relay ( a heavy solenoid in fact) that connects the block to battery negative during t preheating and cranking.

If the sail drive is isolated then there is, IIRC, an insulating joint and insulating spacers around the fixing bolts.

Just to add to the issue. I have recently heard of a similar instance on a Bav. The anode has completely eroded, the leg and prop is heavily corroded. The boat has not got or ever has had a ground plate to the hull. I know not whether this is a contributory issue but most boats I know do have the ground plate fitted. This is the first year it has happened to this boat and it sails five days out of seven with the only difference this season is the antifoul, hard coat with maximum copper content.

 

[VicS]

Just to add to the issue. I have recently heard of a similar instance on a Bav. The anode has completely eroded, the leg and prop is heavily corroded. The boat has not got or ever has had a ground plate to the hull. I know not whether this is a contributory issue but most boats I know do have the ground plate fitted. This is the first year it has happened to this boat and it sails five days out of seven with the only difference this season is the antifoul, hard coat with maximum copper content.

I dont think the absence of a ground plate is part of the issue.
They are only relevant to the use of certain types of radio equipment or lightning conductors aren't they.

 

[grumpygit]

I dont think the absence of a ground plate is part of the issue.
They are only relevant to the use of certain types of radio equipment or lightning conductors aren't they.

I'm just throwing my three pennyworth in Vic. Is this a common occurrence with sail drives and not just Bavs? There seems there might be a common link somewhere and it's just a case of having all the information at hand. "Aren't they" (Bavs) immune to lightning strikes and radio equipment, if not why no ground plate?

 

[VicS]

I'm just throwing my three pennyworth in Vic. Is this a common occurrence with sail drives and not just Bavs? There seems there might be a common link somewhere and it's just a case of having all the information at hand. "Aren't they" (Bavs) immune to lightning strikes and radio equipment, if not why no ground plate?

Dont know.
I've only ever seen ground plates mentioned in connection with SSB radios. (I assumed then connected directly to the earth terminal of the radio rather than the Dc negative and anodes etc.) and with lightning conductors



Something new you have thrown into the discussion and which I will now have to find out about.

 

[Nostrodamus]

I know when we bought our boat and had it surveyed the previous owner had it taken from the Med back to the UK. It may have been the end of the season so it was in the water for longer than its usual haul out and anode change. As a result the saildrive leg anode had worn so much that there was pitting on the prop and saildrive. The surveyor said both prop and saildrve needed replacing otherwise there was a risk of failure.
This cause problems as they do not make 120 legs anymore. They are now the 130 which has different gearing making it difficult to match a prop.

I was told the saildrive anode should be changed each year. At least with the 130 it can be done in the water as it comes in two half's where as the 120 was a one piece thing.

Why don't they make an oversized anode that last longer than a year?

 

[grumpygit]

Dont know.
I've only ever seen ground plates mentioned in connection with SSB radios. (I assumed then connected directly to the earth terminal of the radio rather than the Dc negative and anodes etc.) and with lightning conductors



Something new you have thrown into the discussion and which I will now have to find out about.

As far as I'm aware the ground plate is wired to the boats common grounding system, and certainly on ours. I always presumed any leakage in or out was to prevent it from conducted through shaft/prop/leg. What effectual capacity it has to this leg/prop corrosion I know not, I may be way of base with my presumption though. I'm sure you will come up with the solution though, as always.

 

[grumpygit]

I know when we bought our boat and had it surveyed the previous owner had it taken from the Med back to the UK. It may have been the end of the season so it was in the water for longer than its usual haul out and anode change. As a result the saildrive leg anode had worn so much that there was pitting on the prop and saildrive. The surveyor said both prop and saildrve needed replacing otherwise there was a risk of failure.
This cause problems as they do not make 120 legs anymore. They are now the 130 which has different gearing making it difficult to match a prop.

I was told the saildrive anode should be changed each year. At least with the 130 it can be done in the water as it comes in two half's where as the 120 was a one piece thing.

Why don't they make an oversized anode that last longer than a year?

Well Mark they might do when they make antifoul last longer and we don't have to be lifted annually. Our Copper Coat users will probably tell us how they go on with their anodes. From my experience there is a difference in the manufacture of anodes. I have used Zspars for a couple of years and they only just made the season of around 8 months. This year I used a Plastimo shaft anode and a McDuff 4kg hull anode and I will get another season out of these without doubt. Saying this, as far as McDuff is concerned it's reflected in the price!