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Sacrifical Anode Corrosion

My boat uses a Volvo Penta saildrive.  Volvo's anti corrosion system depends on the saildrive being electrically isolated from the engine.  They actually use plastic washers on the bolts to ensure this.  I've checked and the saildrive is correctly electrically isolated.

However, I've been seeing rapid corrosion of the 3 piece anode just forward of the prop.  Strangely, the larger two piece anode on the saildrive isn't being affected.  The boat is in fresh water and I'm using magnesium anodes.

I've asked my club to check if there are stray current either from another boat or the electrical distribution system.   The boat is usually left plugged into the nearest power pedestal which might be an issue.

However, I'm really puzzled why only one set of anodes is being affected and not both???  Any ideas?

I have a VP in salt water and the three piece anode seem to deteriorate quicker than the one piece anode that goes in front of the prop.  The three piece anode is replaced every year while the bigger (one piece) in my case usually lasts 2 - 3 years.  At another club where they was a lot of stray current people used to suspend bigger sacrificial anodes off of the wharf.  In my case I just felt the smaller anodes ate away quicker because they were smaller and it took longer to eat away at the bigger much thicker anode.

I hadn't thought much about it until now but there was something else different with the lower unit besides the fast erosion of the three anodes on the prop.  The bronze prop is being coated with some kind of deposit.  It's only on the bronze not the vertical aluminum of the lower unit.  The deposit is light coloured, very rough and difficult to remove.  It's really bonded to the bronze.  I wiped some acidic water line cleaner on the prop and the deposit foamed and dissolved.  The bronze hasn't been affected.

I originally thought this was some kind of algae (the boat is docked in a backwater lagoon with lots of weeds) but I'm thinking the erosion of the magnesium anodes on the prop and the accumulating deposit on the bronze is related.  Am I exchanging my magnesium anodes for someone else's lower unit???

A couple of thoughts on the above posts:

1. Obviously stray current leak from dockside pylons can be an issue: checking for ground faults may uncover a problem that can be fixed by your marina

2. One of your neighbors in the vicinity of your boat may have a corrupted power cord draped in the water

3. If someone near to your boat dropped a 12V battery over the side by accident, I have heard that can accelerate anode degradation

4. You can try hanging a zinc fish (or similar for your location/situation) attached to your shrouds, which are grounded to your keel.  We do this and the zinc fish does erode over time, likely saving anodes on the prop

5. Assuming you have already checked the isolation of your transmission/saildrive from the engine, these might be the next steps in correction.  In the Volvo MD202, I am thinking that the only way the metal of the engine is isolated from the inner gearing of the motor is via the rubber "damper" between the engine and the transmission.  There is also a gasket that isolates between the engine casing and the transmission casing.  Perhaps something has compromised the damper in your engine?

When I had our boat in NJ, there was a Farr 395 with a saildrive that had excessive degradation of the saildrive casing (Aluminum) even when she was moored out in the harbor.  The owner kept using Bondo to patch up and smooth the saildrive casing.  Oy!

An update...by the way, the boat is in fresh water not salt.  I use manganese anodes instead of zinc.

  • We checked for stray voltage using a multimeter with the ground lead connected inside the boat to the lower unit and a special silver probe on the positive lead dropped over the side a few feet down.  We measured 1.29 volts DC.  The marine mechanic said any reading over 0.5 volts is a problem.
  • I took the boat out into the lake and checked again.  The multimeter read 0.019v DC so there's something definitely wrong at/near my dock.
  • The club brought in their electrical contractor to check the shore power.  The contractor responded in writing that was everything was working correctly.
  • We recycled the power of a nearby locker building without changes.
  • There are no boats connected to shore power within at least 100 feet.  It makes no difference whether my boat is plugged in.  It also doesn't matter whether the engine is running.
  • I confirmed the lower unit on the saildrive is electrically isolated from the boat ground.  In fact, the boat has a galvanic isolator that's working properly.

The current consensus is a nearby boat has a wiring issue.

Note that we're measuring DC voltage.  I found the comment about someone dropping a battery over the side interesting???

Also, the deposit on my bronze prop assembly is likely the manganese from the disappearing anode.

I have a manganese anode bar from another boat.  I've wired that to ground and dropped it in the water beside the boat.  It will be interesting to monitor it over the next week or so.

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