Galvanic corrosion on saildrive

[WantTo]

I had my Marlow Hunter 40 hauled for bottom painting and there is serious damage to the saildrive, most likely due to galvanic corrosion. The zinc I had put on in late June was totally gone. I keep the boat at my dock in my backyard and do not keep it plugged in to shore power. None of my neighbors have a boat in the water nearby; all are on lifts. Is there a way I can test the water at my dock to see if there is stray electricity? I have a diver clean the bottom monthly, and he does not have any issues in the water. I am not sure why he didn't notice the corrosion or the zinc problem (he usually tells me when the zinc needs to be replaced), but the water is a bit murky in the summer.

[belizesailor]

Man, that sux.

Is there power to your dock?

Stray power does not have to come from an external source, it can also come from your own boat.

[Capt Clark]

WantTo, You may want to install a galvanic isolator.

Capt Clark

[MartinR]

I know many here will not agree with me on this, but in my opinion you should never ground your engine/saildrive to anything remotely connected with electricity.

For me the same goes for through hulls. The risks much outweigh the benefits.

[WantTo]

The boat is a 2014 model and has galvanic isolator; however, it may have been damaged by a lightning strike last year. I got all new electronics for that one. Geico handled that great and it only cost me $500. I don't know if the yard checked the isolator back then, but I am asking them.
I agree with MartinR.

[fstbttms]

Originally Posted by WantTo

I had my Marlow Hunter 40 hauled for bottom painting and there is serious damage to the saildrive, most likely due to galvanic corrosion.

Why do you suspect galvanic corrosion? Much more likely to be electrolytic (stray current) corrosion, which can damage aluminum very rapidly.

https://www.youtube.com/watch?v=88ax4YKe9H0

Originally Posted by WantTo

Is there a way I can test the water at my dock to see if there is stray electricity?.

Yes.

https://www.marinesurveypros.com/scs...20system%20and

Originally Posted by WantTo

I have a diver clean the bottom monthly, and he does not have any issues in the water.

It is unlikely that he would. Stray current in saltwater is usually unfelt by swimmers. Certainly people are not good detectors of stray current in saltwater.

Originally Posted by WantTo

I am not sure why he didn't notice the corrosion or the zinc problem (he usually tells me when the zinc needs to be replaced), but the water is a bit murky in the summer.

Might be time to find a different diver.

[WantTo]

You are correct. Stray current not galvanic. Not sure where my brain was earlier.

[CarlF]

fstbttms- that video is amazing.

The OP said he wasn't plugged in and no nearby boats. Is this due to DC current from onboard the boat?

How do you check for that?

[fstbttms]

Originally Posted by CarlF

fstbttms- that video is amazing.

In this case apparently there was a problem with the shorepower system and/or a neighboring boat. The owner changed marinas and the problem went away.

[boatpoker]

Originally Posted by fstbttms

Why do you suspect galvanic corrosion? Much more likely to be electrolytic (stray current) corrosion, which can damage aluminum very rapidly.

+1 from a retired Certified Corrosion Technician.
99.999% of the time, vessels suffering from stray current corrosion are the source of their own issue.
PS. OP says boat not plugged into shorepower therefore galvanic isolator has no effect on the situation, working or not.

[boatpoker]

Originally Posted by MartinR

I know many here will not agree with me on this, but in my opinion you should never ground your engine/saildrive to anything remotely connected with electricity.

For me the same goes for through hulls. The risks much outweigh the benefits.

Volvo will not warranty if the engine/saildrive is bonded.
Yanmar will not warranty if it is not bonded.

[boat driver]

check for an internal short in an invertor, or a stray/cracked wire sitting in bilge water.
sail drives are really susceptible to corrosion- so agree with not bonding the sail drive, but if rest of boat is bonded check the bonding connections. From past experience an internal shorted charger/invertor killed three drives before the unit was identified. Another vessel had a bonded system that the bonding was corroded from the zinc- and the saildrive was the next sacrifice.

[FabioC]

Originally Posted by MartinR

I know many here will not agree with me on this, but in my opinion you should never ground your engine/saildrive to anything remotely connected with electricity.

For me the same goes for through hulls. The risks much outweigh the benefits.

There is a school of thought that does not recommend grounding and is aligned with your position. In fact, some production boats (e.g., Jeanneau's) are not grounded.

[Capn Rick]

Look for DC stray current, this will most likely be coming from your boat. Naturally occurring galvanic corrosion is between two dissimilar metals submerged in an electrolyte (salt water) and is measured in millivolts (thousandths of a volt). Higher voltage is stray current and can destroy underwater metals in a matter of weeks (sometimes days).

A galvanic isolator protects your boat from low levels of DC current coming thru the green grounding wire when plugged into shore power. It does nothing if you are not connected to shore power.

With a digital multimeter check the engine block for voltage by placing the red lead on the block and the black lead on a battery negative post. You can also check the bilge water by sticking the lead in the water. If you have DC voltage, try turning on and off DC breakers, on/off switches and battery switches to isolate the source. The most likely source for stray DC current is faulty wiring or a failed DC appliance. A good place to start is the starter solenoid and the alternator.

The source of stray current can sometimes be hard to find. If you had a recent lightning strike it is possible the stray current is related.

[guyrj33]

How would you be able to keep the sail drive isolated? Isn't it connected to the engine which in turn is connected to the starter motor and battery?