The real damage from current leakage

[Skipper]

See pics.
This is a drive shaft from a 35(ish) foot Tiara express in our harbor.
Snapped like a twig.
Diver had been telling owner there's a corrosion problem. Owner ignored.
Turned out to be AC current leakage. Zincs unable to handle it.
$6,000.

 

[MitchK]

Any idea what the cause of the current leakage is/was?

 

[Maine Sail]

It would take years & years & years of AC leakage to cause that type of damage and I would strongly doubt you even could. DC on the other hand, that type of damage can occur in days or weeks... The owner of that boat had better start looking for the DC leak or hire a professional actually trained in marine corrosion.

 

[weinie]

It would take years & years & years of AC leakage to cause that type of damage and I would strongly doubt you even could. DC on the other hand, that type of damage can occur in days or weeks... The owner of that boat had better start looking for the DC leak or hire a professional actually trained in marine corrosion.

Interesting.
Why does AC do less galvanic corrosion than DC despite the much higher potential?

 

[Skipper]

Any idea what the cause of the current leakage is/was?

The diver told me refrigerator AC. Although it's possible it's fridge DC, looking at MS comments.
The diver was there to help rem the shaft in water.
In any case the diver said mechanics identified the problem, after the owner took the bruising from the drive shaft estimate.
The diver may have just misinterpreted what type of current.

 

[dlochner]

Interesting.
Why does AC do less galvanic corrosion than DC despite the much higher potential?

The short simple mostly accurate answer is that the current is alternating, it removes and then replaces electrons. Whatever is attached is negative for 1/60 second and positive for 1/60th of a second, so the electrons go back and forth.

DC is direct current, positive is always positive, negative is always negative. Electrons leave the positive side and move to the negative side causing the corrosion.

 

[dlochner]

The diver told me refrigerator AC

He may have meant Air Conditioning and not AC circuit. Easy to confuse the two.

 

[capta]

A quick question. What does the other shaft look like? Did anybody mention this to the owner? I would think that both shafts would have about the same damage.

 

[Skipper]

A quick question. What does the other shaft look like? Did anybody mention this to the owner? I would think that both shafts would have about the same damage.

I thought the same thing but I have no info on the other shaft.
The incident description and pics I took were obtained by chance, when I saw the diver whom I know walk past me with the shaft in a cart. We chatted till we reached his truck, then I took pics and departed.
I'll have to ask him next time I see him but this thread will be forgotten by then.
The $6k may have been for both shafts.

 

[dlochner]

A quick question. What does the other shaft look like? Did anybody mention this to the owner? I would think that both shafts would have about the same damage.

Depends where the fault lies. A while back I was talking to an ABYC Electrician and Corrosion specialist. He told a story about a boat with a similar problem. One engine and prop shaft was toast, the other in decent shape. The issue was a fault on one engine and not the other. Sorry can't provide more details.

 

[CaptKimble]

OUCH!

 

[Skipper]

Update:
Diver said today that the port-side shaft and prop are unscathed.
Corrosion beginning coincides with owner having a 120v-only fridge installed in Mexico, 2.5 years ago. Fridge has been running 24/7, when at the dock.
I mistakenly said previously that mechanics had isolated the problem to the fridge, but I had misunderstood based on what the diver had previously told me. Turns out the assumption is fridge. No firm diagnosis yet.