Hot Prop

[gfrazzle]

My diver cleans and inspects my hull every month. Last week he reported that I had a hot prop- severely damaged the bronze 18" 3 blade on my 2007 41DS since the last cleaning. The boat has a Galvanic Isolator so dock power should not be a factor. A marine electrician inspected the boat and used a Seaguard corrosion tester. The tester reading limits should be between .500 and .700 for a bronze prop, my boat showed .125. The electrician said the problem is that I have no zincs. My diver claims there are and sent a picture from the last cleaning showing a zinc on the shaft and rudder but I had him put another on the shaft at the request of the electrician. He retested a few days later and the reading was .900 so he requested the diver remove the original zinc to get the reading within the normal limits. These readings did not fluctuate when dock power, all breakers, and batteries were turned off.

My question is this- could the zincs actually be to blame and if so, why doesn't the problem appear as the zinc is reduced during normal electrolysis which would logically bring the reading below the lower limit. The electrician insists that this is the only problem ( he hasn't rechecked the readings since the extra zinc was removed ). I'm highly suspect and need to be certain before I buy a new prop!

If this ends up not solving the problem, is there a procedure to trouble shoot the electrical system? I have been unable to find any helpful testing information online and there are no ABYC marine certified electricians in my area that I'm aware of.

Any thoughts? Mainsail?
Thanks

 

[shemandr]

A picture of a zinc doesn't mean it's working as it should. That zinc could be at the end of it's useful life and still retain it's form. I would go with the electrician's assessment.

 

[fstbttms]

That zinc could be at the end of it's useful life and still retain it's form.

Having installed many tens of thousands of anodes over the last 25 years, I have never experienced this.

 

[jssailem]

Asked and answered... @gfrazzle
hot-prop.1249921949
Posting the question a second time in a different forum may not yield different answer...
@Brian D

 

[SG]

Is the source of the electrolytic damage FROM your boat, FROM the shorepower, or from the marina?

The marina could be another boat (or boats), or the marina's shorepower system itself.

Sometime a zinc (or aluminum) shaft anode doesn't have continuity with the boat or the shaft itself. (You're shaft coupling might be an issue if it isolates it from the boat's ground. Or, if the shaft is coated with antifowling paint or something else, the anodes may not be directly contacting the shaft.

Generally, I wouldn't disconnect the zinc (or aluminum) anode. I'd add protection until you figure-out what's going on.

Are the zincs "melting away"? Are the doing they their sacrificial job.

 

[dlochner]

Did he test the GI? Modern GIs fail safe, that is they allow a ground connection when they fail and they no longer provide stray current protection.

What condition are the zincs? Have they eroded? Zinc can form a hard crust that will insulate the zinc and it will no longer provide protection.

The Boat Zincs website had some good reference material: Corrosion Quiz: Question 1

 

[Brian D]

Asked and answered... @gfrazzle
hot-prop.1249921949
Posting the question a second time in a different forum may not yield different answer...
@Brian D

That is correct. We really frown on posting the same question in multiple forums.

 

[capta]

Has a metal (steel/alloy) boat moved in near you recently? Also, it is possible that some liveaboard is causing this, but the device which causes the problem may not get turned on until he gets home after work, hence it doesn't show up when the diver is there.

 

[Mechone]

A few clubs I go to have ground fault monitoring per dock( not Finger ) this is hooked to a raspberry Pi4 that emails members if a boat plugs in and is leaking. Most boats that are leaking 90% of them its either the hot water tank element or battery charger. Must unplug until fixed. When they first started they had quite few boats they tracked down unplugging all boats, then plugging in one at time finding which boats were causing the issues.

 

[Mr Fox]

Adding more zinc won’t change the corrosion potential between the bronze and the zinc, you can add 100 of them it won’t “overprotect” anything. Something else is up; I’d be surprised if galvanic corrosion would do “severe” damage in a month.

 

[Mr Fox]

Answered in the other post but I’ll put here too- the corrosion potential between two metals will not change with the addition of more of one, it is a physical constant. You cannot overprotect the prop with the addition of more zinc.

You either have an electrical leak or your isolator is not working and neighbor does, or your marinas ground is faulty.

Did you check every breaker? Off and on and watch for fluctuation?

 

[LeslieTroyer]

Got a new neighbor in the marina, or one redoing their own electrical system.