Sacrificial anodes going way too fast

[Cap Morgan]

I just replaced the zinc anodes on my prop shaft just over 2 months ago. Had to pull the boat out of the water again this week to address some issues with through hulls. I was shocked to see how much the anodes have been eaten up already!
Question
Is there a sequential checklist of things to check in order to isolate what’s causing this high rate of electrolysis?
My CAL 39 had three anodes on the prop shaft so I replaced all three. I noticed that it is the anode closest to the prop that’s wearing the most if that indicates anything. Also, since my boat has been anchored in Biscayne Bay for 6 weeks of that time and traveling the rest of time it can’t be any neighboring boats causing the problem, it’s internal.

Appreciate your thoughts.

[michaeld]

We're you anchored near any docks? If so, I would suspect the culprit is shore power leakage.

[tkeithlu]

If you've got that kind of risk I recommend purchaase and use of a silver reference electrode. You hang it over the side and connect your voltmeter between it and the boat's ground. Look for 1.0 volts or so to know that your anodes are still working.

Is your prop shaft bonded into the system?

Is the bonding being used as a conductor?

https://www.boatzincs.com/corrosion-...electrode.html

[boatpoker]

Originally Posted by tkeithlu

If you've got that kind of risk I recommend purchaase and use of a silver reference electrode. You hang it over the side and connect your voltmeter between it and the boat's ground. Look for 1.0 volts or so to know that your anodes are still working.

Is your prop shaft bonded into the system?

Is the bonding being used as a conductor?

https://www.boatzincs.com/corrosion-...electrode.html

This is the only answer whether done by yourself or by an ABYC Certified Marine Corrosion tech who will already have and understand the equipment and what else to look for.

It is extremely unlikely that another boat is the issue, theoretically possible but I've never seen such a case. You are not suffering from "electrolysis", that's a process that does not occur on boats and is simply "docktalk" by the uneducated.

PS. I am a retired Certified Marine Corrosion tech.

[Cap Morgan]

MichaelD

I’ve been anchored miles away from any docks the entire time.

[Cap Morgan]

Tkeithlu,
You’ll have to pardon my lack of knowledge of electrical engineering terms. I’m not certain what the term bonded means but I will say that there is a small metal plate resting on the prop shaft which is wired. I can’t remember what the other end of the wire is connected to but will check today when I visit the boat.

[boatpoker]

Originally Posted by Cap Morgan

Tkeithlu,
You’ll have to pardon my lack of knowledge of electrical engineering terms. I’m not certain what the term bonded means but I will say that there is a small metal plate resting on the prop shaft which is wired.

Photo ? Sounds like part of a bonding system.
If bonding system is improper or compromised it could be the issue.
You need a pro, you don't have the knowlege to track this down and you are not going to gain
that skill here, it is way too complex for a forum

[fstbttms]

Whatever is going on, it isn’t electrolysis. Electrolysis is a something that happens to the water in which the boat sits, not the underwater metals in question.

[Capt Clark]

Cap Morgan,

Do you have a working galvanic isolator? I had the same issue years ago and added the isolator which solved the problem.

Capt Clark

[boatpoker]

Originally Posted by Capt Clark

Cap Morgan,

Do you have a working galvanic isolator? I had the same issue years ago and added the isolator which solved the problem.

Capt Clark

A galvanic isolator prevents low current DC leaving or entering the vessel via the shore power cord ground. The OP is at anchor so a GI would have no effect.

[wrwakefield]

If you cannot find a certified corrosion tech and need to persue this yourself, then along with the previously referenced corrosion reference electrode, [and a decent multimeter] read The Boatowner's Guide to Corrosion by Collier.

I'm no expert, but feel I have gained a much better understanding of how to diagnose and mitigate issues as you describe, along with other corrosion issues we boat owners are all familiar with.

Best wishes resolving things.

Cheers! Bill

[Bowdrie]

Originally Posted by Cap Morgan

I will say that there is a small metal plate resting on the prop shaft which is wired.

They were used as a "brush" in order to create a low resistance path to the water as sometimes the oil-filled gearbox can render a high resistance path to ground from the engine.
Bonding schemes are not a panacea/band-aid for other problems; indeed, they have the potential to create their own "battery" or galvanic cell.
When you connect steel tanks, aluminum tanks, stainless tanks, metallic objects/housings, engines, seacocks, keel bolts, mast steps, chainplates, rigging, etc. all together, and then slap on some zincs and add salt water, you have, (in effect,) created a "pea soup" of all kinds of various potentials just waiting for some currant leakage into the system that can lead to the scenario you speak of.
The idea that electricity follows the path of least resistance is false, it divides its flow according to the various resistances, (resistors in parallel).
I would hazard a guess that somewhere/somehow there is some currant leakage into the bonding system from the boats DC system.
A galvanic isolator can help, (block DC/pass AC,) but if there is some leakage elsewhere the isolator is effectively bypassed and does no good.
Questions, are there any 12V sending units in tanks for measuring fluid levels?
A bilge pump float-switch connection that's wet?
Perhaps a "leaky" diode on an alternator?

[Cap Morgan]

Definitely sounds like I need to consult a Pro. As I won’t be able to get one while in the boatyard will be interesting to see if I can find one that will travel to the boat in my Dinghy.

[michaeld]

Capn' Morgan. I had this issue while at our dock in Pompano. The guy across the canal sold the house. A new owner came in and reworked the dock. My problem went away. I have no other suggestions. Good luck. BTW, we'll be in your area at the end of April for the Miami to Key Largo Race, sponsored by the Miami Yacht Club. It's a lot of fun.

[Knotical]

Originally Posted by Cap Morgan

Definitely sounds like I need to consult a Pro. As I won’t be able to get one while in the boatyard will be interesting to see if I can find one that will travel to the boat in my Dinghy.

Before you hire a pro I suggest you do a lot of reading yourself and learn to use a multimeter, you will be pleasantly surprised how easy it is to diagnose simple to fix issues in a systematic way, you can start by reading the above mentioned book. Most pros that I have come across wanted to sell me something and weren’t interested in diagnosing old problems, they may also spend hours doing things that you can do yourself with some knowledge, believe me, it will go a long way.