Bonding Efficiency

[denpooch]

Hello Folks.
When I took her out this past week (the boat...not the admiral!) there were circular 'stains' surrounding any thru-hull fittings. It is my understanding that this indicates a level of galvanic corrosion and poor grounding.
I employ the use of zinc anodes on the shafts, trim tabs, each of the engines and there is a stern 'block' of zinc. Last season I gave the latter a good tightening.
Question: Is there a 'sure-fire' way to determine if the 'green' bonding wires inside the boat are actually doing their job? Those wires are attached to all thru-hull components as well as the engines. I believe that they terminate aft at the zinc block.
A guy at the yard said my boat was the worst of the lot and he tells me there is definitely a grounding issue with the boat.
What say you? Thanks.

[km1125]

You can measure resistance from any of the through-hulls to the common bonding on the boat to make sure they are all connected.

[Viper]

Ya you can do a resistance reading. Voltage drop testing at connection points along the bonding system will also tell you if there are poor connections or wires. The voltage should be the same at all points. The point is that all the hardware is at the same electrical potential.

Make sure there are no bare DC wires or non-waterproof connections lying in water in the bilge such as bilge pump wiring.

Have you checked behind your breaker panel to see if you have a galvanic isolator?

Are your neighbours having the same problem? What is the condition of their anodes compared to yours? How often do you and them replace anodes?

[KeithS]

Paint "halo: around the fittings may not indicate a corrosion issue. It is common with high copper paint and a slightly over-zinced boat. You can check with an ohmmeter to be sure you have less than one ohm from each fitting to your transom zinc, but if you have halos around all your underwater hardware, this is probably not your issue. The best way to be sure of a galvanic corrosion issue is to find an ABYC certified corrosion tech in your area and have him use a silver/silver chloride cell to test your boat in the water. Galvanic corrosion can be a complicated issue and there is a lot of bad advice around the dock.

[bud37]

I went thru this with our old boat,(a 350 Mariner) took me a lot of reading up and many hours testing to check and resolve all the bonding system. Got it in the end but very time consuming and it would be hard to explain simply in a forum setting.

Turns out it was the boat in the next slip dumping into the water. You can buy a reference electrode but like keith said, getting a certified proper tech to test things out is maybe the best route.

[denpooch]

Thanks everyone. Your advise is much appreciated.

[denpooch]

I just got back from the boat and took these images. Do we still agree with comments made? Anything else to think about?
Thanks again.

[km1125]

That does not look good to me. I'd be cleaning everything off there and seeing if there's been material damage to any of the underlying metal.

Do you know what kind of bottom paint that is? Wonder if that could be some of the issue.

[bud37]

Have a look at this....Pascoe site with pics....

https://www.yachtsurvey.com/corrosion.htm