Steel hull/Coppercoat/Stray current - ouch!

[KellysEye]

>A GI wont offer protection against this sort of thing... they are only effective at preventing the effects of galvanic action from dissimilar metals.

Galvanic isolators will also protect the boat from stray current corrosion caused by a bad earth in the marina and the current created when salt water mixes with fresh water.

 

[FurryWritingDesk]

Quick update

Thank you to everyone for contributing opinions - it certainly is an interesting one and has given me a steep learning curve around physics and chemistry that I thought I'd left behind with my school science lessons!

The current strategy is to get a surveyor down to check our electrical systems, comprehensively, early next week - whilst our boat is still out of the water. We'd hope to find nothing wrong on our side - but are equally cognizant that we could be the source of an electrical issue ourselves. As part of the survey, we'd also hope that they can suggest what remedial work needs to be done on the hull. @Passerell - our thoughts chime with your suggestions.

The marina has stated that their electrical systems are "fine" and they won't permit us to contact the owner of the MFV (formally) until we have a survey in hand that indicates that our own boat isn't the problem. So, we are where we are for now.

I will definitely keep you posted as to what comes to pass. Thank you all again. It is hugely appreciated.

 

[JumbleDuck]

... the current created when salt water mixes with fresh water.

Say what?

 

[pohopetch]

It is hard to tell from the photos, but if you are getting blisters erupting from underneath the copper coat you should consider the possibility that you are over-zinced. While over-zincing doesn't cause a problem on fibreglass hulls it is a definite issue for wooden or steel hulls. A well prepared steel hull has almost no steel directly exposed to the water, and only the usual shaft and prop which needs protecting. Too much zinc moves the potential past the optimum point and can cause a tiny cell to form at points on the hull surface under the paint which eventually pops out as a blister. If you carefully grind some of these down you will see a small shiny area (around 5 to 10mm diameter) with a black spot in the middle which is a tiny pit. If you can't spot this under some of the blisters then it is probably another issue. It takes some years for the blisters to appear but if the number of zincs was increased some years ago "for good measure" it could be contributing. Of course a steel fishing boat with an unfaired hull that is badly painted, or already rusty is a different story since it has a huge exposed steel surface which is why you see these boats absolutely loaded with zincs.

We have had our 43ft steel sloop for 10 years and at the first haul out about 20% of the hull was showing blisters. We reduced the zincs to one on the side of the keel and one on the end of the prop shaft and ground out the blisters, re-faired, painted etc. There were still a few left at the next haul out but now 10 years later the hull comes out clean and nearly perfect. You can easily check the correct zinc levels by using a silver/silver chloride electrode and a multi-meter by hanging the electrode over the side when the boat is in the water and measuring the potential on the multi-meter. The electrodes are not expensive and allow you to get things just right.

 

[VicS]

>A GI wont offer protection against this sort of thing... they are only effective at preventing the effects of galvanic action from dissimilar metals.

Galvanic isolators will also protect the boat from stray current corrosion caused by a bad earth in the marina and the current created when salt water mixes with fresh water.

It will only block current from the very low voltage sources one encounters with galvanic corrosion. It won't block the current from higher voltage sources, not even from the vessels 12 volt system, and certainly not from "mains "voltages, the whole point of them being that while blocking current from galvanic sources they allow current from mains voltages to pass so preserving normal functions of the protective earth

Not quite sure what you mean by "the current created when salt water mixes with fresh water" . Are you suggesting that if I mix some fresh water with some salt water a current will flow. If so between what two points.
What happens to this current when a river flows into the sea.