electrolysis checking

[jamie langstone]

The boat was lifted for its annual maintenance and it discovered that the anodes had gone and also the fixing bolts had dissolved as well. Any way what is the best way to check the electrical system for signs of the problem. we disconnected the +ve from the battery and connected a multimeter to the +ve terminal and the +ve battery post and had a reading of 8.09 Volts with everything turned off then we pushed the cabin light pushed fuse in and it went up to 12.3 volts. we then pushed the fuse back out and it dropped down to 8.09 again. I read a post that it should read 0 volts so am I correct that if there is something that is left on that we don’t know about that would give us a reading. Any help appreciated.

 

[superheat6k]

The place to learn more of this technique is Nigel Calder's Boatowners Mechanical and Electrical Manual.

If you are measuring a voltage between these two positions there is a circuit when they are connected (as they normnally are), so current is flowing somewhere.

So where to look ?

Turn off all circuits except permanent services, e.g. primairlly the bilge pump/s. My money is on a flap float switch. The seawater gets in and then a stray current arises as the +VE leaks into the surrouding salty and thus conductive bilge water, this in turn then finds something attached to the bonding circuit, that can make the bonding circuit, and with it the anodes, very slightly positive, and the anodes then start to fizz.

The links I PM'd to you should also help explain this further, including the subtle differences between leakage, stray and galvanic currents.

 

[superheat6k]

If you find the root cause/s please come back here and let us know what you found.

 

[jamie langstone]

If you find the root cause/s please come back here and let us know what you found.

I will do .

 

[Jamie Dundee]

My prop anode disappeared back end of the season, I haven’t done any digging yet as to cause but did find that the float switch to the bilge pump had packed in. Now removed and being replaced by a new auto bilge pump.

 

[jamie langstone]

The boat in question is a corvett it’s not my boat it’s a friends . I have a merry fisher 805 and the anode on the propeller last 6 months max so it get changed twice a year . I had a ocqteau 625 that done the same thing on the prop anode so I am used to changing the prop anode .

 

[superheat6k]

The boat in question is a corvett it’s not my boat it’s a friends . I have a merry fisher 805 and the anode on the propeller last 6 months max so it get changed twice a year . I had a ocqteau 625 that done the same thing on the prop anode so I am used to changing the prop anode .

On my Corvette the bonding strap cable across the R&D coupling failed and the shaft brushes I fitted went high resistance. Nearly lost the stbd prop, but fortunately just some pitting.

But in that case the anodes were not dissolving much at all.

The worse corrosion of all occurs if there is a 240vac stray current eakage into the water anywhere near the boat. For this reason I always haul out immersed shorepower cables wherever I see them.

 

[yamaha115]

Hey
Just new here, wonder if these are Negative grounded ..?

 

[superheat6k]

Every boat I have ever dealt with has been negative ground. Some boats may have more than one ground where the DC is not connected to the AC earth, but this is not viewed as necessarilly safe.