Bravo 3 magnesium anode in fresh water

Big blue

Big blue

Member
Nov 19, 2017
35
Cardiff u.k.
Boat Info
310da sundancer 1998 mercruiser D4.2
Engines
Mercruiser D4.2
Hi all
Just trying to get some idea on how often other owners with bravo 3 drives in fresh water are changing there anodes.
Owned various alpha drives in the past and have had anodes last at least 2 years.
I am only getting 10 months from my anodes and they are eroded down to at least 50%, cav plate anode is about to fall apart, is this about right?
 
Lots of variables with drives and anodes.....Bottom line....replace them when they are worn to 50%
 
You can always try to add a prop nut anode. Boatzinc.com has a kit for them. With all the stainless in the water you may need extra protect. Also, does your setup have the Mercathode system? I am not sure when they started adding them but if it does make sure that it is functioning.

-Kevin
 
Salt water wet slipped B3 here....

My prop anode and the horseshoe one bolted to the anti-cavitation plate need to be replaced every 6 months. The others will last 2 seasons (12 months)

Every scenario is different.

The bottom line is the anodes are doing their job (assuming your housing isn’t corroding too)

Weather they last 6 months or 2 years, replace them when 50% worn.
 
Boat will be lifted this month for some maintenance and try sort out other issues, but it seems a shame that I put a 2 year Antifouling paint on hull but need to pull boat for the anodes
 
Anodes seem to erode at the same rate, just quicker than I thought, I have checked my mercathode the best I can, 12v present at the controller and the electrodes on Shield are clean, carried out test with multimeter and probe in water in front of the props and getting 750 mv.
I have the prop nut anodes fitted, for some reason I have the later prop shafts with the threads drilled into the end to mount them on
 
Also struggling with corrosion on drives and paint blistering, they need completely stripping back again and re painting
 
Unless the fresh water is VERY clean (free of minerals and what not), sometimes Mg anodes can overprotect and it does cause paint blistering as one of the symptoms. I'm not saying that's absolutely what is causing this, but it "could" be... especially with the fast deterioration of them. You might want to try aluminum anodes as they are often a better choice for fresh water (get either the Merc/Quicksilver brand or, even better, the Performance Metals brand) and see how they work for you. Just monitor closely until you get a comfort level with it. FYI, you can change the anodes while in the water.
 
With the way your anodes are deteriorating, I'd be suspect of a leak, either from your boat or a neighbor.
I had a similar issue where my anodes went south fast for 2 years until my slip mate left. Now, after a year, they are hardly eroded. The guy on the other side of him had the same problem.
 
Lazy Daze said:
Unless the fresh water is VERY clean (free of minerals and what not), sometimes Mg anodes can overprotect and it does cause paint blistering as one of the symptoms. I'm not saying that's absolutely what is causing this, but it "could" be... especially with the fast deterioration of them. You might want to try aluminum anodes as they are often a better choice for fresh water (get either the Merc/Quicksilver brand or, even better, the Performance Metals brand) and see how they work for you. Just monitor closely until you get a comfort level with it. FYI, you can change the anodes while in the water.
Click to expand...

I tried the Performance Metals navaloy aluminum anodes with excellent success in fresh water. No OD blistering at all.

Bennett
 
bbwhitejr said:
I tried the Performance Metals navaloy aluminum anodes with excellent success in fresh water. No OD blistering at all.

Bennett
Click to expand...

I had the same experience with the Navalloy aluminum anodes on my B3 drive in fresh water. That reduced the blistering. Another thing that helped was installing all of the parts in the continuity kit. Many of the parts were missing from my drive. http://www.mercruiserparts.com/99940a2-circuit-kit
 
I was under the impression that aluminium anodes on the B3 was a big no no, to the point that they are now used in salt to control the poor corrosion resistance that present on the early B3 drives.
I check the drives over with a multi meter, all earth bonds are present and working, always check resistances between all anodes and from drive to drive, and always it’s good.
Galvanic Isolator Also checked out good.

Another idea that I am pondering over to help is to paint the props with prop o drev Antifouling paint, If i can seal the stainless under a layer of paint it shouldn’t be in contact with the water.
Any thoughts on this
 
Just had a look at the navalloy anodes, think I am sold on them, I have anodes on the trim tabs too which I normally use magnesium to match the drive anodes, do they sell navalloy anodes for tabs or is it best to go to aluminium
 
Zinc should only be used in salt water. Aluminum can be used in either. FYI, Merc's have been coming with Al anodes on them, direct from the factory.

I have no experience with that Prop O Drev, but a quick google search returned it's data sheet and it says:

"Compatibility/Substrates For use on Aluminium/Alloy, Zinc-sprayed Surfaces and Lead. Also suitable for factory enamelled finishes."

Are you sure you can use that on stainless?

A properly maintained B3 will do just fine in salt water. But it does take some "looking after" on a regular basis to keep it that way. I don't disagree with your thought of painting the props, though. But I'd look for something that is more permanent than typical antifouling paint - possibly something epoxy based.
 
Big blue said:
Just had a look at the navalloy anodes, think I am sold on them, I have anodes on the trim tabs too which I normally use magnesium to match the drive anodes, do they sell navalloy anodes for tabs or is it best to go to aluminium
Click to expand...

Yes, I got the Navalloy anodes for the trim tabs as well. If i am not mistaken, all needed came in a kit.

Bennett
 
New set of navalloy anodes fitted this weekend with matching trim tab anodes, so see how it goes, local marine engineer seems to think they will not work in fresh water and will from a white glaze and stop working like traditional aluminium anodes. I hope he is wrong
 
Tens of thousands of Bravo III's are out there in fresh water with "traditional" aluminum anodes and good results. Hard to argue with those numbers. The Navalloy are even better. HOWEVER, as I somewhat eluded to above, there can be differences from one body of fresh water to another. The #1 best way to determine what anodes work best for you is look at history... look at other boats with a similar setup to yours and if it's been working well for them for at least a few years, it should work for you. Unless, of course, there are extenuating circumstances such as stray current (or a neighbor with less than adequate protection), as mentioned above.
 
I am on a fresh water river in Pennsylvania. Magnesium anodes are a requirement for the B3 in our pool.
If aluminum is used, you will have noticeable corrosion after just one season. Ask every new boat owner at our marina that forgot to have the dealer change them before launching for their first season - every one of them needed to sand and repaint.
Magnesium anodes will make it 2 seasons here (~5 months in the water).
 
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