Outdrive Electrolysis !!!

tony1b2000

tony1b2000

Active Member
Oct 10, 2007
243
Salem MA
Boat Info
Carver C37
Engines
Mercury Diesel 4.2 TDI, inboards
Wow, had the the boat hauled out of the water for winter storage today.
The guy power washing said I had a problem.
I am in disbelief. 95 % of my outdrive zincs were gone.. All 4 of my trim cylinders had pits in them and one looked like a chunk of metal was ripped out. Also some corrosion on the bell housing and other area's. See pictures.

OEM cylinders are running at $500 a piece. SEI has units that run about $160. Anyone have experience with their products???
Now back to the electrolysis.. Is there any way I can check with a meter or something at the dock to see whats going on?
Lesson learned, I may get a mid seaso haul out going forward to change the zincs. I have had the boat for 5 season and NEVER seen this before.

Last, how can I protect the areas of the bell housing that have been chewed up from getting worse ?? Rustoleum paint??

Thanks!
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Last edited:
Same slip for 5 seasons?

if so, something has changed drastically. Maybe stray current in the water, maybe the mercathode stopped working?
 
maybe one of your neighbors has a hot boat!
 
atrick said:
maybe one of your neighbors has a hot boat!
Click to expand...

I’d say that’s a good bet.

I’ve had great experiences with SEI drives, but not trim rams.

No way I’d put my boat back in until I figured out what’s what around the dock. Anyone else nearby have similar circumstances?
 
Yup, 5 years. But a few new (old) boats with inboard around me. Both my drives have independent mercathode systems.
How do you check for stray currents that can do this?
 
Your anodes were doing their job until they got depleted, then the exposed aluminum became the anode. If your anodes have been lasting the season, then something has changed - could be a new boat nearby or wiring at the dock or maybe your boat. Read this (http://www.mercstuff.com/bravo3corrosion.htm). You need to check your boat while it is in the slip using a volt meter with the test probe - this will help determine if you have a stray current issue at your slip. Next you need to verify things on your boat are working correctly - specifically the Mercathode system. As far as fixing the damage, it's a matter of priming and painting any areas of exposed aluminum - probably need to replace that badly corroded trim ram.

I'm not going to try and explain everything about electrolysis corrosion between dissimilar metals - but in general you have big stainless steel propellers (cathodes) attached to an aluminium outdrive (anode). When you connect two metals (ie seawater), electrolysis will cause the less noble metal (anode) to erode away as electrons flow to the more noble metal (cathode). This electrolysis speeds up when the finish on the anode is damaged - ie chips in the outdrive paint. To counteract this, you introduce an even less noble metal than the metal you are trying to protect - zinc is less noble than aluminum (remember the periodic chart of elements - A to Zinc). Mercruiser has developed the Mercathode system which basically introduces a small electrical current into the water around the drive to disrupt the electrolysis "flow".

To further complicate things, one of your neighbors boats could be "hot" - what this means is that their shorepower is not grounded properly (could be their boat or the dock) and your boat has become the ground for that other boat or boats.

If I know more than I should about this it is because I went through similar problems on my 290 - two destroyed BIII drives after 3yrs - checked the dock, etc, etc, hull potential thru the roof - ended up being a faulty Mercathode. Two new drives (under warranty) and a new Mercathode controller and no more corrosion problems. It was an annual ritual of changing anodes twice yearly and being obsessive over paint on the outdrive.
 
^^^ that's a good deal and a must have if you own a BIII and keep the boat in the water.
 
Make sure you used the proper metal for the anodes. For Bravo 3s in salt you should be using Aluminum.

Also make sure you protect the drive before launch with a couple of heavy coats of Trilux 33. Paint it on heavy an make sure all metal it covered. The paint does not make the drives look pretty - it is intended to protect. You also need a 1-1/2 inch border between you bottom paint and the transom assembly.

Lastly check all the bonding connections.

-Kevin
 
Could also have voltage leaking from the trim sender wiring units. They are constantly powered when the ign is on. There is a simple diode fix for it and a small rewire
 
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