Zincs (or Magnesiums)

[zellerj]

I have two boats right beside each other at a Lake Erie marina, a sailboat and a Tiara 2700 with twin I/Os. The sailboat has a zinc on the prop shaft. When I pulled it out for Winter the zinc was in good shape, very little evidence of electrolysis. This zinc is about 10 years old. I very seldom plug the sailboat into shore power, because I motor enough to keep the batteries charged.

The Tiara 2700 is plugged in to shore power almost all of the time - I don't really know why I do this, because I don't have anything that needs iit, and the bilge is always dry. When I pulled the Tiara, the starboard zinc trim tab was about half gone, and the larger zinc below the hydraulic lines was pretty corroded. The port engine was not as bad, I will probably keep the trim tab zinc without changing it out, howerver the zinc by the hydraulic lines was pretty corroded.

I would expect that stray current in the marina would affect both boats in a similar fashion.

My questions: Does shore power being connected somehow cause the electrolysis to occur, and if so , how can I check this out and fix the issue? Why would there be a difference between the port and starboard engine?

 

[alldodge]

Your in fresh water so you do not want zinc, that is for salt

For fresh you want aluminum or magnesium and preference on magnesium

The sail boat doesn't have much dissimilar metals, and not being plug in helps. The Tiara having I/O's means there is a lot more aluminum and stainless in the water. These 2 items have enough trouble keeping the dissimilar metals from reacting. Add shore power and this only increases the risk.

Each drive has a anode on the bottom of the drive along with the zinc. The anode produces an opposing current to protect the drive. If the Mercathode is not producing enough power, not connected or the anode wire is damaged, that drive will have more corrosion.

Merc makes a probe which is attached to a multimeter and the boats battery to test the output

 

[poconojoe]

I don't know a whole lot about the Mercathode and only know generally concerning sacrificial anodes.

Just thought I'd mention a little thing I noticed...
I had started seeing a little corrosion on my drive and noticed a bonding wire was disconnected from the side of the drive. The wire had broken off the Stak-Kon crimp. As you might already know, there are a few of these wires and maybe something similar has happened to one of your drives.

 

[QBhoy]

All dodge has it spot on here, but shore power has an influence in some cases. Especially on an out drive boat.
Local to me. There is an ongoing issue at the most expensive marina around the Loch. Almost all of the mercruiser outdrives in the marina suffer from the trim fittings into the drive corroding quickly and alarming alt quickly. It’s the local merc dealers bread and butter all year.
The marina denies any blame but it’s almsot certainly caused by the pontoons, shore power and their configuration.
Other things on my drive worry me, as all dodge said. So many disssimilar metals in use.
I have the dual purpose (or at least for fresh and brackish) Aluminium anodes on mine and see very little wear on them. I’m in solely fresh water with no salt influence. I should likely have magnesium only.

 

[poconojoe]

This thread reminds me of those horrible photos of outdrive and outboards almost completely dissolved.

 

[Scott Danforth]

There are also blended zincs

However a bad connection in the bonding system on shore power is the equivalant of dropping your boat in acid

 

[dingbat]

No concernable erosion of a “zinc” after a year of use isn’t a good thing.

 

[zellerj]

I am in search of grounding cable diagrams for a 1988 5.7 L. I took a good look at the inside transom assembly and there are ground wires that were not reconnected after engine re-install. There was also a ground wire not connected on the outside of the transom that connects forward of the magnesium located at the bottom of the outdrive unit. How many grounding wires are there on a 5.7 L alpha drive? Does anyone have a diagram ? I do not believe that I have the mercathode system. Fresh water Lake Erie boat.

Also.... when replacing the magnesiums that look like nut covers, I found that the studs on either side of the outside transom assembly that the mags go on were stripped. How can a soft metal like magnesium or zinc strip a bolt? Has this bolt been corroded in the past and I am just finding old issues that have been covered up? Is this bolt easy to replace?

 

[harringtondav]

From memory - flywheel housing to transom plate to steering arm. Then bell housing to out drive.

 

[alldodge]

Will add, exterior transom plate to gimbal ring

Also.... when replacing the magnesiums that look like nut covers, I found that the studs on either side of the outside transom assembly that the mags go on were stripped. How can a soft metal like magnesium or zinc strip a bolt? Has this bolt been corroded in the past and I am just finding old issues that have been covered up? Is this bolt easy to replace?

Sounds like you have an older alpha transom assembly. With the bolts being stripped then you need to replace the bolts, they are made of aluminum (item 18 part 97-8M0030655 ). That said, if they are that loose, there may be other issues with the transom

What year is the boat?

https://www.mercruiserparts.com/bam/subassembly/30872/2203/30

 

[zellerj]

The boat is a 1988 alpha with a 5.7 L merc. The transom and stringers were replaced in 2016. I bought the boat in 2017. Runs good, but I noticed some erosion of the magnesium anodes. Thanks for the part number. According to the schematic,part 8M0030655 has threads on both ends. O on the bow side, The anode screws into the stern side of the part.. What does the bow side screw into?

 

[alldodge]

What does the bow side screw into?

Lock washer and nut (items 16 and 17)

https://www.mercruiserparts.com/bam/subassembly/30872/2203/20

 

[zellerj]

Thanks AllDodge, I understand now.