Beneteau Cutlass Bearing Mounting Corrosion

[sailtjk]

I'm a relatively new owner of a 1999 Beneteau 321 which we keep in San Diego. In January the boatyard installed a new cutlass bearing, flexofold propeller and zinc on the prop shaft. The diver who cleans the bottom monthly advises me that the stainless steel? tube in which the cutlass bearing is mounted is actively corroding. See attached picture.

When I look inside the boat, I see no metal fitting on which to attach a bonding wire. Somewhere in the fiberglass skeg, the metal tube housing the cutlass bearing transitions to a fiberglass/epoxy tube. There is a dripless Volvo shaft seal. So if something is to be done to stop the corrosion, it appears it has to be done outside the boat.

The diver is going to attempt to install a small zinc using one of the setscrews that retain the cutlass bearing. There are extra setscrews added by a previous owner. Does this seem right? Have others had this problem? Is this galvanic corrosion or crevice corrosion? Is this a flaw in the design? Am I missing something?

 

[Gunni]

Do you have a galvanic isolator installed?

Bonding would presume that your boat electrics are creating the corrosion, but if you are in a marina and plugged up to the dock ground, your tube may be part of the marina "battery".

 

[gord331victoria]

Get your diver to mount a 2 inch zinc on that culasss bearing, this is what I have. What I read about these cutlass bearing is they were originally fibreglass, but were not strong enough to handle a dinghy line wrap, and the charter operators demanded they upgrade to a metal one, and now you must protect it.

gord

 

[sailtjk]

The diver is installing a zinc. Hopefully it solves the problem. I'll also look to see if a galvanic isolator is installed. Thanks for the input.

 

[sailtjk]

My diver installed a 2" collar zinc on the cutlass bearing tube and it appears to have solved the problem. He checked it two weeks after it was installed and did not find any evidence of corrosion. It was installed right behind the fiberglass skeg, so should not affect performance.

The zinc on the propeller shaft and this new zinc appear to be eroding at the normal rate so it appears there is no problem involved with being tied into the marina ground through the safety ground wire in shore power so a galvanic isolator may not be needed. The diver will continue to monitor this for me on a monthly basis.