prop anode

[nzfisher]

I had the prop nut zinc falling off on my R25sc in earlier years. Since then I "fit" the zinc to the prop nut by putting a light coat of thin white paint to see where it was "touching" the nut assembly. I found that the nut was hitting the cotter pin excessively which caused it to not "seat" properly on the nut. Since then I have formed the cotter pin to no longer hit the zinc and found a larger thinner stainless washer(s) that provides for a compression thrust fit that provides some extra spring thrust against the zinc as it wears. Since then I no longer lose prop zincs. I also set the prop nut with a bronze hammer and file flat the mating surfaces to get as much contact as I can between them. How tight to torque the bolt remains an open question. My current birth is pretty good for stray current and I also put zincs on the rudder. Some combination of the above has seemed to make a difference in lost zincs. I replace all zincs when I pull the boat and the boat is only in the water 5-6 months.

 

[Red Raven]

When I bought our boat (R27) it had been only 4.5 months on the (salt) water since new (2014). The prop zinc was gone when it came out of the water. The factory took the boat in on trade so they replaced the prop zinc before selling it to us and delivered this last spring. The boat was in the water (at a marina) 5 months and when hauled out and the prop zinc was gone again. There was significant corrosion on the rudder (but not on the prop!). After investigation of the bonding system I found that the system relies solely on the prop zinc for protection of every bonded item (including the rudder which has a lot of bronze surface area and mass). While independent zincs protect the thrusters and trim tabs (trim tab zincs are questionable value since the the trim tabs are not bonded and have no dissimilar metals under water until the zincs are added!). All other metals below the waterline depend on the prop zinc alone for protection.

I do not know if the prop zinc corroded off each year or if it just fell off from the high revolutions/forces or a combination of both. While I have installed a new prop zinc (very tight with fender washer and lock washer that comes with it from the factory) I have zero faith that it would get through another 5 months next summer and I really want to avoid a new rudder! Since the factory did not recommend a rudder zinc (due to the complex surface, or performance issues, or what I don't know) I chose to install a "mini divers dream" transom zinc on the starboard side similar to where the stern thruster is on the port side. I also add a Galvanic isolator to deal with potential marina issues (though for the first seasons 4.5 months the boat was not at a marina). The new transom zinc is bonded to the rudder (like nearly everything else). I also plan to purchase a silver chloride reference electrode to use during the summer season for testing to make sure things are OK until I pull out.

Bottom line is that I would not rely on the prop zinc alone for protection in salt water based on my admittedly short experience, too much risk of it coming loose and falling off or just corroding away and resulting rudder or prop damage.

I'll report back next season on how well the attempted solutions work out.

 

[knotflying]

I also installed a galvanic isolator and use the sacrificial large fish zinc that is connected to the battery ground and then placed in the water at the marina. Big difference. Also as mentioned in a very early post I placed copper pennies along side the nut that the prop zinc goes over and my prop zinc seems to hold up well.

 

[Levitation]

Hey Mike, very creative; you created a Cu-Zn battery cell (Daniel Cell) With a bit of ingenuity you could have the pennies wired in series and epoxy an LED bulb on the end of the bolt and have underwater mood lighting :mrgreen:

And let me comment that even though the trim tabs are electrically isolated from the rest of the boat there is corrosion (rusting actually) of the screws holding the tabs on and corrosion of the hinge portion of the tabs - pin holes eaten completely through. Like most is us I figured the tabs were not an electrolysis issue until I noticed the damage. Last year I put Zinc anodes on the tabs. The anodes over the 4 week period before I hauled out, had significant erosion! So I am adding a bonding wire from the tabs to the negative side of the battery along with my fish (which takes a beating).

denny-o

 

[Red Raven]

Mike,

Any idea how long your prop zinc lasts without the added fish. I'm curious how how long it would last if it didn't fall off.

Curt

 

[knotflying]

I would say at 6 months I replace it. I can't say how much has been sacrificed at that time. I would have to do a before and after weight. The good part is that it is still there and not loose.

 

[Red Raven]

Thanks Mike!