Hacked the quick dissolving prop zinc problem.

[Haifisch]

I have been running a 6 month experiment on preventing quick decay of the prop zinc. Already have the galvanic isolator and am well informed on issues with hot marinas, neighbor boats etc. I have been focusing on controlling the controllable.

Here's what I came up with... I was getting 6 or less mo. on the prop zinc; the divers dream, thrusters, and tab zincs last much longer. For the past 6 months, I have been using a hanging zinc. It is a large anode of zinc attached to a long plastic coated cable with a clamp. At west marine, they sell them shaped like a fish, but boatzincs.com have a version that is just a big ass giant chunk of zinc with a threaded rod running the full length. Rather than attaching it to the bonding bolts, I have been clamping direct to the shaft just forward of the stuffing box.

Just dived the boat this weekend, and at 6 months prop zinc is less than half consumed, while the hanging zinc is definitely getting some Galvanic action. I know that a shaft brush is another way to accomplish this, but this validates that getting some backup to the prop zinc definitely helps. Just need to remember to remove it before starting the boat. While in the marina and leaving the boat for a week or more, it's pretty easy to deal with. When on a trip, I leave it at the dock.

I took some photos of the clamp and hanging zinc, but thus far have been unable to navigate the Byzantine posting instructions and albums on Tugnuts. Left me know if you need more info.

 

[BB marine]

Your repair or fix for the issue validates the use of a shaft grounding brush. If the grounding or bonding between the shaft, shaft coupling and transmission is compromised because of the transmission mechanical components the prop is a stand alone metal component under water. The prop anode is the only protection the prop has. There is a lot of metal surface for the anode to protect and therefor erodes faster than any other anode. The thruster anodes are stand alone only protecting the thruster housings, The trim tab anodes are only protecting the trim tab planes both of these components are not part of the bond wiring. The divers dream is there to protect all under water metals that are attached to the bond wiring including the prop shaft and prop if the prop and prop shaft have a good connection to the engine ground and the engine ground has a good connection to the bond wiring. The best way to test the bond wiring to confirm the divers dream is doing its part is to do a continuity test with a ohm meter between the divers dream and each metal component that is wired to the bond wiring. The resistance should be no higher than 1 ohm. This measurement is best taken from the divers anode surface confirming a good connection between the anode and the anodes ground wire. If all checks confirm 1 ohm or lower to all components your divers anode is protecting everything.

A simple test for your boat to confirm that the prop anode was eroding faster because it was stand alone is check the resistance between the prop shaft and the divers dream on the transom. If the resistance is below 1 ohm the divers dream is protecting the prop and prop shaft. If not than the small prop anode is doing it all and the large fish is just adding anode weight and working with the prop anode.

If your fish is made of Zinc I would recommend each time you remove it from the water and let it sit in the air you brush it off before using it again. This is a common mistake many boaters do when using Zinc anodes. They look at them and if the anode is not eroded badly they will relaunch. Zinc anodes oxidize after being removed from water. There is a thin layer of white crust that forms on the exterior surface. This oxidation coating reduces the anodes protection and in some cases the anode is doing nothing. When ever a zinc anode is introduced to air it should be brushed off and clean to a shiny material. ( warning the dust particles should not be inhaled use a dust mask when doing this)

I believe the shaft bonding brush would be a good addition for your issue. You would be essentially using the divers dream to do the same operation as the Fish.


If a shaft brush is used and is working properly and the prop anode is really not needed. I just had a confirmation about this with a marine surveyor, actually two a hull surveyor and an engine surveyor both confirmed that the use of a prop and prop shaft anode with a shaft bonding brush in use that is connected to the boats divers dream anode. The Prop anode is not recommended. It could cause over protection.

Good experiment ! You have proved that the prop anode in many cases is a stand alone anode with out a grounding brush.

 

[Red Raven]

In addition, this validates the need for a transom zinc or equivalent in addition to the prop zinc or shaft brush. Many of the earlier boats (including my R27) had no other protection besides the prop zinc for the underwater metal on the other side of the transmission (rudder, etc.). Not only is the connection from these items to the prop zinc potentially poor but if the prop zinc drops off the protection is non-existent. I installed a mini divers dream transom zinc and a rudder zinc. An alternative is to add collar zincs to the swim step brackets below the water line. I still need to check the prop zinc periodically as I do not have a shaft brush. With a little tightening of the prop zinc through the season (and a few other prop zinc hacks noted elsewhere in Tugnuts topics) it lasts the needed 6 months before I pull the boat for the winter.

Curt

 

[Haifisch]

Thanks for the advice on this... Will look at doing the brush this winter. Related to Brian's comments on oxidation of the hanging zinc clamped to shaft: By being able to pull the hanging zinc out of the water, I can rough it up with a wire brush between cruises which easier than doing the same with the diver's dream since my boat lives in the water. I'll check the resistance next spring when I have the boat pulled for bottom paint.

Curt: Do you know the internal diameter for the collar zincs that will be mounted to the swim grid?

 

[Red Raven]

No, I’m not sure on the swim grid brackets diameter. I am not near the boat to measure. Perhaps someone else here on Tugnuts can measure. My guess is one inch. If you already have a divers dream you should not need the swim grid collar zincs (assuming both the dream and the swim grid brackets are bonded). Just another alternative for others to avoid adding a transom zinc.

Curt

 

[Trisailor]

Mine were 1 inch. I placed collar zincs on the swim step supports about 6 inches up from the lower transom mounting points. As of last week they were showing very little sign of wear. During the time they've been installed I've gone through one set of zincs (3 .5 inch) on the trim tabs and 4 prop zincs (4th just installed). The swim step support zincs have been in place about 30 months. The shaft brush install is now on the list as a future project. I do not have a divers dream zinc.

 

[Horigan]

...By being able to pull the hanging zinc out of the water, I can rough it up with a wire brush between cruises which easier than doing the same with the diver's dream since my boat lives in the water...

Be aware that brushing a sacrificial anode with a steel brush can reduce its effectiveness. It’s better to use quartz sandpaper.