View Poll Results: What protects your prop shaft(s) and propeller(s)?
Prop Shaft Brush with hull zinc only 5 22.73%
Prop Shaft Brush with hull zinc and Prop shaft zinc 8 36.36%
Prop Shaft zinc only 8 36.36%
Not part of my job description 1 4.55%
Voters: 22. You may not vote on this poll

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Old 03-22-2017, 01:01 AM   #21
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Originally Posted by angus99 View Post
This is so timely. I'm at the boat now completely replacing the bonding system--all new #8 and #6 wire, new lugs, heat shrink tubing, aluminum shaft, transom and rudder anodes etc.

The existing bonding system on my boat was a spaghetti plate of wires of different colors and gauges serving both bonding and safety ground functions, tying the cases of electrical components (including AC-powered air conditioners) to the bonding system. Metal tanks are also connected and there may have been an attempt at lightning protection as well since metal seacocks 18 inches above the water line are also bonded.

I found a lot of tarnished wire, corroded and loose terminals and self-tapping screws connecting wires to the long copper bonding strip in the engine room. (ABYC requires machine screws and tapped/threaded holes in the copper bar or nuts/machine screws to secure the terminals.) I could spin many of the terminals secured with self-tapping screws by hand.

The guy I'm consulting with (also an ABYC-certified marine corrosion expert whom you know, Steve) suggested I forego shaft brushes. He considers each prop shaft it's own separate bonding system and uses two (sometimes three) anodes per shaft, depending on the length and how fast they're sacrificing themselves. He says his company was never able to get shaft brushes to work consistently enough to have confidence in them--certainly never as a substitute for anodes.

Through hulls, prop shaft struts, rudders/steering gear and stuffing boxes are all tied to the bonding system. Once all the terminations are tight, each gets a spray of Boshield to limit future corrosion.
Sounds as if you are on the right track Angus, I'm glad to see it. While I agree I don't like to rely entirely on the standard carbon brush arrangement (it's far better than the piece of bronze half pipe held in place with a spring, no kidding, that's an off the shelf part, but not as good as the slip ring) for galvanic protection, and thus the isolated, protected shaft has its merits, bonded underwater metals are less susceptible (not immune) to stray current corrosion. It's one of several good reasons to bond. A bonded underwater metal offers stray current a more direct low resistance path back to its source, which will either trip a circuit breaker or blow a fuse, or cause the wire source of the stray current to rapidly corrode and part, thereby stemming the flow of electricity.

I have a manganese bronze prop siting outside my office, as a door stop, that has been corroded almost down to the blade roots (it's pictured in some of the articles on this subject). It's a victim of stray current corrosion. Again no guarantees but if it had been bonded with a brush, my bet is the source would have been shut down more quickly. In light of that, using shaft anodes and a brush isn't a bad idea, and of course, as I mentioned earlier, all of the anodes must be maintained to prevent the shaft anodes from carrying the full load.

Steve D'Antonio Marine Consulting, Inc.
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