A client sent me an excerpt from your eBook “Marine Electrical Systems,” and I was a surprised to see the following line of thought.
Use zincs to protect against the galvanic currents that are set up by dissimilar metals on your boat that are immersed and that are in electrical contact with one another. The best example is your bronze propeller on a stainless steel shaft. The best protection is to put a zinc right on the shaft next to the propeller, or a zinc on the propeller nut. An isolated bronze through-hull doesn’t need protection because it is not in electrical contact with another immersed dissimilar metal. If electrically isolated, high quality marine bronze is electrochemically stable in seawater; nothing good can come from connecting wires to it.
While the simple corrosion immunity supposition is correct for a true bronze alloy, it ignores the potential for stray current corrosion. Should the thru-hull become energized, by a defective bilge pump or wire for instance, it can become part of a stray current cell.
A bonding system provides a direct path for this current back to the source, thereby avoiding damage to bonded thru-hulls and hopefully tripping a breaker or blowing a fuse. It also has the opportunity to discharge lightning current harmlessly into the sea, but only if bonded with correct size conductor (minimum 6 AWG).
As an aside, the “leaking” wire doesn’t have to physically touch the thru-hull. Take, for example, a hot lead in the bilge water, and an unbonded thru-hull: the path of the electrons flows from the battery negative into the outside water through a bonded connection (a shaft for instance, which is bonded by default), back into bilge water via the unbonded fitting then back to positive of battery. Anodes are the wire in the bilge and the outside of the through hull. Cathodes are the bonded item carrying electrons to the water and the inside of the thru-hull receiving electrons from the outside water.
This article I wrote goes into more detail about bonding system advantages,stevedmarineconsulting.com/wp-content/uploads/2014/03/BondingSystems138_05.pdf .
If a thru-hull like this can never be accidentally energized, then stray current is not a factor. But the lightning protection remains valid. Anecdotally, I find that even where no dedicated lightning protection system (LPS) exists, well-bonded vessels suffer less damage as a result of direct or near strikes.
Excellent point. Out of context, the advice “nothing good can come from” connecting wires to an electrically isolated through-hull is misleading. The point is clarified in the eBook from which this passage was excerpted. If the through-hull is well isolated, connecting it to more metal as part of a bonding or lightning protection system can offer some additional protection against corrosion (and possibly a direct lightning strike).
In our view, if your boat is otherwise expertly wired, features high quality silicon bronze thru-hulls, and shows no sign of chronic corrosion problems—stray current or otherwise—then crawling around the bilge and connecting every single bronze thru-hull with 6 AWG wire is a good idea, but it might not be a top priority. Bonding remains optional under the standards adopted by the American Boatbuilding and Yacht Council.
That said, stray current can pose considerable trouble. As we’ve pointed out in past reports (see PS Advisor, January 2006), stray DC current can do significant harm to vulnerable components like aluminum saildrives over a surprisingly short period of time. However, it is important to remember that if you go the trouble of bonding all your through hulls, it is important that you maintain the bonding connections and regularly inspect the “circuit.”
Wooden boat owners might want to keep thru-hulls isolated. Having all the bronze fittings bonded together can exacerbate a form of decay caused by the chemical reaction between the metal and the wood.