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Old 08-23-2001
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Kathy Barron is on a distinguished road
Bonding Thru-Hulls

The shaft and hull zincs on my boat disappear after a very short time. Because the boat has all of the thru-hulls bonded, I'm concerned that I may have a severe electrolysis problem that will sink the boat if I don't replace my zincs soon enough. Am I right about this?

Kathy Barron responds:
Thanks for the question. There's nothing like opening up Pandora’s box. There are, in fact, two distinct approaches to the subject of bonding thru-hulls as Kevin Zinn of the American Boat and Yacht Council (ABYC) suggests: "…ABYC recognizes that there are two schools of thought on this issue; to bond or not to bond. ABYC does not dictate a method for addressing galvanic corrosion, rather ABYC provides guidelines for a particular method if chosen to be used, e.g., a 200 millivolt shift relative to the potential of the least noble metal being protected."


There are also two possibilities as to the source of your particular zinc loss:

  • You could be in a "hot" marina with lots of stray electrical currents in the water from bad shoreside wiring or neighboring boats.
  • The electrical leak is on your boat due to faulty wiring.

Most small zincs used on prop shafts are designed to protect against currents created by dissimilar metals immersed in sea water when the two parts are electrically connected. Zincs cannot provide enough galvanic potential to protect against substantial electrical currents that may be flowing in the boat or in the water. These stray electrical currents will cause electrolytic corrosion to electrically bonded thru-hulls and other metal parts.

Electrically isolated, good quality marine bronze is electrochemically stable in salt water and, if isolated (not bonded), does not always need galvanic protection, particularly when they're not in contact with other dissimilar metals also submersed in salt water.

There are several variables in making the decision to bond: for instance, speed of water movement across the hull and vessel usage (less frequently used vessels require less protection). Increased water conductivity increases galvanic activity, and the need for galvanic protection increases with the pH, salinity of the water, and the wearing away of protective coatings. Also, if the thru-hulls are in an area of the bilge where they lay in bilge water most of the time—below the level of float switch activation—they should be bonded to minimize stray current corrosion. ABYC says, "The need for a cathodic protection system for metal appendages on nonmetallic hulls may not be justified if the metals coupled are galvanically compatible. However, individual testing on a case by case basis is necessary."

However, this is only half the answer. The ABYC also recommends that boats should be equipped with a lightning protection system, and a major part of that system is to bond all underwater metal parts into the lightning path. This could involve not only the thru-hulls, but also aluminum, copper, bronze, stainless, Monel, and even brass, all tied together—talk about an electrolytic basket case. But ABYC does warn that this type of bonding can lead to an increased possibility of galvanic corrosion.

So now what do you do—leave the boat unprotected against a lighting strike to prevent corrosion, or bond to save the boat from lightning and keep replacing the zincs to save the thru-hulls? Consider the location of the boat—is it in a high-strike area for lightning? If so, you might want to opt for the lightning protection system.

Regardless of the choice you make, consider doing the following things:

  • Ensure that all AC and DC wiring on your own boat is up to par and is not the source of the current "leaks."
  • Reduce the amount of DC galvanic current flow at your dock by providing a galvanic isolator in series with the grounding conductor of the AC power.
  • Add more zinc by hanging a block of zinc or a "fish" (found in most marine stores) over the side of the boat with a #10 green ground wire connected to the prop shaft or a thru-hull with an alligator clip. This provides additional protection and, because of its size and density, you won’t need to replace the shaft zinc as often. Just don’t forget to reel in the fish before backing out of the slip.

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