EXPOSURE cites Stan Honey’s excellent article, which recommends NOT connecting ISOLATED Thru-Hulls to the boat’s Bonding System. TALBOT doesn’t bond his “Isolated” Thru-Hulls. In fact, most experts don't recommend the bonding of electrically isolated
metal thru-hulls and seacocks.
WHEELS, however, recommends bonding all Thru-Hulls.
This is a complex, and sometimes contentious issue - To Bond or Not to Bond (isolate)!
Are your Bronze Seacocks, in fact, Isolated?
Metal valves and plumbing
systems connected by hoses to any water
source (including bilge
pumps & raw-water-cooled exhaust
systems) may NOT be Electrically Isolated. The water within the non-conductive Hose may provide an electrical
path between the otherwise (assumed to be) Isolated components.
BTW: Forespare DOES recommend Bonding Marelon (Non-Metalic) Seacock Fittings. “Properly bond or ground the bronze thru-hull. It is still subject to electrolysis and corrosion, even with the Marelon® valve installed.”
ABYC doesn’t dictate a method for addressing galvanic corrosion. They do provide guidelines for a particular method if chosen to be used. "Where metallic thru-hull fittings and drain plugs are installed below the normal accumulation of bilge
water, consideration shall be given to the installation
of a bonding system. See ABYC E-11, and ABYC E-2.
boat with an aluminum stern drive is kept in a marina and is supplied with AC electrical power from the dock
. A boat nearby on the same dock
is also built of fiberglass
, but its propulsion
system uses a standard inboard engine
driving through a stainless propeller shaft and a bronze propeller. This boat also is supplied with AC power from the dock. Both boats, either purposefully or inadvertently, have electrical continuity between their on board AC safety
ground systems and their underwater metal structures. Since both boats also have continuity between their AC safety
ground systems and the dock safety ground system, there is electrical continuity between their underwater metal structures. Because the aluminum stern drive is more reactive than the bronze and stainless drive hardware
, the stern drive will corrode, acting as a sacrificial anode, and will deliver galvanic electrical current
to the other boat's propeller and shaft via the AC safety ground system.
AC BONDING & ISOLATION:
There should be no direct (low resistance) connection between any of the AC shoreline conductors and a vessel's underwater metal structures.
There are several solutions to the problem of exposure of a boat's underwater metals to influence by electrical currents passing through the AC shoreline distribution system. The most effective (and expensive) is the installation
of a properly sized AC ISOLATION TRANSFORMER aboard the vessel. This transformer should be installed as the first device downstream of the AC shoreline receptacle, ahead of the ship to shore switch and the electrical distribution (circuit breaker) panel. The transformer should be installed on an electrically isolated mounting with the AC shoreline safety ground conductor connected to the metal housing of the transformer. The shoreline safety ground conductor should not be connected to any on board AC device that is being supplied power from the secondary side of the transformer and/or to the under water metal structures of the vessel. Use of a properly designed and installed isolation transformer electronically isolates the electrical power being used aboard the boat from that supplied by the shoreline. This will effectively prevent corrosion currents from using the shoreline as a conductive pathway between the boat with the device and any other vessel using the same dock power system.
Another solution, though not quite as effective as a transformer, is to install a GALVANIC ISOLATOR
in the AC shoreline ground conductor between the boat's receptacle and the first AC device, such as a ship to shore switch or distribution panel. This device allows AC current
to pass unimpeded, while blocking low level (galvanic) DC currents. A galvanic isolator
will block approximately eighty-five percent of the potential problems that occur due to safety ground interconnection between boats.
I look forward to further stimulating & (perhaps) controversial discussion.
SOME ON-LINE REFERENCES
“Corrosion, Zincs, & Bonding” - By Micheal Kasten (Metal Boat Qurterly)
Marine Electrical Check List - by Robb Zuk