When we hauled our tug last fall, areas around three thru-hulls revealed what looked like corrosion. Some people call them haloes. Investigation found that there is no consensus about the cause. I talked with a corrosion specialist who believes it has something to do with electrical current because it occurred around the thru-hulls. Some people believe they most likely occur due to interaction between the copper bottom paint and the bronze strainer or the thru-hull. Even though the strainer is not bolted to the thru-hull, corrosion can still occur because salt water is a conductor of electricity.
"Some People call them haloes."
The yard assumed that improper bonding caused the haloes. Bonding is electrically joining all the metal parts that penetrate through the hull. Done properly to today's American Boat and Yacht Council (ABYC) standards, there should be a green wire of fairly good size that runs from each fitting, including the rudder, prop shaft, and engine, to a common ground point inside the boat. It appears that when our tugs were built around three decades ago, the accepted practice was to bond to the engine. At some point hence ABYC dictated that the engine should not be used as a ground buss.
The solution: create a dedicated buss bar that should not be connected to the shore power neutral white wire. I removed all bonding connections from the greasy old wet engine and put them onto a nice neat clean buss bar conveniently located. I then relocated the electrical grounds that were on the engine to a point on the ceiling directly above it.
It's important to remember that the engine still needs to be grounded.
More on the subject regarding Haloes
ReplyDeletefrom BoatU.S. Magazine February|March 2015
NO ANGEL HALO: "Haloing" of bottom paint is sometimes observed on boats with bonding systems that are in poor condition or are providing overprotection of the bronze components. It's most prevalent with vessels using black or other dark-colored bottom paint and is more common with certain brands containing higher amounts of copper. If found on your boat, inspect your bonding system connections (if you have one) to ensure they are clean, tight, and corrosion free. Another good idea is to conduct a corrosion survey to ensure proper galvanic protection is being provided.