Quadrant Marine

 

The Hot Dock

You keep your boat in a crowded marina and there’s a lot of talk about corrosion and bad wiring. It’s all the marina’s fault, right?

Not necessarily, and the reason why is due to a general lack of understanding of what happens in a marina full of boats. After all, this is not an easy thing to understand, and it took a lot of research to find out what was going on here. Yes, the marina’s wiring is involved with all such corrosion problems, but it may or may not be the cause.

The key to understanding corrosion problems in marinas involves several things.

• First, all the boats in the marina are tied into the marina’s grounding system.
• Second, all the boats are also grounded at a second source - the water.
• Thirdly -- a major point here -- the power system on the boat (green wire) is also grounded to the boat’s common ground point, usually established at the engines. This means that your boat’s common grounding and bonding system is electrically connected with the dock’s grounding system.

Before we get any further into this, let’s be sure that we understand our terms here. The shore system has three wires , one of which (black) is obviously the current carrying conductor. The others are the ground (white), or neutral, and the grounding wire (green), which is the safety ground intended to deal with short circuits and protect people from electrical shock. Also called a "bond," it is attached to the frames of all appliances and other electrical devices. The ground, which is the negative (-) side of the system, is supposed to be at earth potential, but this is a normally current carrying conductor, whereas the grounding wire isn’t.

Anodic means that a metal is less highly charged than others. Stray current will seek a path to an anode, and from there travel to ground, the water. When an anode is energised by an outside source, as the electricity leaves the anode, it carries molecules of metal away with it. This electrical erosion is true electrolysis. Cathodic means that a metal is more positively charged; current will flow from it, and on to one that is anodic, with no resulting metal loss or erosion. Thus, with a zinc attached to a stainless shaft, the zinc erodes while the stainless is protected. Adding an outside source of current accelerates the process.

Galvanism, or galvanic current, is caused by dissimilar metals, metals with a greatly different electrical potential that are electrically joined. Galvanism plays the primary role in the "boat battery" problem described below.

 

Next, consider that all the boats in the marina are connected together electrically by both of these wires, the neutral and the ground or bond, one of which is also joined to the underwater metals of the boat (the bond). The boat’s neutral is never grounded to the boat itself, but always earth potential of the dock ground. The underwater metals of the boats, of course, can vary between such things as brass, bronze, stainless steel -- and aluminium, as in outboard motors, aluminium boats and stern drives. All of these metals can develop different electrical potentials galvanic ally in the same manner as a battery does.

We could have, for example, ten boats on a pier all in a row, and all tied together via these ground systems. Each boat in this chain creates a galvanic "cell," just as the plates in a battery does. The average inboard boat develops about volt or less via its submerged metals, so that ten boats connected together has the potential to develop 5 volts since cells in series increase overall voltage in a line by the value of each cell.

Electricity is obsessed with finding a path to ground and will always do so. It will follow the path of least resistance. If the 5th boat in the row were an aluminium stern drive boat, as shown in the illustration below, what do you think would happen? Yes, all those boats with bronze and stainless parts on the bottom, which are far more noble than aluminium, are going to set up a nifty little battery with the stern drive or aluminium boat as the sacrificial anode! The inboard boats will do just fine, but the stern drive or aluminium boat will see its drives or hull damaged or destroyed.

Much the same thing can happen to boats that underwater metals of inferior alloys such as low grade bronze or stainless steel such as the kind that often comes from the orient. Boats with cheap "active" alloyed metals are going to be anodic to those with high quality passive alloys. Alloys are often termed as active or passive based on the amount of alloying metal that raises or reduces its electrical potential. Only passive metals should be used on boats. Stainless shafts are passive, whereas other alloys may not be, or less passive.

What we have then is a veritable "boat battery" where each boat on the circuit acts like a battery plate. So it is that this has nothing to do with the dock’s wiring, though the marina often gets blamed for these problems.