approach. Many European boatbuilders take
this approach. In the United States, we favor
bonding; the ABYC advocates bonding, and
I support that position for most (but not all)
boats.
In some cases, bonding is just not practical
or necessary. For example, small trailerable
boats with only a few through-hull fittings
aren’t really a concern. They can be easily mon-
itored, and if the boat spends most of its life
sitting on a trailer, bonding is overkill. For
wooden boats, bonding may not be a great
choice either; faulty cathodic protection sys-
tems on wooden hulls can wreak havoc with
the wooden structure of the boat.Conducting a Corrosion Survey
Most corrosion surveys are performed
because the boatowner is having a problem:
either excessive corrosion or, more commonly,
rapid anode consumption. Troubleshooting
the problem requires first gathering some
data: water temperature, current, depth, and
salinity; boat usage; and the presence of new
boats in the area. These can all impact how
long an anode lasts, and you (or, in com-
plex cases, a corrosion specialist) will spend
quite a bit of time researching changed con-
ditions when the complaint is rapid anode
consumption.
For the purposes of this book, we’ll confine
ourselves to learning how to answer a few basic
questions:- Is the boat cathodically over- or under-
protected? - Is the boat equipped with a galvanic isola-
tor? If so, is it functioning properly? - Is there any electrical current leakage into
the bilge water or the bonding system? If
so, which circuit is causing this induced
current?
We’ll begin by confirming the calibration
of the DVOM; it should be correct to within
less than 50 millivolts (mV). We will be
measuring very small values, and sensitivity is
quite important.
Here’s how to check calibration using a
zinc anode and a silver/silver chloride refer-
ence electrode:1 Using a small-engine pencil zinc anode as
your test piece, attach it to your DVOM’s
positive lead.
2 Connect the silver/silver chloride refer-
ence electrode to the DVOM’s “com” or
negative jack (top photo opposite).
3 Set the meter to DC volts and submerge
the zinc and reference electrode in salt
water. Be sure the electrodes are not
touching the bottom; 1 foot or so below
the surface is just right. The reference cell
and the zinc anode should be within sev-
eral feet of each other.
4 The meter should read approximately
–1 volt DC (bottom photo opposite),
indicating that the meter is calibrated
correctly. A reading of 50 mV or so one
way or the other indicates the zinc alloy
varies slightly, which changes its actual
voltage potential.
5 If your meter readings are not approxi-
mately –1 VDC, try another anode; the
one you are using may not be zinc, but
rather magnesium or an aluminum alloy.
6 If a new anode doesn’t help, see if another
DVOM makes a difference. If it does,
then your meter is at fault.It’s important to note that the voltage
potentials given here assume you are working
in a saltwater environment. In fresh water, due
to its relatively low conductivity, voltage
potentials will be much lower. For example, a108 electrical systems troubleshooting