Your brand-new bilge pump blows a fuse every other trip, and your VHF goes dim whenever you try to transmit. An easy way to find the source of your problem is to get out your multimeter and do some electrical investigation.
But first, read the instructions and
be sure you understand them.
If not, you could hurt
yourself and your
you must have a
basic understanding of wiring and
what is dangerous.
This includes the differences between AC
and DC electricity and
wires on your boat.
Got it? Great! Now,
let’s review the basics of using
a multimeter to measure DC current, resistance, and voltage.
Current is the rate of flow of electricity running through
the wires in your boat. Think of it like a garden hose, where
current is the amount of water coming out. It is measured
in amps. When a current is interrupted by something like a bad connection or
improperly sized wiring causing a device to stop working, or at least stop working
as it should, this is known as resistance – like a kink in the hose. And voltage is, to
put it very simply, somewhat similar to the amount of water pressure in the hose.
The more pressure, in theory, the more power the current can produce.
A multimeter can be used to measure all of these things and more. Some units
are more complex than others, performing more functions. Some have different
features to better measure the same functions (such as auto-ranging vs. manual).
Some are digital and some are analog (with a needle). Most of us have simple needs,
such as determining basic current, resistance, and voltage, and don’t need to spend
the extra money for complicated features. However, it’s worth spending money for
quality, regardless of usage, because electricity can be very dangerous.
68 | Multimeter 70 | Ungrounding 74 | Windlass Service 76 | Water-Pump Maintenance
Do you really need a multimeter? Yes! Once you understand
the basics, it can save you time, money, and hassle when
trying to locate the source of a power problem
BY LENNY RUDOW
Many boats have
AC power, which
can kill. Ensure all
AC power is turned
off to the boat
or isolate shore-power connections,
inverters, and generators. Be certain
of your capabilities, and if in any
doubt, consult a
If the problem
you’re having isn’t
getting devices to
turn on but, rather,
keeping them on
because of a circuit
breaker tripping or
a fuse blowing, you
might be drawing
too much current
through a given
older boats with
devices can exceed
the capacity of the
If you don’t have
good documentation on how many
amps a given
device draws, here’s
how to use your
multimeter to find
out. This test uses
the meter itself
to complete the