Instruments and Instrumentation

Tech Tips
by Bill Whitney
This column is a forum for sharing the vast range of practical should have the compass / vessel system checked by a
experience accumulated by our membership and not just my professional compass adjuster. The main point here is to check
favorite boat maintenance topics. It is intended to be the place your compass at least annually.
where you, the reader, can ask technical questions and either Another group of instruments that we tend to ignore are those
obtain direct answers in this column or direction to appropriate that monitor the engine functions. 99% of the time we take
reference material. these instruments totally for granted, until something suddenly
Since you have not asked for specifics since the last newslet- doesn’t seem right and you really need know what is going
ter, I’m suspicious that you may not know how to contact me. wrong. Now you’re really aware of the gauges. But are they
You can contact me via e-mail at wmewhitney@earthlink.net, right?……Do they even work? Have you ever checked them?
or better yet, see me at one of the FSS. meetings. For most of us the honest answer is no, we never have. We
Just to refresh your memory the planned topics for this always took their readings at face value and never questioned
column were; electrical wiring, alternators and regulators, their accuracy.
batteries, grounding and bonding, radios and antennas, and From a practical standpoint, you don’t really have to calibrate
instrumentation (navigation aids, depth, speed, GPS, LORAN, these instruments, but you do need to check their operation
etc.). We have already covered wiring, alternators and regula- before starting the engine and during the warm-up process. You
tors, batteries, bonding and grounding, radios and antennas; so also need to know what the “right” reading should be. Do you?
let’s talk about instruments and instrumentation. This is a big Electrical gauges have the unique characteristic of having both
topic, so it will probably be divided into two or more a mechanical zero and an electrical zero. This property makes
installments. them easy to check for basic operation. Before you turn
To begin with, let’s start with a statement: “All instruments anything on (like the ignition key), look at where the needle
are subject to errors and/or misinterpretations.” A warning: “If __ rests. This is the mechanical zero. When you first apply power
you don’t know how to use an instrument, or understand its’ to gauge, but before you start cranking the engine, the needle
limitations, you are better off without it.” And some advise: will move (sometimes jump) to the electrical zero position.
“Caution and commonsense when using quality instruments When you see this movement it indicates that the winding
will usually yield good information.” But enough of this, inside the meter movement are good and the bearings that
hopefully you’ll get my point as we look at instrumentation support the needle are OK. If you don’t ……well something
systems more closely. could be wrong. (Or, in the case of a temperature gauge, you
The basic “instrument ” aboard all vessels is the marine may have an older style mechanical gauge that does not have an
compass. Its’ a truly remarkable, reliable piece of gear once it is electrical meter movement.) An electrical meter movement
Properly adjusted for the boat that surrounds it. Since there are should move freely.
numerous good books on compasses and their use, including the The actual value that it reads is less important as long as you
venerable “Chapman’s”, there is no necessity to explain how know what it displays when the engine is warmed up and
this simple instrument works. But its simplicity can also be its operating normally. In fact many modern meters don’t even
weakness if you don’t understand magnetism. If you have display a numerical value, just an approximate value. The word
moved any major ferromagnetic (i.¢. iron, steel, etc.) compo- “Hot” doesn’t really tell you much, except that this relative
nents on the boat since the last time the compass was checked _value is above the “Normal” range of the engine. But again, the
you may have a problem and not know it. Likewise, if you have main point here is not the technical complexity of the engine
moved any electrical wiring or changed the current flow in _gauges. Its that they have to work, and that you really need to
wires close to the compass, you could also have a problem. know what a “normal” reading is BEFORE the information
Even the storage of your toolbox or placement your favorite supplied by the gauge becomes critical.
binoculars too close to the compass could significantly alter As with the compass, these instruments need to be checked
your port of arrival. for proper operation. My recommendation is that you take a
You should check your compass at least annually, preferably serious look at them every time you run the engine. This way
after all your gear is in its normal stored location. There are you know what the normal reading is and you will be aware of
several ways to do it. I usually consult the almanac (or anything that is changing over time, like a heat exchanger that
Eldridge) and use the sun’s azimuth at sunrise but another is slowly becoming less effective, or an alternator that isn’t
simple way is to use a fixed range on a calm day. You can also recharging the batteries.
use your LORAN or GPS, but remember to pick a position that In the next article we will get into the navigation instruments
is at least 10 miles away from your current position. You also and the things to be aware of when approaching that unfamiliar
need to make sure you have programmed the readout for a harbor in the dark on a rainy night.
magnetic heading. If you have set up the GPS or LORAN for a
true heading verify that you have set the variation correctly. If
the compass heading has changed more than 2 or 3 degrees you