| Getting A Good Fix
by Andrew Howe
Editor's note: Most sailors are probably unaware of the breadth of their skills and knowledge. But the fact is, to be even halfway competent as a sailor, you must have a grasp of a wide variety of skills, from sailhandling to knots to engine troubleshooting. Not the least of these skills is the ability navigate effectively by combining and evaluating as much information as you can. The following article describes how to put together a complete navigational picture.
It's hard to recall a time when "fix" didn’t
mean a latitude/longitude readout on the display screen of a GPS receiver. But it
wasn’t so very long a go when sailors everywhere were struggling to plot loran
time-difference lines, all the while watching out for good crossing angles, proximity to
baselines, ground-wave conversions, and other such esoteric stuff. And then the switch was
made to lat/long readouts, and we’ve never looked back.
But a look back is necessary on occasion, especially since the intelligent navigator never relies on a single source of information. So, in addition to
the ubiquitous lat/long readout, here are some other worthy additions to the all-important
fix.
Soundings: Often overlooked (except to note that
“it’s not too shallow yet”), depth is put on every chart for a reason.
Using a depth sounding in conjunction with another piece of information, say a visual bearing or a
radar range, can make the difference between certainty and doubt.
This is especially useful in coastal piloting situations when depth
curves show up on the chart. All it takes is a quick glance at the depth sounder when
taking a bearing or grabbing a range off the radar, and a decent fix results. Sometimes a
depth sounding alone can provide sufficient information for the navigator. For example,
sailing in the Fort Lauderdale/Key West Race, I can recall tacking in toward the outlying
reef and making the decision to tack back out based on the depth curve (while, of course,
also checking the GPS position and the radar and visible images of the outlying lights).
Visual bearing: Get out that old hand-bearing compass
or learn how to use the bearing compass in that fancy new set of binoculars. Practice
taking bearings so that, when a round of bearings becomes necessary, the mechanics will be
second nature. A visual bearing can be very precise, but sometimes a quick and dirty
bearing is all that is practical due to weather or motion. As a way to get a coastal
navigation fix, nothing is easier than a bearing. The best part about visual bearings is
that they force the navigator to look outside and see what’s going on.
To use visual bearings, the navigator must first check out the chart to
find objects along the route that will likely be visible. (Circling them on the chart
helps find them again when it comes time to plot.)
With a multiple-bearing fix, a good rule of thumb is to take the
bearings positioned more off the beam first as they will change the most rapidly. Choose
bearings with good crossing angles to give the best fix geometry. And, if possible, shoot
three or more objects for the best possible results. To correctly plot a visual bearing,
first remember that the bearing is toward the object, so an LOP should be drawn pointing
in that direction. The plotted bearing line doesn’t have to be very long (three to
four inches is okay), but it should have an arrowhead indicating where the object is on
the chart, the time, and the degrees magnetic (assuming a magnetic compass of some sort is
the source). This labeling protocol helps keep a clean record on the chart and enables the
navigator to check his/her work. Just like practicing with the bearing compass, plotting
practice is a must. Try a few bearings when you are confident of your position, plot them
and compare with a GPS fix, to develop confidence. You’ll be surprised how accurate
you can be.
Radar ranges: If radar is installed, learn how to grab a quick radar
range. Like visual bearings, a bit of chart study beforehand is advisable to find good
radar targets. Keep in mind that sandy beaches and swamp areas prevalent along some coasts
make poor radar targets, as the radar beam can “skip” on shore for several yards
before returning, yielding an inaccurate result. Better to use a prominent bluff area,
bold rocks, or manmade structures like lighthouses at the water’s edge, all of which
make a clean radar target. A review of chart symbols will serve to remind the navigator of
what makes a good target.
A correctly plotted radar range is similar to a visual bearing, except
the navigator draws a short arc with a compass, with time and range written on the arc. It
should be pointed out that most yacht radars are only accurate to within +/-5°; so, if
possible, combine a radar range with a visual bearing or use multiple ranges rather than
relying on a radar bearing. Again, practice and plot to develop confidence in the gear and
your skill.
Natural range: This is one of the greatest cockpit
navigation tricks going. Study the chart and discover where land forms intersect to show
ranges. Does Pond Island disappear behind Fish Point at the corner of the channel? When
does the water tower appear over the end of the breakwater? From this information, often
grabbed on the run, the navigator can quickly orient the boat on the chart. Of course, at
some time a "real" fix is needed, which can use the range information as the
basis for the fix, adding other information as available. Familiarity with the chart can
make this a very reliable navigation tool.
Radio beacon: They're still out there, and they
work just as well as they did when that new Ray-Jeff RDF came on board! Have some fun and
tune them in. Radio beacon LOPs plot just like any other LOP, with a line drawn toward the
beacon, properly labeled with time and degrees. Look in any navigation text for a review
of how to use them, especially if you’re planning on voyaging to foreign waters,
where they are still much in use. For navigators with children aboard, the radio beacon
can provide a good introduction to the world of navigation since beacons make a sound,
which makes demonstrating good and bad tuning easy.
Combinations: When was the last time you grabbed a GPS
position and also recorded the depth? Do they agree? Or ran out on deck to sight a visual
bearing over the steering compass before blithely turning to a new course plugged into the
GPS? If the channel is tight, the visual bearing may be more accurate (unless differential
GPS is installed). At night, with the radar on, have you ever asked the on-deck watch to
confirm the lights they see? Everyone on deck should be capable of identifying the lights
en route and taking a simple visual bearing using a handbearing compass or a bearing taken
over the steering compass.
There are many choices open to every navigator. Learning to take
advantage of all the tools available increases confidence of the navigator and crew and
increases the safety of the boat.
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