Chuck Husick: Techno-Talk, November 2003 BoatUS Magazine - updated August 2009
Marine radar is the most significant maritime collision
avoidance device ever developed, notwithstanding the fact that the radar-equipped
Andrea Doria and the Stockholm collided in the Atlantic in 1956. More
recently, a tug and barge collided with a radar-equipped sailboat skippered
by one of the nation’s foremost authorities on boat safety.
Do these unfortunate events mean that radar is not an effective an anti-collision
system? In fact, radar can be wonderfully effective. These collisions
were caused by failure to plot and track the changing positions of the
Radar works best when the area around a boat is repeatedly checked to
determine if any other vessel is on a course that will bring it uncomfortably
close. The good news is that radar target tracking can be easier and
more accurate than visual tracking.
On the Screen and Why?
What you see on the radar’s display is not an electronic image
of what you can see with your eyes, anymore than a medical x-ray is
a photograph. The image on the radar screen shows the relative ability
of objects to reflect a tiny fraction of the radio frequency energy
pulses the radar has transmitted back to your radar antenna.
Dense materials such as land and metal reflect the radar signal very
well, fiberglass boats and daymarks mounted on wood pilings are, in
radar terms, stealthy, reflecting little if any of the energy. The degree
to which we can “see” an object on our radar also depends
on the direction in which the radar energy bounces off an object. A
small reflecting surface that bounces energy back in the direction from
which it came will be much easier to see than a large surface that reflects
lots of energy off in some other direction.
Learning to identify what the blips on the screen represent can take
some time. Use your radar on clear days so that you can match the on-screen
image with the visual scene. You can learn a great deal even when your
boat is anchored or tied to the dock, especially if other boats are
moving about nearby.
and Tracking Targets
The plotting and tracking of targets (blips) on a recreational boat
radar screen is done manually. The process is no different from taking
a visual bearing on another vessel and then checking a minute or two
later to see if the bearing has changed. Radar’s ability to precisely
display both the relative bearing and the distance to the target makes
it easier to plot the target’s position than when taking a visual
bearing, provided you have practiced enough to recognize the target’s
Even the smallest of today’s radars typically provide two means
for precisely measuring the relative bearing angle and the range to
a target, the cursor control and a combination of the Electronic Bearing
Line (EBL) and Variable Range Marker (VRM).
The cursor control works like a computer mouse, creating a cross mark
or a small circle on the screen. The cursor’s position relative
to your boat (at the center of the screen) is shown in degrees or relative
bearing and distance in nautical miles. (If your radar is connected
to the GPS, the latitude and longitude of the position may also be shown).
You might write down the target’s range and bearing and compare
the recorded position with the target’s position a minute or two
later. Or you might lightly mark the target’s position on the
screen with a grease pencil. Alternatively the EBL and VRM controls
can be used as an electronic substitute for the grease pencil mark.
Connecting the dots created from successive position plots will show
the target’s course. If the target’s relative bearing has
not changed and the range is decreasing you should change your course
or speed or prepare for a close encounter.
Regardless of how you do it, the bottom line in collision avoidance
is simple. Assume any boat you can see, either visually or on the radar
is “out to get you” and keep track of its relative position
until you satisfy yourself that it poses no immediate threat of collision
Automatic Information System (AIS)
Many of today's radar displays, especially those integrated with chartplotters can present the target information obtained from a vessel's AIS. Identifying the targets that represent AIS reporting vessels will greatly simplify the interpretation of the overall navigation display system..
Your radar provides a number of controls that can enhance your ability
to see targets in difficult conditions. These controls must be used
with care since overuse of any of them can reduce your chance of seeing
a possibly critical target.
Sea clutter, the radar energy reflected from the nearby sea surface,
can overwhelm the signals reflected from real targets, including other
vessels and navigation aids. The STC “sea clutter” control
reduces the sensitivity of the radar’s receiver to varying strength
signals returning from close-in targets, making it easier to see signals
from boats, buoys and other “hard” targets. A low STC setting
will make targets easier to see, use too much and you risk not seeing
something you may wish you had seen. Some radars provide an automatic
The FTC (fast time constant) control, often called the rain clutter
control works by discriminating against the rapidly changing signal
reflections from raindrops. As with the STC, use too much of it and
you risk not seeing a real target. Turn it off when you want to track
the movement of a rainstorm. Your radar may have an interference control
(IR) mode that will reduce or eliminate the occasionally annoying curved
dashed lines caused by signals from other boat’s radars.
Remember to use the STC, FTC and IR controls with discretion since they
may reduce the sensitivity of the radar to returning signals. Experimentation
is the best way to learn their proper use.
constantly monitoring the radar screen, noting, plotting and tracking
targets is the best way to use radar for collision avoidance, there
are times when there is little or no nearby traffic. At such times,
the radar’s automatic guard zone can be used to alert the user
to the sudden appearance or gross movement of a target. The guard zone
is centered on your vessel and depending on how you set it up, a target
entering or a target leaving the zone will sound an alarm.
The guard zone can resemble a donut and completely encircle your position
with selectable inner and outer radius or consist of a sector, such
as ±30 or 45 degrees from your bow. If you are traveling in the
company of another boat you might establish a target-leaving alarm guard
zone ahead of your vessel, containing the radar image of the vessel
you are following. Set that way, the guard zone would alert you if you
fell too far behind or came too close to the vessel ahead.
Sailors, who often wish to limit their use of electrical power, will
particularly value the “watch” mode included in many radars.
The watch mode keeps the set in standby mode, consuming very little
power until the interval timer turns it on to scan for targets.
Not Just for Big Boats
The risk of collision is no less and perhaps slightly higher for small
boats than for large yachts that often make open water passages where
there is less traffic. Boats as small as 20 feet can and in many instances
should be equipped with radar. You don’t need to spend a great
deal of money to enjoy the benefits of radar navigation. Sets costing
not much more than $1,000 with a maximum range of 12 to 16 nautical
miles are perfectly adequate for most coastwise navigating in recreational
vessels, including boats 30 feet and longer.
Whether your radar’s maximum range is 16 miles or 72 miles you
will use ranges greater than six to eight miles only occasionally. The
minimum, often 1/8-mile range provided by many radars, can be invaluable
when looking for navigation aids in poor visibility or at night and
the two-mile range may be the one most often in use.
Radar’s primary use at longer ranges is for tracking rainstorms
and detecting flocks of birds circling over schools of fish.
Small radars are easy to install. The antenna unit that contains the
transmitter and receiver is connected to the display with a multi conductor
cable. Providing you don’t have to cut the cable to pass it through
a mast, all you have to do is plug it into the display. For the basic
installation, the plus and minus 12-volt power leads are the only two
other wires that need to be connected.
The radar antenna should be mounted on a mast above other structures
where it will have an unobstructed view in all directions. Placing it
at least five feet above anyone’s head will satisfy radiation
There is little to be gained by installing the radar antenna on a very
tall mast. The distance to the radar horizon varies with the square-root
of the height of the antenna. At nine feet above the water, the radar
horizon will be 3.6 miles distant, at 18 feet it will be 5.1 miles.
Placed too high up on a sailboat’s mast, the radar might miss
seeing a nearby target on the windward side when a boat is heeled over.
Sailboat installations can use an eight-foot radar mast mounted on the
stern or no more than 20-25 feet up the forward side of the main or
mizzen mast. Don’t worry about the blind spot caused by the mast,
it will only affect radar visibility over a small aft facing sector.
Mount the radar display at the helm of the boat, in clear view and within
easy reach of the helmsman. Using radar involves lots of button pressing
and knob turning. The screen must be easy to see in the brightest sunlight
and also able to be dimmed for use at night.
Connecting the radar to the boat’s GPS or Loran C will make the
active waypoint visible on the screen as a “lollypop,” a
small dotted circle connected to a line extending from the boat’s
position at the center of the screen. Connecting the boat’s electronic
heading sensor, usually a part of the autopilot, to the radar, will
allow the radar “picture” to be seen in a North Up presentation
that corresponds to the usual way we view charts.
You don’t have to be on an ocean voyage to benefit from what your
radar will show you. Its greatest value can be in heavily traveled bays,
harbors , rivers and lakes. When deciding when to use your radar remember
that familiarity breeds competence. The rotating antennas you see on
big ships are not there to scare off the birds, they are providing the
watch with invaluable navigation data.
By Chuck Husick
Chuck Husick is a pilot, engineer, sailor and former president of Chris