Sailing Sense, Reflections on Radar

Shortly before midnight on July 25, 1956, two ocean liners, the Andrea Doria and the Stockholm, collided off the New England coast and 50 people lost their lives. The events that led to the collision remain poorly understood, but it is known that both ships were using radar. The tragedy is often referred to as history’s first radar-assisted collision.Radar is, of course, an important part

Shortly before midnight on July 25, 1956, two ocean liners, the Andrea Doria and the Stockholm, collided off the New England coast and 50 people lost their lives. The events that led to the collision remain poorly understood, but it is known that both ships were using radar. The tragedy is often referred to as history’s first radar-assisted collision.

Radar is, of course, an important part of the navigator’s toolbox; some sailors consider it to be more useful than a GPS/chartplotter. But like any piece of gear, if it isn’t used correctly, it can get you into trouble. Here are the things you need to be aware of when using your radar.

Size and shape on a screen. Because a radar screen displays reflected radar waves, what you see doesn’t always correspond to the size and shape of the actual object. Rather, the image you see is a function of the reflectivity of the target and the angle at which the radar waves hit the target. For example, a metal buoy may be very distinct on your screen, while a pile of floating wood—say, a wooden boat—might not be visible even though the boat is much larger than the buoy. Keep in mind that metal reflects better than wood and that a sharp angle reflects better than a curve. The best way to develop a feel for how different objects appear on your screen is to practice: In good visibility, look at the screen and then at the actual object(s).

Direction and speed of a target. The speed and direction at which a target—another boat, for example—moves across the screen is not its actual speed and direction (unless your vessel is stationary). Failure to understand this point has caused many collisions. Small-boat radar units display relative motion, the motion that is taking place between you and the target. Think about this as analogous to the relationship between true and apparent wind. Your anemometer measures apparent wind (which can be converted to true wind if you know boat speed and direction), just as your radar measures relative, rather than true, motion.

Professional radar operators use many plotting techniques to avoid collisions, but most sailors don’t have the extensive training and practice needed to become proficient. In the absence of plotting, the following rule of thumb can be extremely useful: When an object is moving along a straight line toward your position—usually this is the center of the screen, unless you have offset it—there is a risk of collision (Fig. 1). This constant radar bearing is equivalent to observing that the bearing of a visual target remains unchanged. Although you can “eyeball” this motion on the screen, it’s better to put an electronic bearing line (EBL) on the target, as well as a variable range marker (VRM) or the cursor. Seeing the target moving along the EBL toward the center of the screen will alert you to the fact that a collision is possible (Fig. 2).

Because a radar unit has a limited number of EBLs and VRMs, you have to select which targets to track; a large vessel usually takes precedence over a smaller boat. If there are numerous targets around you, it’s useful to turn on echo trails. An echo trail is not as precise as an EBL, but it can give you a general sense of the target’s relative motion (Fig. 3). More-sophisticated (and expensive) radar units have built-in Automatic Radar Plotting Aids (ARPA) that can be very helpful for collision avoidance.

Radar can’t see everything. Several years ago, I was sailing in dense fog about 50 miles offshore. While I was on watch, we came within about a hundred feet of another sailboat; the boat didn’t show up on the radar screen until we were nearly on top of it. It turned out to be a wooden boat. We got a good scare, but neither of the boats sustained damage. This incident drove home the point that radar does not see everything.

Radar can see things that don’t exist. Even a properly adjusted radar set can display spurious images. For example, a radar signal might bounce back and forth several times between the metal on your boat and the metal on another boat, making it appear that there are multiple targets. Another common problem, the result of “spreading out” of the transmitted radar signal, is side-lobe effects. This can lead to a series of false images in line with the actual target (see Fig. 4 for an example).

It is usually less dangerous to see something that doesn’t exist than to not see something that does exist, but the experience can be disconcerting. The best way to deal with these errors is to read your manual and practice with your unit, in conditions of good visibility, so that you can learn to minimize, recognize, and safely discount spurious images.

Bad weather can degrade performance. Many sailors are unaware that dense fog and/or heavy rain can decrease radar sensitivity. As you gain familiarity with your unit, you can learn to compensate (somewhat) for this effect. Slightly adjusting the gain, for example, can sometimes help in fog, and most units have rain-clutter filters. However, these adjustments can do only so much. The best solution is to install the most powerful unit you can afford. For example, in adverse weather conditions, a 4 KW unit will likely pick up weak targets that a 2 KW unit will miss.

What is close is more important than what is far away. It’s common to mount a radar antenna as high as possible in order to increase a unit’s range, but the tradeoff is an increase in the minimum distance at which you can see a target when it is close in (Fig. 5). As a practical matter, the difference of a mile or two on the long-range scales is inconsequential. On the short-range scales, however, a difference of 10 or 20 feet can be very important—for example, when you’re running between channel buoys in a dense fog. I believe (and not everyone agrees) that if close-in work is important, the radar antenna should be mounted only as high as is necessary to keep the crew from being radiated. An added benefit is that this location helps decrease weight aloft.

Consider the other boat’s radar. You should never assume that seeing another boat on your radar means that the other boat’s radar can see you. Sailboats in particular tend to be poor targets, which is why it’s so important to mount a radar reflector on your boat.

Stabilize your radar. Stabilizing your radar refers to interfacing the display unit with a heading sensor. A major benefit of stabilization is that it decreases smearing an image if your boat is yawing. Stabilization also permits a north-up (as opposed to head-up) screen display; this can be very helpful when comparing the screen image with a chart.

Never become overconfident. Radar has led to accidents because the increased ability to “see” leads to overconfidence. A number of years ago, on a delivery from Nova Scotia, the owner of a radar-equipped luxury yacht insisted on plowing ahead at full speed, through dense fog, into Southwest Harbor, Maine. I still remember being awakened by a sudden shudder as we ran hard aground. Unfortunately, despite the (over)confidence of the owner, radar doesn’t see barely submerged rocks.

Know how your unit works—before you need to use it. The only way to increase your proficiency is to build up your practice time with the equipment. Do it when conditions are good, not in dense fog or darkness. That’s not the time to learn what all those funny-looking images on the screen mean. There is no question that radar is a great tool for navigation as well as collision avoidance. And as with any tool or instrument, practice is the only way to create genuine competence.



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