In this era of GPS and AIS, many voyagers might think radar is beyond them. Some mariners could well believe that radar is an expensive piece of equipment that’s difficult to operate, has limited usefulness on a recreational vessel and is practical only on large ocean-going vessels.
But consider Chesapeake Bay where I sail my 42-foot Hunter sloop Dolly G. The bay is a sailor’s paradise: beautiful, full of little coves and gunkholes, usually benign conditions. It can also be challenging given the merchant shipping, barge traffic, numerous recreational sail and power boats, fish traps, summer haze, sudden violent squalls, moderately common fog, commercial and recreational fishing fleets and numerous sailboat races. In this environment, I find that radar is the most useful single piece of electronic equipment on my boat.
A simple standalone radar system costs about $1,500, installation extra. Top-end recreational marine radar integrated with a GPS chartplotter and other electronics, such as fish finder and automatic identification system (AIS), can increase the price tenfold.
So what does a mariner get from this investment? What is the benefit of radar compared to just using electronic charts and AIS?
Situational awareness: In darkness and conditions of restricted visibility, radar is essential. Even in clear calm weather radar is such a useful aid to my situational awareness that I have come to depend on it. My radar is on whenever I take Dolly G out. To conserve power when I am sailing it is not always transmitting, but it is always warmed up on standby ready for instant use.
First and foremost, darkness, light drizzle and any fog up to extreme pea soup do not adversely affect radar detection. These are all situations that would seriously compromise unaided vision.
Second, radar provides depth perception not possible from eyeballs alone. Looking at a big vessel on the water, you have no visual clues to help determine the distance to the ship. Radar tells you exactly how far away it is. Moreover, with a little effort, you can determine its relative course and speed. Radar also allows you to identify coves and mouths of rivers from a couple miles out when the visual shoreline is a hazy green blur.
Third, radar extends the area you can keep under surveillance. Standing at the helm, my visual horizon is roughly three nautical miles in front of me with an arc of good vision of roughly 40 degrees. Visually I can monitor about three square miles without moving my head. With the radar antenna mounted at 30 feet above the waterline the radar horizon is about six nm and the arc of coverage is 360 degrees. With radar, I can monitor about 110 square miles. The 40-to-1 increase in monitored area is a huge safety advantage.
Fourth, radar detects and tracks severe weather. Because thunderstorms and squalls are high in the atmosphere, they can be detected and tracked well beyond the six-mile (on my sailboat) radar horizon, allowing you to determine if the storm is heading for you and providing ample time to prepare. This capability is quite useful on Chesapeake Bay in the summer when afternoon thunderstorms are common and summer haze often restricts visibility so much that you cannot see approaching storms until they are on top of you.
Target identification: Equally important as learning how to adjust the controls to balance target detection and clutter from rain and waves is learning to identify all the blips on the screen. Target identification offshore is not very difficult because there usually will not be many targets. The huge reflection is coastline, the large well-formed blip is a large vessel, and the larger amorphous echo is severe weather. In a crowded environment like Chesapeake Bay, however, identifying the dozens of small blips on the display is difficult. Fortunately, all modern recreational marine radars provide a very useful tool sometimes referred to as “echo trails” and sometimes as “wakes.”
Normally, every time the radar antenna sweeps by a given bearing, the system erases whatever is on the screen at that bearing and displays the targets detected on the current sweep. That is, there is no memory in the display. With the wakes feature activated, everything that was detected on the past several antenna sweeps is shown (the duration of the memory is adjustable). With wakes, the system displays each target as a line rather than a dot. The direction of the line is the target’s course relative to your own ship’s heading and the length of the line is the target’s relative speed. Since the most recent blip is color-coded (my radar displays the current sweep in yellow and the previous sweeps in blue), it is obvious if the target is approaching or receding.
Knowing the relative course and speed of each blip, you can identify stationary targets such as buoys and anchored vessels as well as other vessels making way. A stationary target becomes a vertical line, the length of which corresponds to your own ship’s speed (which you quickly learn to recognize). A vessel on a parallel, or reciprocal, course shows up as a vertical line, the length of which corresponds to the relative speed, and the color-coded most recent hit indicates whether the vessel is on a parallel or reciprocal course. Very long wake lines (fast target) pointing right at the center of the display represent a risk of collision (constant bearing, decreasing range). You should examine these targets more closely.
Determining risk of collision: The display with wakes activated is a good start to determining risk of collision but keeping track of a particular target on a display cluttered with numerous blips is difficult. Fortunately, there is another very useful tool, Variable Range Marker/Electronic Bearing Line (vrm/ebl), which allows you to focus on one or two targets. This function allows you to draw a bearing line on the display at any bearing and a range circle at any range.
To determine risk of collision, you would draw the bearing line and the range circle through the target of interest and watch the target blip. If the blip drifts above the bearing line as it approaches, the target will pass in front of you; if it drifts below the bearing line, the target will pass behind you. If it marches down the bearing line toward the center of the screen, you should take action to avoid a collision. You can track severe weather in the same way as you track other vessels and determine if an approaching thunderstorm is a threat or will pass at a safe distance.
Another tool provided by marine radars is automatic radar plotting aid (ARPA), or mini-automatic radar plotting aid (MARPA). ARPA is a sophisticated processing tool that tracks targets, projects the future course, determines point and time of closest approach, draws the target’s projected course on the display, and alarms if a risk of collision exists. International performance standards for ARPA exist. You will find ARPA on the large oceanic vessels. The MARPA function found on recreational radars does essentially the same thing as ARPA, but there are no fixed standards and different manufacturers provide slightly different functionality. That being said, MARPA determines closest point of approach (CPA), time of CPA (TCPA), bearing, range, relative speed and risk of collision for any blip you select.
Although MARPA is a very useful tool for determining risk of collision, it can be less effective than other techniques in some situations. First, it takes several sweeps of the antenna for MARPA to acquire enough data to lock on and start tracking. Other techniques may be faster, which becomes important in close quarters conditions such as exist on Chesapeake Bay. Second, MARPA requires that the system detect the target on at least half of the antenna sweeps. Weak returns from a small or distant vessel, or when you have reduced the receiver gain to compensate for wave or rain clutter, may not provide enough hits. Viewing a “wake” with the EBL activated may allow an operator to track a target visually in conditions in which MARPA would fail to establish lock.
Navigation: Navigation is the process of establishing where you are by determining a fix. Traditionally one takes three compass bearings on prominent landmarks using a hand-bearing compass, transfers the lines of position (LOP) to a paper chart, and determines the fix as the point of intersection of the LOPs. Using radar, you can quickly determine LOPs by placing the cursor on each landmark and reading off the bearing. Compared to using a hand-bearing compass, radar allows one to obtain a fix more accurately, with greater safety, and in conditions of poor visibility. Even better, placing the radar cursor on a landmark gives you range as well as bearing. Triangulation using range is much more accurate than using bearing. Moreover, using both range and bearing allows you to obtain a fix from two landmarks rather than three. Even one landmark will do in a pinch. Of course, you still have to do your paper chart work and obtain the latitude/longitude from the chart.
In many situations, a lat/long fix is not as useful as range and bearing. When I contact a friend by VHF radiotelephone and ask for their location, the last thing I want is a lat/long. First, I need to remember or write down a string of numbers. Then I need to transfer the lat/long to a chart. Then I need to plot my own location. Only then do I know where they are relative to me. I would much prefer to hear, for example, that they are 1.1 miles bearing 270° from Bloody Point Lighthouse. I can visualize this and immediately know where they are. Similarly, bearing and range to a tanker bearing down on me is very useful when I contact them because I can tell them that I am the sailboat half a mile bearing 125° from them. They know exactly where I am and can probably identify the radar blip that indicates my vessel. You easily obtain range and bearing by placing the cursor on the object and reading the cursor data box.
Pilotage: Getting from one point to another safely is the essence of pilotage. Since the radar display shows coastlines, land, buoys, and other vessels, with your own ship at the center of the display, you can easily steer your vessel just from the radar display. This is similar to piloting using a GPS chartplotter when you drive the own-ship icon around the chart display but with several differences. On the one hand, the GPS chartplotter display shows depth contours, whereas in radar piloting you need the depth sounder. In addition, the chartplotter can identify buoys and other fixed objects. On the other hand, radar shows buoys, land and other fixed objects where they are, not where the chartmaker thinks they are and, most important, shows non-fixed objects like other vessels. You can steer the boat and avoid collisions from the single radar display. Moreover, the very nature of radar precludes the possibility of chart datum mismatch or charting errors.
Maintaining distance off a danger point: The variable range marker/electronic bearing line (VRM/EBL) function is a simple and practical way of determining if something is on a collision course with you. The VRM is also useful for maintaining safe distance off a danger point such as a prominent rock or point of land. Simply establish a VRM circle at the range you want to stay off the danger and steer your vessel so that the blip corresponding to the danger point stays outside the VRM circle.
Finding a good place to drop anchor: Similarly, the VRM/EBL function enables you to find a good spot to drop anchor in a cove. Simply set a VRM circle to your anchor swing radius, adding a safety margin for how far shoal water extends from the land, and maneuver your vessel until no land falls within the VRM circle. Then drop your anchor and sleep confident that regardless of how you swing you will not end up aground (assuming your anchor holds). While at anchor, especially as darkness falls and the wind picks up, I find it very reassuring to turn on the transmitter and measure the distances to shore and nearby anchored boats to verify that I am not dragging.
Proximity alert: Guard zones with timed transmission are two tools that are very useful for alerting you to the presence of another vessel. Once you establish a guard zone, usually a doughnut shaped area centered on your own ship, the radar will alarm if it detects anything inside the guard zone. Timed transmissions are useful for minimizing wear and tear on the radar scanner and for conserving power: the radar transmitter comes on every five minutes (or whatever time you set), the scanner makes a few sweeps, alarms if anything is detected within the guard zone, and then goes back to standby mode to conserve power. This guard zone technique is useful only when you do not expect to find anything near you, for example, when you’re offshore.
Radar and GPS chartplotters
Integrated radar and GPS chartplotter: Several additional tools are available if you can afford an integrated radar and GPS chartplotter system. Integration allows you to transfer marks from radar display to chartplotter display and vice versa. This means that you can put the radar cursor on an unidentified blip and then look at the chartplotter display where the mark shows up allowing you to determine if the radar blip corresponds to any charted object. You can also transfer marks from chartplotter to radar screen allowing you, for example, to see what blip corresponds to a specific buoy.
Even more useful is the ability to transfer MARPA graphics from radar to chartplotter. In this situation, once the radar is tracking a target, the chartplotter will display the target as well as its projected course. You can quickly see if the target is in a buoyed channel and if MARPA projects it to stay in the channel. You can even overlay the entire radar display on the chart, allowing you to identify charted marks, off-station buoys, missing buoys, etc.
Radar vs. GPS chartplotter: A GPS chartplotter displays a nautical chart of the local area with the GPS-obtained location of your vessel superimposed (in North American waters the fix accuracy of a WAAS-capable GPS is better than three meters). This is a marvelous tool for trip planning and avoiding allisions with fixed and charted objects, and is essential for following a desired track between waypoints with an autopilot. However, if you want to avoid collisions with other vessels, avoid allisions with uncharted objects such as fish traps, uncharted or off-station buoys, drifting buoys, or track severe weather, you need radar. Moreover, with radar you can maneuver in the blind just as easily as you can with a GPS chartplotter, even better because you see everything around you, not just the charted objects. If I had to choose between radar and a GPS chartplotter, I would take radar without hesitation.
In my opinion, radar is the preeminent tool for avoiding collisions and allisions, and it is an outstanding tool for navigation, pilotage, severe weather tracking, and general situational awareness. Every boater should have radar if possible. You must learn how to use it effectively, but you will find that the benefits are well worth the effort. n