When asked which electronic tool would be their first choice for outfitting a boat for sea, most captains would probably choose radar. All other devices, from loran to differential GPS, offer information that the skilled navigator can determine on his or her own. Radar, on the other hand, provides information that no amount of skill can compute. Only radar tells us what is really out there, in the dark or in fog, and allows us to take early action to avoid a collision with another vessel.

Knowing how to plot another vessel’s relative motion gives the navigator the power to make decisions. Without knowing a target’s course and speed, operating a vessel in restricted visibility becomes little more than guesswork.

Because both our vessel and other vessels are moving, our small-boat radar will show us relative motion. Radar units for larger vessels usually have the capability to show true motion, but this type of radar is rarely found on voyaging boats. Since a relative-motion display shows us situated at the center of the screen regardless of our speed, we need to plot and analyze the course and speed of the other vessel relative to us. Then we can determine if the risk of collision exists.

According to Coast Guard statistics, the number-one root cause of accidents between two vessels is an operator’s failure to properly understand relative motion. We will discuss the RTM triangle (more on what RTM stands for below) of radar plotting for collision avoidance on a head-up radar, one that is not stabilized by a compass (the heading flash always matches the vessel’s heading).

To begin, let’s imagine that we want to freeze our radar screen at a particular instant in time. To do this, we need to mark the target’s position relative to us.

First, ask the helmsman for a "mark"; that is, ask him to report to you when he has settled the boat’s heading on the ordered course. Set the variable range marker (VRM) and electronic bearing line (EBL) in the general vicinity of the target before the mark is given. This saves fumbling with too many dials during the moment that the mark is given. This is especially important in rough weather when the helmsman might have difficulty keeping the course steady for any longer than a few seconds.

When the helmsman sings out "mark," arrange the VRM and EBL in such a way that they intersect at the target, and then note the time. For purposes of plotting, we only need minutes. The VRM circle should just barely touch the target at its closest point to us, and the EBL should pass right through the middle.

Note the distance away and relative direction of the target and write this information along with the time on the plotting sheet or on a separate piece of paper. The idea is to record this information off the radar scope since the radar information can be changed easily if the dials are bumped or another crewmember comes along and plays with the set. Keep in mind that we are plotting relative bearings. In other words, the "0" at the top of the maneuvering sheet really represents our heading. (This does not affect our plotting except when we want to figure out what the other vessel’s true course isbut we’re getting ahead of ourselves.)

Then, using the first round of range and bearing numbers you recorded, plot the position on the radar plotting sheet exactly as it appears on the screen. Label it "R," the first letter in our RTM acronym, and write the time next to it.

From R, draw a line straight down, parallel to the heading flash line on the plotting sheet, and label this "T." This line should be as long as the distance traveled by our vessel in six minutes. For example, if we’re moving at 10 knots, the line will be one mile long; if we’re moving at six knots, the line will be six-tenths of a mile long. We use six minutes because it is so easy to figure out distance traveled in this time: six minutes is one-tenth of an hour. To figure the distance, simply multiply the speed times 0.1.

Six minutes after R was marked, ask the helmsman for another mark and repeat the process of plotting. Label this new point "M."

Analyze the target

If the target were stationary (a buoy or a vessel not making way), we would note that the target moved exactly to position T because that was our own course and distance traveled over six minutes. If the target did not move on our screen, it means that the other vessel was moving at exactly the same speed as our vessel and in the same direction. Relative motion would be nothing.

We now have all the information we need to deduce the target’s closest point of approach (CPA) to our vessel and the target’s true course and speed. In most scenarios, all that will be required is a quick rendering of the other vessel’s CPA. Even the most energetic navigator will soon grow weary of plotting fully every target that appears on the screen. Plotting only the CPA takes just a few seconds.

We’ll go the distance here, though. Connect points R and M with a long line and label this line "DRM," for direction of relative motion. At the end of the line, draw in a half arrow to indicate in what direction this target is moving.

To find CPA, draw a line perpendicular to the DRM line through the center of the maneuvering sheet (our position). This distance, from the center of the sheet to the point on the DRM line where the perpendicular intersects, is how close the target will get to us if we both maintain our respective courses and speeds.

Using calipers set at the distance between R and M, walk down the DRM line to the CPA. Each interval represents the distance that this target has moved in six minutes. The time of CPA can be determined by multiplying six times the number of intervals. For example, if R was at minute 00 and M at 06 and we measured five intervals with our calipers from M to the CPA, the time of the CPA (TCPA) would be minute 36.

It’s tempting to imagine this vessel with its bow pointed straight down the DRM line. But keep in mind that the DRM line only represents the motion of the other target relative to our motion. It’s the combined direction of our two courses and speeds.

To find the vessel’s true course and speed we need to complete the triangle of which we have already drawn two sides. Connect point T to point M with a third line (think of it as the True-Motion line). It will help clarify the plot to draw this line longer than the two points, adding an arrow at the end of the line to indicate the direction that the vessel is traveling. Use dividers to measure the distance between these two points and multiply that number times 10 to figure out the speed of the target. Remember that this distance is only how far the target has traveled in six minutes.

To find the vessel’s true course, place one edge of a set of parallel rules on this line and walk it through the center of the circle. Be sure to read the compass rose in the same direction as the arrow of line T/M (not the reciprocal course). Then add our heading to this number. If the new number is more than 360° we simply subtract 360° from it. For example, if our heading is 030° and the T-M line is 350°, we get a combination of 380°, or a true course of 020° (380 minus 360). We don’t really need to know the vessel’s true course, but it helps to know in which direction it is headed if we try to identify it by its lights.

Taking action

Once we know the target’s CPA and TCPA (and its true course and speed), we are equipped to make a decision. We can now decide how to react to this other vessel’s position and motion. Without this information, we can only watch helplessly as the target gets closer and closer to us and we are potentially faced with a close-quarters situation that could have been avoided with six-minutes’ worth of simple navigation.

To decide how to react to the target, imagine that the radar screen is divided into three sections: the area directly ahead and on our starboard side (clockwise to 112.5°) is section A; the area directly ahead and on the port side (counterclockwise to 112.5°) is section B; and the entire area astern and abaft 112.5° on both sides is section C.

We do this so that we can prioritize targets according to the nautical Rules of the Road. Targets in section A should receive our immediate attention (we would see their red running light), section B next (we would see their green running light), and last we’ll react to targets in section C. However, be mindful of targets on the entire screenit is too easy to lose track of the big picture when your face is buried in the radar. Using these sections allows the operator to realize quickly which targets are of the most pressing concern.

We can also prioritize targets based on their distance from us. When a target is in the outer third of the screen (regardless of the range setting we are using), we should perform all plotting and decision-making. When a target has progressed to the middle third, some sort of action should be taken, whether it means holding course and speed or dramatically altering course or speed to avoid an eventual close-quarters situation. The closest third of the screen will serve as a safety zone, allowing us to take last-minute action, if needed.

Oftentimes, these quick-reference practices of judging (determining a quick CPA and TCPA and noting which section of the screen the target is in) if a target is threatening are the best a navigator can expect; the minute-to-minute chores required in maintaining a watch, especially at night near the coast or in fog, fill up one’s time to the extent that a full RTM plot is not feasible. A determination of a target’s CPA and rough estimate of the time of CPA is usually plenty.

However, plotting the RTM triangle enough times that it becomes second nature will ultimately prove useful.

If you cheerfully spend six minutes drawing triangles on a maneuvering board, studying the target with binoculars, and mentally reviewing the Rules of the Road, however, not only will collision be avoided, but your confidence will soar. n