Avoiding Disaster

True Motion displays like this one have come down from the big-ship world. For collision avoidance, switch to the unit’s relative-motion mode.

One foggy May morning, a few years ago, a 900-foot container ship heading westwards at 25 knots collided with a 40-foot yacht called Wahkuna, which until a few minutes before the collision, had been heading north at about 7 knots.

Considering those speeds and the visibility at the time of barely 50 yards, you might expect the result of the subsequent accident investigation to be as much a foregone conclusion as the result of the collision. But although the investigators found plenty to criticize about the conduct of the ship, they also found that if both vessels had held their courses and speeds, the collision never would have happened because the yacht would probably have passed at least three quarters of a mile ahead of the ship. She didn’t. Wahkuna’s captain misinterpreted the information provided by his radar and decided to stop—right in the path of the oncoming vessel.

Collisions between ships and pleasurecraft are rare, but when they happen, they are almost always serious. And in most cases they could have been avoided if the vessels’ radars had been used properly. If you find yourself in a potential collision situation, you need to take maximum advantage of your radar as a collision-avoidance tool by considering the following.

Turn True Motion Off

Until a few years ago, we could have said “your vessel is always at the center of the radar screen, heading straight up-wards”—what we commonly call Relative Motion mode. But that’s not true any longer. True Motion displays, in which the center of the radar picture moves across the screen in step with the boat’s movement in the real world, are becoming increasingly common. For collision avoidance, though, your first move is to switch the True Motion function off and revert to Relative Motion mode.


All radars can display a head-up image in which the boat’s heading is represented by a vertical line pointing upwards from the center of the screen. Many captains are quite happy with this presentation—it has the advantage of placing things that are in front of the boat at the top of the screen, things on the starboard side to the right, and so on. The trouble with head-up is that as the boat yaws, the picture yaws, and the distinct blobs that represent ships or other boats turn into vague smears. The solution for this is a stabilized picture, in which a compass input is used to rotate the head-up picture to counteract the effect of the boat’s movement.

Course-up looks very much like head-up, but with the effect of yaw removed. In both formats, the boat’s heading is still represented by the straight line of the heading mark. It’s a matter of personal preference which presentation you use; the important thing is to account for yaw.

Tracking Targets

For well over a hundred years the regulations that we now know as the International Navigation Rules have included the advisory that “risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change.”

On a relative-motion radar display, the logic of that advice becomes obvious. Ships are represented by blobs, and our own vessel is represented by the center of the radar image. So if a blob appears to be heading straight for the center of the screen—us—then sooner or later we’ll be in danger of sharing our little patch of water with another vessel.

The easy way to tell if a ship is heading straight for the center is to move the electronic bearing line (EBL) so that it points straight at the approaching blob and apply what’s known as the steady bearing test. If the blob appears to be sliding down the line a few minutes later, its bearing relative to us is steady (unchanged) and there is a risk of a collision. It’s important to appreciate, though, that this doesn’t mean that the ship is heading straight towards us. It’s not. It’s heading straight towards a point somewhere ahead of us, where the potential collision is waiting to happen.

Another effective technique for tracking vessels is radar plotting, which means monitoring the movement of each blob either by marking its position directly on the screen or by measuring its range and bearing and marking them on a paper plotting sheet. This works fine but is a bit outdated as almost all modern radars have functions such as tracks, trails, or wakes, in which the past position of each blob is shown as a pale trail on the screen. It’s particularly useful if you can set the length of the trail to a fixed time, because then the length of the trail gives an instant impression of how fast each blob is moving. It’s even better to set your trail length to six minutes, because six minutes is one tenth of an hour. So if the trail is two nautical miles long, you can immediately see that the closing speed is two miles in a tenth of an hour, or 20 knots.


An old-fashioned compass can tell you if there’s a risk of collision but not much more. It can’t tell you whether you are likely to miss the approaching ship by half a mile or two miles. Radar can—again just by keeping a record of the movement of the blob across the screen or by displaying the direction of the wake, track, or trail behind it.

The essence of the steady-bearing test is that if the blob has been sliding down the EBL towards the center of the screen for a few minutes, it will continue to do so unless someone takes evasive action. Suppose, however, that it isn’t aiming for the center of the screen but is slipping along a straight line that passes half a mile from the center.

The same principle applies: unless someone does something to change the situation, the target will keep moving across the screen in the same direction and at the same speed until it passes half a mile from the center and then slides away on the other side. Once again, the point at which the blob is closet to the center of the screen is called the closest point of approach or CPA.

Predicting the future movement of a contact like this can also give us another very useful piece of information. If the target looks like it will cross the heading marker, the ship will pass ahead of us and we will pass astern of it. If it’s going to pass behind us, we will be cutting across her bow. That simple test might have been enough to save the Wahkuna by telling her captain that stopping was just about the worst thing that he could do.

In a restricted-visibility situation, like fog, Navigation Rule 19—summarized by this diagram—takes over from normal steering and sailing rules.

Give way

Other plotting techniques can be used to calculate a target’s course and speed, assess the effect of altering course and speed, and even calculate a course to intercept. But one of the most useful is another simple check that tells us almost instantly how to react when we encounter another vessel in fog. The normal “steering and sailing rules” (found in the U.S. Coast Guard’s Navigation Rules book) don’t apply in fog. For instance, you don’t give way to a vessel crossing from your starboard side or hold your course and speed for one that is approaching from your port side.

Instead, Rule 19 takes over. Among other things, it says that if you have detected another vessel by radar alone and have decided to take avoiding action by altering course, you must avoid the following: “(1) an alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken (2) an alteration of course towards a vessel abeam or abaft the beam.” In other words, the Rule encourages you to alter course to starboard for a vessel anywhere forward of the beam or on your port quarter and to alter course to port for a vessel on your starboard quarter. I’ve been on a boat where the owner had a simple decal that he stuck to the bulkhead next to the radar. It consisted of a circle, divided into four quarters. From six o’clock through to three o’clock, it was green to suggest altering course to starboard; from three o’clock to six o’clock, it was red, to suggest altering course to port.

Ultimately it’s simply common-sense rules that will allow you to stay out of harm’s way, and equally important, let other helmsmen anticipate your maneuvers when they can’t see you.

This article originally appeared in the January 2010 issue of Power & Motoryacht magazine.

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