Point of Contact Page 2
One foggy morning a 900ft container ship, heading west at 25 knots, collided with a 40ft sailboat that, until a few minutes before the collision, had been heading north at about 7 knots. The investigators criticized the ships conduct, but they also found that if both vessels had maintained their course and speed, the sailboat would probably have passed at least three quarters of a mile ahead of the ship. Unfortunately, the sailboat skipper misinterpreted his radar display and decided to stopright in front of the ship. The lesson here is that the clearest radar contact in the world wont do you any good if you cant make sense of it.
True or relative: Until a few years ago ones own vessel was always at the center of a radar screen, heading straight up. However, thats no longer the case. True-motion displays, in which the center of the radar picture moves across the screen in step with the boats movement in the real world, are increasingly common.
In a collision avoidance situation, its best to switch off the true-motion function, and go to the relative-motion mode. Your vessel appears to be stationary at the center of the radar screen, while the rest of the world moves past it. There are a number of options to consider.
Head-up or North-up: All radars can display a head-up picture, in which a line pointing straight up from the center of the screen represents the subject vessels forward motion. This is useful, because things in front of the boat are at the top of the screen, things on the starboard side are on the right, and so forth. But if the boat yaws, the picture also yaws, and blobs representing other vessels turn into vague smears.
Data sent from an electronic compass to the radar allows it to rotate the head-up image to produce a cap north-up picture where north is at the top of the screen. In both formats, the boats heading is still represented by the straight line of the heading mark. Which display you prefer is up to you.
Will it hit me?: For over a century the Navigation Rules have advised that risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change. On a relative-motion display, where our own boat is at the center of a radar image, the logic is obvious. If a contact appears to be moving straight toward the center of the screen, there is a risk of collision.
One easy way to tell if this is happening is to use the feature known as tracks, trails or wakes, in which the past positions of each contact are displayed as a pale trail on the screen. If you can, set the trail length to six minutes or one tenth of an hour. This makes it easy to calculate a contacts speed. For example if a trail is two miles long, the closing speed of the other vessel is two miles in a tenth of an hour, or 20 knots.
Close approach: A conventional compass can tell you whether there is a risk of collision, but it cant tell you if youre going to miss an approaching vessel by 500 or 2,000 yards. Radar can do this, as long as you keep a record of the movement of a contact across the screen, or if you look at the direction of the wake the contact leaves behind it.
If a contact is heading straight for the center of the screen for a few minutes, unless someone does something, the contact will continue to converge. Suppose, though, that the contact isnt heading for the center, but is going to pass 1,000 yards from the center? The same principle applies here: unless someone does something to change the situation, the contact will keep moving in the same direction and at the same speed, until it does pass 1,000 yards from the screens center. The point at which the contact is closest to the center of the screenthe other ship is closest to your boatis called the closest point of approach, or CPA.
Remember, when predicting a CPA, if it appears the contact will cross the heading marker, the ship will pass ahead of us and we will pass astern of it. But if the contact is going to pass behind us, that means we will pass in front of itjust like the boat that was hit by the container ship. It is important to understand this distinction.
Giving way in fog: Normal steering and sailing rules do not apply when visibility is restricted by fog or other conditions. This is when Rule 19 takes over. The rule says, among other things, that if you detect another vessel by radar, you should:
Avoid altering course to port for a vessel forward of the beam, other than for a vessel being overtaken.
Avoid altering course toward a vessel that is abeam or abaft the beam.
In other words, in restricted visibility, the Navigation Rules encourage you to alter course to port for a vessel on your starboard quarter, and to turn to starboard for everything else.