Point of Contact Page 2

Radar has been around for more than 100 years and has progressed from being a scientific novelty to becoming the most versatile of marine electronics. You can use radar to find out where you are, to find your way into unfamiliar harbors, to dodge thunderstorms, or even to find fish by picking out flocks of feeding seabirds. By far its most important use it that for which it was first
Author:
Publish date:

Collision avoidance

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 ship’s 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 stop—right in front of the ship. The lesson here is that the clearest radar contact in the world won’t do you any good if you can’t make sense of it.

True or relative: Until a few years ago one’s own vessel was always at the center of a radar screen, heading straight up. However, that’s no longer the case. 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 increasingly common.

In a collision avoidance situation, it’s 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.

radar.3

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 vessel’s 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 boat’s heading is still represented by the straight line of the heading mark. Which display you prefer is up to you.

radar.4

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 contact’s 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.

radar.7

Close approach: A conventional compass can tell you whether there is a risk of collision, but it can’t tell you if you’re 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.

radar.8

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 isn’t 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 screen’s center. The point at which the contact is closest to the center of the screen—the other ship is closest to your boat—is 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 it—just 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.

Related

arc18-3981

Stories from the Cruisers of the ARC

Each December, the docks at Rodney Bay Marina in St. Lucia are abuzz as the fleet of the ARC—the Atlantic Rally for Cruisers—arrives to much fanfare. No matter what time of day or night, the staff of the World Cruising Club, organizers of the 33-year-old rally, are there to ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com A sign from outside the box  Rev counters on modern engines are driven electronically from a terminal on the alternator. If all is well, as soon as the engine fires up the revs will read true. If, ...read more

emSelf-tacking-jib

Ask Sail: Are Self-trackers Worth It?

Q: I’m seeing more and more self-tacking jibs out on the water (and in the pages of SAIL) these days. I can’t help thinking these boats are all hopelessly underpowered, especially off the wind, when compared to boats with even slightly overlapping headsails. But I could be ...read more

01-LEAD-hose-leak-CREDIT-BoatUS

Know how: Is Your Bilge Pump up to the Job?

Without much reflection, I recently replaced my broken bilge pump with a slightly larger model. After all, I thought, surely an 800 gallon-per-hour (gph) pump will outperform the previous 500gph unit? Well, yes, but that’s no reason to feel much safer, as I soon discovered. The ...read more

190314-viddy

St. Maarten Heineken Regatta: A Source of Hope

The tagline for the St. Maarten Heineken Regatta is "serious sailing, serious fun." However, for the inhabitants of St. Maarten, the event is more than just a festival of great music and some of the best sailing around. Local blogger Angie Soeffker explains the impact the race ...read more

SPOTX-1500x1500_front

Gear: SPOT-X Satellite

Hits the SPOT The SPOT-X two-way satellite messenger is an economical way of staying connected to the outside world via text or e-mail when you’re at sea. As well as the messaging service, it has a distress function that not only alerts authorities if you’re in trouble, but lets ...read more

_8105684

A Kid’s Take on the Northwest Passage

Going North—and West Crack! Crunch! I woke with a start to the sound of ice scraping the hull of our 60ft sailboat, Dogbark. In a drowsy daze, I hobbled out of the small cabin I was sharing with my little sister. As I emerged into the cockpit, I swiveled my head, searching for ...read more