Bearing Drift: A First Sign that Danger Exists
To judge the potential for a collision with another vessel, you need to know the bearing between your vessel and the other, and whether and how that bearing changes over time.
Take a series of bearings, called drift bearings, to some fixed point on the other vessel such as its bow, stern, or—at nighttime—a light.
Make sure you take the bearing to the same part of the vessel (or same light) each time.
A series of drift bearings taken over a few minutes will show the vessel’s bearing drift relative to your boat.
If the vessel drifts to the right, it is said to have right bearing drift .
If the vessel drift s left it is said to have left bearing drift .
If the bearings show no change over time, the vessel has a steady bearing drift, which means you have a high risk of collision.
You must also observe the rate of drift . This drift rate is not a mathematical formula, but an observation.
If the bearings change slowly, the vessel has a slow drift rate.
If the bearings change rapidly, it has a fast drift rate.
The faster the drift rate, the lower the risk of collision. Use drift bearings, bearing drift , and drift rate
together to decide whether you must maneuver to avoid a collision.
One of three situations always exists between vessels:
Meeting: Two vessels are moving toward each other, end to end. This includes vessels bow to bow, bow to stern, or stern to stern.
Crossing: One vessel moves against the other. Vessels cross from left to right or from right to left.
Overtaking: A faster vessel is coming up from behind a slower vessel.
In a busy channel, harmless meetings between vessels can quickly change to dangerous crossing situations. For this reason, you must maintain a visual watch on any vessel until it no longer presents a risk of collision.
Will that vessel off our starboard bow cross ahead or astern of us? How about the large power vessel coming toward us down the channel? Will we pass safely, port-side to port-side? Use the following techniques to determine if another vessel will cross ahead or astern of you or if the threat of a collision exists.
To determine bearing drift and drift rate:
Stanchion bearings. Use a stanchion or other fi xed point to sight the other vessel. Watch the way the vessel drift s relative to the stanchion. Does it appear to drift to the left , to the right, or stay steady? If drift ing, is its rateof drift fast or slow?
Compass bearings. Use your handbearing compass to shoot the bow of the vessel. Wait 30 seconds and shoot it again. Repeat the procedure one more time. Note whether the bearing drift s to the left or right.
In either of the above cases, if the bearing drift s toward our bow, the other vessel will cross ahead of us. If the bearing drift s toward our stern, it will pass astern of us. And if the bearing has little to no drift and remains constant, a risk of collision exists.
DOES A RAPID DRIFT RATE EVER INDICATE DANGER?
Rule 7 in the Navigation Rules for International and Inland Waters offers examples of what you use to determine if a risk of collision exists. For instance, it cautions that drift bearings could change rapidly in three cases and still result in a collision:
∆ Extremely large vessels
∆ Vessels close to you
∆ When you approach a vessel that is towing
To track large or close vessels with drift bearings, pick a prominent point on one end of the vessel. Shoot only this point each time you take a bearing. Do not hesitate to take defensive action if needed (see more on this below). We will discuss collision avoidance strategies for tugs with tows later in this chapter.
ACTION STEPS AND OPTIONS TO KEEP YOU SAFE
Mariners always have two action steps to avoid a collision:
Change course: Come right or come left
Change speed: Slow down (or stop) or speed up
Choose one or both action steps to control the situation. Take action earlier than you think necessary. Th at way, if things don’t work out as planned, you have time to try something else.
If you change course, alter your course 60 to 90 degrees or more. If you increase or decrease speed, do so in a dramatic way. The goal is to make any action you take crystal clear to the other vessel. Most collisions result from confusion between vessels.
HOW TO CHANGE A DRIFT BEARING
A decreasing range along a steady bearing indicates that a risk of collision exists between vessels. You must get that bearing moving to the left or right. Remember that the action steps of changing course and/or changing speed are always available to you.
In a crossing situation, slow or stop your boat. In a meeting or overtaking situation, change course away from the other vessel. If needed, combine this with a speed change.