Nedroma Constant Bearing, Decreasing Range
http://solent-art.co.uk/newpass/newpass.php How can you tell if you are on a collision course with another vessel?
Let’s suppose that you are taking a little cruise on your local waters and you see a boat off your starboard side, approximately 1000 feet from you. It is angled slightly in your direction, but you cannot ascertain whether or not there is any eventual danger of collision. Rather than waiting until you get very close to the vessel to find out if you are going to collide, you can obtain an early warning by locating a fixed object on your boat, such as a cleat or stanchion, and visually aligning it with the other boat. In a few minutes, moving along at the same speed and the same direction, you align the same fixed object with the vessel on your starboard side. If it is at the same alignment as previously, you know that you will eventually collide. Therefore, you must change speed and/or direction to avoid closer contact.
Whenever you change course, it is best to do so emphatically. If your turn will be to port, leave no doubt in any vessel operator’s mind as to where you are heading. Also, you will learn the navigation rules on meeting, passing and crossing in the Boating Safety Class, but keep in mind that not everyone knows or obeys the rules. Stay alert to the movement of other vessels, even if you have the right of way. Keep in mind that you must do all that you can to avoid a collision, even if it means that you must break the rules!
Also be aware that under the “Good Samaritan Law”, you are legally obligated to help another boater in distress, unless it puts you, your crew or your vessel in danger. This law has been practiced on the water for time immemorial, and it protects you as well as any other boater who needs help.