The most certain method of deciding if a risk of collision exists is to take a bearing on the other vessel as soon as it it sighted, then repeat this at 2-3 minute intervals. In open water this should be as soon as it comes over the horizon, if the vessel is making full speed there is only a short period to make decisions in. It is a sign of inexperience to wait until the vessel is close before starting to take bearings.
As part of training the crew, this task should be delegated, initially under supervision. This will help the crew to be more alert to other vessels and to keep a more effective watch when they discover how hard it is to make this judgement by eye.
In confined waters, there are other problems. Often the vessels will be following a winding channel, in this case, they may be heading well clear, then suddenly turn towards an unsuspecting vessel. For this reason, the skipper must be aware of possible routes for shipping to take when entering confined waters and not just assume that vessels will maintain their present course.
The other difficulty in confined waters, is that there is much less time to take the bearings. However, in these situations a quick check is to look at the movement of the background in relation to the other vessel:
If the background appears to be moving in an aft direction, you will pass astern of the other vessel.
If the background appears to be moving in a forward direction, you will pass ahead of the other vessel.
If the background is steady in relation to the other vessel you will hit the other vessel.
This rule reminds us that proper use of radar should be made, with checks at a variety of ranges and use made of plotting to determine the closest point of approach.
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