For determining risk of collision, why we need take true bearing of the target and not the relative bearing (as per rule no 7) ?
Let us say we are on a ship on a true course of 045 Degrees. We see an approaching target. Now we take its true and relative bearings. The true bearing is 090 Degrees and relative bearing is 045 Degrees.
After sometime we again take the true as well as relative bearings. This time again the true and relative bearings are same. That is, true bearing is 090 Degrees and relative bearing is 045 Degrees.
Now we conclude that there is risk of collision and we need to take action. We decide to alter our course to starboard and make it 090 Degrees. For some reasons the other ship altered her course to port. What will be the bearings now ?
The true bearing will still be 090 Degree and will imply that there is a risk of collision. But what about relative bearing ? The relative bearing now will be 000 Degrees. As the relative bearing has changed, it may imply that risk of collision does not exist now.
If we rely upon relative bearing, we will not know that risk of collision still exist until we take another bearing after few minutes to realise that though relative bearing had changed earlier but now the relative bearing is not changing again.
So what I want to say is that in case both the ships keep on changing their course, their relative bearing will change even when the risk of collision still exists.
For this reason, as per rule 7 of the COLREGS, the risk of collision shall be deemed to exist when the compass bearing (true bearing) of the approaching vessel does not appreciably change.
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