(Rule of thumb. Not for navigation)
If the bearing is changing, you are not on a collision course
When you see a ship on the horizon, the most important thing to determine is if you are on a collision course and if you need to take evasive action. This is very easy to do, and takes only a few minutes.
While you continue on your course, and assuming that the other vessel is continuing on it's course, simply watch the bearing of the other vessel over a period of a few minutes. You can use the steering compass, a hand bearing compass, or simply to line up the vessel with a shroud or a stanchion or any point of reference on your own boat. If the bearing changes after a few minutes, and neither of you have changed course or speed, then you should be OK to continue on your course.
If the other vessel's bearing has moved toward your bow, then the vessel will pass ahead of your bow, and conversely, if the bearing of the other vessel moves toward your stern, they will pass behind you.
This rule applies regardless of the speed and course of either vessel, provided that neither change speed or direction during your evaluation.
If the bearing stays the same, you are on a collision course
It does not matter which direction the other vessel is coming from, if the bearing stays the same, and neither of you change course or speed, you will eventually collide.
The only slight exception to this rule is if the other vessel is on a parallel course at exactly the same speed, and mathematicians will tell you that parallel lines actually meet at infinity too.
You need to pay extra attention on a parallel course, because the danger stays with you for longer, unlike other crossing situations, which soon pass.
Taking evasive action
The easiest way to remember who has right of way is that the burdened vessel (the one that has to move) needs to
alter course to starboard and pass behind the other vessels stern.
With this rule in mind, it is pretty obvious who needs to move.
Regardless of all the rules of the road, the overruling factor is that
might has right. Don't rely on your right of way with a 100000 ton tanker bearing down on you. It is quite possible that they are not aware of you, and could not stop or turn, even if they were.
Avoid crossing in front of big ships, particularly on your starboard side (right of way), unless you are sure that you have a good few miles of sea room between you. The speed of some of the fast ferries nowadays can be deceptive and hard to judge, especially when they are coming straight at you.
When and if you decide to take evasive action, make an exaggerated move to make your intentions obvious to the other vessel.
In a crossing situation, I normally alter course to take aim for just behind the transom of the other vessel. This makes it clear to them that I plan to pass behind them, and I keep changing course to aim for their stern as they move past.
Howard [ Just Imagine ] 13-Sep-2008