There are lots of ways of checking on other vessels for possible collision. The simplest method is to watch the angle between you and the other vessel. If the angle remains the same, you have a collision course. You can do the same thing with radar, by putting the variable range marker on the target. If the target continues down that VRM line, you have a problem.

Or, if you have ARPA-equipped radar – most modern radars have this feature – you can let the radar calculate the crossing vectors. That’s what our Furuno 2117 radar is doing in the photo above. We’ve got two ships heading for us, one to port and one to starboard. This is not the sort of position in which you can make an error.

However, ARPA is not foolproof. Here are some of the risk factors to keep an eye on:

• Target lock. Make sure the target your radar is tracking is the correct one. It is possible for the radar to lock on a different target in busy areas.
• In congested areas be sure the radar does not jump from one target to another nearby.
• Rain and sea clutter can reduce the radar’s ability to lock on and hold a target.
• ARPA depends on accurate course and heading data. If this data is not accurate, the computation of vectors will be off.
• ARPA computes history, so when a target is changing course, it takes 30 seconds or more for the radar to catch up. Some situations will require several minutes to compute.

All of which adds up to this. ARPA is a great tool, but keep a visual check on the bearings between you and the other vessel, as a back up.

For lots more on avoiding collisions, see Practical Seamanship.