AIS is an automatic identification system. Yea, cool, what’s that mean?

It means, on our chart plotter, we can see other vessels, and they can see us. We get information like their bearing from us, their heading, speed, distance from us, “CPA” closest point of approach, TCPA time until CPA. Other information is there length, beam, destination, …..

We write down, the vessel’s name, CPA, TCPA, bearing from us, speed, heading, and the current time.

Then 5-10 minutes later we do the same thing, by comparing the observations we can see if they changed their heading or speed. What should be changing, we hope, is their bearing from us. If it’s increasing they will pass to our right, and if decreasing they will pass to our left. If it’s not changing, ….……. well see below.

If not too much is changing, and their CPA is in the 1NM bracket we will keep a very close eye on them, and call them on the VHF when they are 20NM away or so. We will confirm a port to port or starboard to starboard pass, or if overtaking just let them know we see them, side we/they will pass on, and we/they aren’t about to change heading, ie, tack 120 degrees. Hope that helps…..a little.