Automatic Identification System:
AIS for Yachts

Automatic Identification System

The Automatic Identification System (AIS) is getting more and more a standard piece of equipment onboard yachts. Indeed, prices have come down and you can add them easily to your existing navigation system.

There are 3 sorts of AIS sets on the market. The 2 first are the AIS Class A and Class B, which are both transponders. The third sort is an AIS receiver.

The word 'transponder' by the way, means the same as 'transceiver', which means that the automatic identification system transmits and receives AIS data.

To choose the right set, you have to know the main differences between the 3 sorts of AIS-sets:

Class A:

  • is required for all ships over 300 gross registered tonnes and passenger vessels, irrespective of size.
  • transmits every 2 to 10 seconds while underway, every 3 minutes while at anchor.
  • gives the complete range of static and dynamic data.

Class B:

  • transmits at 30 second intervals. When you're not moving or moving less than 2 knots, the transmissions will be reduced to 1 transmission every 3 minutes.
  • is programmed with the following static data: vessel's name, call sign, dimensions, type and MMSI.
  • gives limited dynamic data: position (latitude and longitude), course over ground, speed over ground and heading.

AIS receiver (= AIS engine'):

  • only receives AIS data, no transmitting.


We, as recreational boaters, will focus on the Class B and the AIS receiver only, because these are specially developed for leisure craft and small commercial vessels.

Apart from an 'Automatic Identification System' set, you also need a VHF antenna. You must know that all AIS signals are in the Very High Frequency-band, which means that the antenna can only see what lies in its line of sight.

You can place an AIS antenna or you can use your VHF antenna, together with a splitter.

This splitter will divide the received signal in 2, one goes to the VHF set, the other to the AIS set.

The splitter will be an active aerial splitter, because when you transmit with your VHF set, the whole signal needs to go to the aerial for maximum range.

So, the splitter prevents that the transmitting signal would go to the AIS set, which could damage it.

The downside of a splitter is that the strength of your VHF signal will be reduced.


The benefit for having an 'Automatic Identification System is that you see the big ships around you and that they see you (if Class B), which comes in very handy when the visibility goes down the drain!

Even in good visibility, it can give you vital information concerning collision avoidance. You know, it's not always easy to determine whether you're on a collision course with a cargo vessel. AIS takes a lot of guessing out and you will be much more relaxed.


cruiseship passing


Another nice feature is the fact that you can set alarms. When a vessel comes in your guard zone, you will be notified. This is particularly handy when you're sailing alone or with 2.

Also, every movement, a turn or a change in speed from either vessel, will show immediately on your screen. So you know very quick the result of a certain action.


Crossing a shipping lane

Remeber though, AIS is an aid to navigation and doesn't replace the act of sticking your head out to see and hear with your actual eyes and ears! Keeping a good look-out...you know?

Computers are just computers and altough you can show up at the other vessel's screen, it's still possible that they haven't noticed you!


Another plus is that an AIS set is easy to mount onboard and consumes very little electricity. It's pretty much an 'install and forget'-story.


From the other side, a lot of boats you cannot see with AIS: all the leisure craft and fishing boats without Class A and B. These can still be pretty big. All vessels, except passenger vessels, below 300 gross registered tonnes don't need to carry an AIS Class A set. Everybody who is not obliged to carry an AIS Class A set, is free to decide to take AIS onboard!

This last remark also proves the need to carry a radar, so that you can see those boats too in restricted visibility...


All AIS data will be shown on your chart plotter, radar display or computer with navigation software.

RMS Queen Elizabeth 2

Did you know that the system can handle more than 1000 vessels simultaneously?!

And that, in extreme cases where lots of yachts and ships are together, the VTS (Vessel Traffic Services) can intervene and block the transmissions of Class B sets? This last feature is to secure the safe navigation of the big ships. Screens can become so cluttered in case of extreme dense traffic, that it's not practically usable anymore. Class A sets have priority.

Also, when you transmit your position, you will be on the Voyage Data Recorder (VDR) that big ships carry. These are the 'black boxes' of shipping. This means that official institutions can held you liable when you don't follow the Colregs!


So, which one should you chose?

Well, if you want an AIS set and you are on a tight budget, go for an AIS engine. This means you can observe the others, but if they don't see you visually or on their radar, you don't exist!

Class B is better, in that way that you see the big ships AND the big ships see you. Prices are higher than the price of an AIS receiver, but, since the last few years, all AIS sets are becoming more affordable.

All in all, if you're a regular sailor and have the funds, I definitely recommend to have an 'Automatic Identification System' onboard!



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