Report Text:

We have recently completed an extensive voyage in our yacht, sailing in British waters.

We found the plotter combined with an AIS “engine” gave us a very valuable picture of traffic especially in fog at night.

There are a few points that came out of its use. Firstly there were some commercial vessels which were not sending a signal. Secondly, we were off the South Coast of Cornwall and found ourselves in the middle of a naval exercise in very bad visibility. Some of the warships were sending AIS and some were not.

We met a few yachts who sent AIS and thought that was uncalled for, how would it be if every yacht in the Solent for example, sent a signal- chaos!!

CHIRP Comment:

There is not generally any reason why a commercial vessel navigating around the UK would elect not to transmit her AIS signal.  Indeed failure to do so would be a breach of regulation.  Although there are exceptional circumstances, e.g. piracy risk, in which a Master may decide not to transmit AIS, these would not normally apply around the UK.

It is possible that a vessel may have a technical problem with its AIS transmitter. However, this is usually considered to be a reliable piece of equipment, although there are sometimes issues regarding the accuracy of the data transmitted. Generally, it is more probable, if a vessel’s AIS signal is not showing on a yacht’s AIS receiver, that the problem is with the reception of the signal.  This can be affected by weather conditions and/or the efficiency of the aerial.

Naval vessels can elect not to transmit AIS. Military exercises are often conducted with a high degree of realism, so we are not surprised that some of the warships encountered by this yachtsman may not have been transmitting AIS.

Whilst we can understand the anxiety of a yachtsman at being in the middle of an exercise in bad visibility, there are a number of mitigating factors regarding the safety of such exercises:

  1. Warning of the exercises is broadcast by the Coast Guard.
  2. The naval vessels operate with a high degree of surveillance and should therefore be aware of the presence of a yacht.
  3. One of the warships has a specific responsibility to act as Guard Ship for the exercise area.
  4. If a yachtsman finds himself in a situation where he is concerned for his safety, he can contact the Coast Guard for advice, initially on Channel 16.

In general, AIS receivers provide useful additional information for use in assessing a traffic situation. However, do not assume that all vessels will be detected or that the data transmitted can be taken as absolutely reliable.  Furthermore, do not rely on a single source of information in making navigational decisions.