“We were sailing from Cherbourg to Southampton equipped with an AIS (Automatic Identification System) transponder and active radar reflector,” the report explains. “As we crossed the eastbound shipping lane, several AIS targets were approaching on the port side and eventually became visible. One was of concern because the closest position of approach (CPA) was almost zero. The speed of this vessel was about 18 knots.”
After monitoring the situation for some time, the yacht in question called up on VHF radio and informed the crew who answered, that according to their AIS the CPA was near zero. The response was 'I agree'. “I then asked if he planned to alter course to avoid risk of collision and he replied in the negative,” the report continues.
"The use of AIS is useful for the identification of a ship, but is not recommended for collision avoidance."
“I politely pointed out that we were a sailing vessel and the stand on vessel under the Collision Regulations and asked once again if he would alter course. The reply was ‘I could do but I’m not going to’. I decided at this point that further discussion was unlikely to be productive, ended the conversation and instructed the helm to turn 20 degrees to port and harden up the sails. This allowed us to pass behind the ship by a safe distance.”
Analysing the incident, CHIRP deciphered several lessons to be learned from the situation. “Do not assume that another vessel will take avoiding action even if it is aware that a risk of collision exists,” CHIRP advises. “The use of AIS is useful for the identification of a ship, but is not recommended for collision avoidance.
“AIS measures speed and course over the ground and not through the water. The refresh rates for AIS also need to be considered. For AIS class A, between two and 10 seconds depending on vessel speed and, for AIS class B, every three minutes where speed over ground is less than two knots, or every 30 seconds for greater speeds. The yacht skipper is complemented for the actions taken and the ship manager for investigating the report on board.”
To read the report in full, please see the CHIRP bulletin here.