Martin and Barbara, along with their teenage son, Rob, have sailed across to the Channel Islands from the Solent in their 10m yacht. Rob has crewed regularly and already has a better idea of how to use the new electronic chart system and AIS than his parents. The system does not include radar.

On the way over to Guernsey the weather was fair with good visibility so it was helpful to be able to see the ships in the shipping lanes both visually and on the chart. They are now on the way back. The forecast when they left was for fog patches but in Guernsey visibility was about 2-3 miles with very little wind.

Martin and Barbara need to return to work so they reckoned that with the AIS they should be able to negotiate the ships in moderate visibility.

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Once in the shipping lanes the visibility dropped to under half a mile. They carefully motored across and thanks to the AIS they managed to clear the lanes and are now heading home at 6 knots. Martin and Barbara are tired after the stress of the shipping lanes so go below to rest, setting the autopilot and leaving Rob on watch with his eyes glued to the screen.

After about 30 minutes they hear five blasts. Martin races up on deck to see a small coaster motoring slowly on a collision course on the starboard bow.

What should Martin do now? Why has the coaster not shown up on the screen? Is it breaking the SOLAS regulations? If the coaster has radar, why has it not altered course to pass behind the yacht?

How to avoid a collision in fog while sailing a yacht?

Martin should slow down and make an immediate and very positive turn to starboard. He should watch the coaster carefully because now that the two vessels are in sight of each other, the coaster may have initiated a turn to port to pass behind the yacht.

Rob might be a wizard with the electronic chart but in this situation an assiduous lookout by sight and hearing is essential. The coaster, providing it is not carrying passengers, is not required by SOLAS to be fitted with AIS if it is under 300 tons. It would normally be fitted with radar.

AIS is hugely helpful for collision avoidance, but can you rely on it for a Channel crossing in fog?

The watchkeeper on the bridge may have spotted the yacht on the radar. Before visual contact, the coaster is correct in slowing down and avoiding a port turn. The rules about collision avoidance in restricted visibility are different to those when vessels are in sight of one another.

AIS is a really useful navigation aid. Seeing ships on the electronic chart and knowing their course and speed is a huge help to the navigator. AIS transmits other information such as the name, call sign and MMSI number of the vessel.

However it only gives some of the picture. There are numerous small vessels including many yachts which do not transmit AIS signals, so without radar these craft do not show up on the electronic screen.

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