Overfalls, headlands and tidal races

Delivery skipper Ben Lowings examines how to prepare for and sail in these sometimes challenging waters

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Sailing around a headland in the British Isles will nearly always involve an encounter with overfalls, a 'commotion of the sea', best visualised as broken or disturbed water.

The first thing to do when considering tackling a headland and an associated tidal race is to look for the overfall symbols on a chart. Imray charts mark them with 'curling waves'. Garmin's Navionics have grey 'shark fins'. Admiralty publications have a single peak 'mountains'. This is similar to the insignia for 'sand waves'. There are several of these, for instance, in Mount's Bay off south-western Cornwall. These denote an underwater feature, not a surface one we are concerned with here.

Small-scale charts can denote overfalls with two parallel wiggly blue lines. Eddies (disturbed water) are identified by a whirlpool symbol. For instance, there are a few chartered ones off St Catherine's Point on the Isle of Wight. UK Hydrographic Office (UKHO) eddies spiral anticlockwise as they go towards the centre.

A reef extending underwater from a headland will interfere with the water in all sorts of ways. If it were smooth like a train station platform, there would be arrow-straight lines of breakers on each edge where it dropped into deeper water. The waves on the side to which the tide is going will generally be steeper.

River outflows can also have this effect. They produce lines of breakers on either edge of the fresh water washing out. The water in between could be almost as smooth as a duck pond. A whirlpool can look almost flat, with just a few bubbles from upwellings, while water on either side can be a maelstrom of very confused seas.

I've rounded a few headlands, and can I've rounded a few headlands, and can speak to the different ways of tackling them, and I've learned that a passage plan starts with a good understanding of what the seabed is like.