The best time to visit Benagil cave: by sea state, not by sunshine

The Algarve is dry. It is one of the sunniest places in Europe and rain is not what stops the boats.

What stops them is Atlantic swell — long waves generated by weather systems hundreds of kilometres out, which arrive here under a completely blue sky. The entrance to Algar de Benagil faces the open sea, so when swell is running it breaks across the gap and nothing goes in, whatever the forecast on your phone says.

The rules add a second condition: no craft may enter the caves when visibility drops below a kilometre, which in practice means sea fog. That is rare in summer and not rare in spring.

Small boats and kayaks feel this first. A catamaran will run a coastal cruise in conditions that ground everything smaller, which is why in winter the cheapest trip is often the only trip.

The sea. Wind builds through the day almost everywhere on this coast, and with it the chop. The first departures of the morning run in the flattest water of the day and are cancelled least often.

The light. The opening in the roof faces straight up, so the column of sun through it moves as the sun climbs. Early it lands on the sand at the back of the chamber, which is the photograph everybody wants. By late afternoon it is on the western wall and the cave is noticeably darker.

The queue. Three motorised boats may be inside at once, or seven kayaks. In July at eleven o’clock that limit is the binding constraint, and your two minutes may cost you twenty of waiting outside. At nine it does not.

The fallback. If a morning trip is cancelled you still have the afternoon to be moved into. Book the last departure of the day and you have nothing.

Benagil is one of the most photographed places in Portugal and the numbers show it. In high summer the sequence at the cave is: arrive, join a loose queue of hulls outside the eastern arch, wait for a gap, go in, two minutes, come out.

The limit is legal rather than practical. Three motorised boats under twelve metres may be inside at once, or one larger boat alone, or up to seven kayaks. In August, at eleven in the morning, that limit is being hit continuously and the wait outside can be longer than the visit.

There are three ways round it. The first is the first departure of the day, which is the answer to nearly everything on this coast. The second is a private hull, which lets you choose the hour rather than take what is left. The third is to come in May, June, September or October, when the same trips run into a cave that is mostly empty.

None of this makes the place less worth seeing. It makes the timing worth thinking about.