Tides explained: springs, neaps and tidal range

Tides are the regular rise and fall of the sea caused mainly by the gravitational pull of the moon, and to a lesser degree the sun, on the oceans as the earth rotates beneath them. Most coasts experience two high waters and two low waters roughly every 24 hours and 50 minutes.

That is the theory. In practice local geography dominates: the shape of a coastline, the depth of a channel and the way water piles into an estuary can double the range in a few miles or shift high water by hours. This is why navigators use tide tables and tidal atlases for a specific place rather than calculating from first principles.

Around the new and full moon, sun and moon pull roughly in line and their effects add together. These are spring tides: the highest highs, the lowest lows, the biggest range and the strongest streams. Nothing to do with the season — the word means to spring up.

Around the half moon, the two pulls are at right angles and partly cancel. These are neap tides, with a smaller range and gentler streams. The cycle from springs to neaps and back takes about a fortnight, so any given week is somewhere on that spectrum.

The practical difference is large. At springs there may be enough water to cross a bar that is impassable at neaps, but a marina berth that dries could leave you sitting on the mud a few hours later.

Tidal range is the vertical difference between one high water and the following low water. Height of tide is how far the water stands above chart datum at a given moment. Add the height of tide to the charted depth and you have the actual depth of water.

Between high and low, the water does not move at a constant rate. The rule of twelfths is the working approximation: in the six hours between high and low water, the tide moves one twelfth of its range in the first hour, then two, three, three, two and one. Most of the change happens in the middle two hours.

For anchoring, add your required depth, the boat's draught and a safety margin, then check what will be left at low water — and remember your swinging circle grows as you let out more chain.

A tidal stream is the horizontal flow of water as the tide floods and ebbs, and for passage planning it usually matters more than height. Streams of two to four knots are common in tidal waters and considerably more in headlands and narrows — enough to halve a cruising yacht's progress or double it.

So passages get timed rather than simply started. Leave to carry a fair tide, aim to reach a tidal gate near slack water, and avoid wind-against-tide conditions, where a breeze blowing against the stream builds a short, steep and unpleasant sea. Tidal atlases and the diamonds printed on charts give the expected rate and direction hour by hour.

Around Barcelona and Menorca the tidal range is very small — often only a few tens of centimetres — so depth planning rarely governs the day, and local winds and swell matter far more. Tidal theory is still part of the RYA syllabus, and rightly so: the moment you charter in the Atlantic, the Channel or the Solent, it becomes the first thing you check.