Tides and tidal streams in passage planning: high water, stream and tidal windows

High and low water

In the North Sea, the English Channel and along the Atlantic coast, the water rises and falls twice a day. About 12 hours and 25 minutes pass from one high water (HW) to the next, so HW and low water (LW) come almost an hour later each day.

The difference between HW and LW is the tidal range: around 3 m in Cuxhaven, practically nothing in the Baltic, usually very little in the Mediterranean, and noticeably more in the northern Adriatic (Trieste averages just over 0.6 m) and the Gulf of Gabès. In the Caribbean the tide is small and often comes only once a day.

Springs and neaps

At new and full moon, sun, earth and moon are in line, their tidal forces add up and the range is large (spring tide). At half moon it is small (neap tide). Springs follow the moon with a slight delay, around two days on the German North Sea coast. At springs the stream runs harder, and the water rises higher and falls lower.

Chart datum and depth of water

Depths on the chart are given relative to chart datum (CD). In German waters this is the lowest astronomical tide over 19 years, a level that the tides alone rarely reach. The depth of water is the charted depth plus the height of the tide above chart datum. A harbour with 1.5 m charted depth is often easy to reach around high water, but perhaps not at low water. Allow a margin under the keel, especially in waves.

Storm surge

Wind moves water. A strong onshore wind in the German Bight pushes it above the predicted level (storm surge); an offshore wind lets it fall lower, even below chart datum. Tide tables know nothing about this, but the water level forecast does. In the Baltic, the wind accounts for almost the entire water level.

Tidal stream

The stream changes with the water level: it sets one way with the flood and the other way with the ebb, and turns in between. In tidal inlets, river mouths and around headlands it reaches several knots. For planning this means:

• You cannot make headway against a stream stronger than your boat's speed. Then you wait for the tide to turn.
• A fair stream shortens the passage considerably. Leaving with the stream often saves hours.
• Wind against tide builds a short, steep sea, even in moderate winds. In narrow passages it can become dangerous.
• A cross-tide sets you sideways, so you have to steer to allow for it.

Upwind, a lot depends on when you start the beat; more in the guide Planning your tacks.

Tidal windows in the Wadden Sea

In the Wadden Sea you can often only cross the bar of a tidal inlet, pass through a tidal channel or enter a small harbour that dries out at low water at certain times. What matters is the shallowest point on the way: charted depth plus height of tide must be enough for your draught, with a margin. This gives you a time window around high water. Miss it and you wait for the next tide, around twelve hours. Channels across the flats shift and buoys get moved; take the latest chart and the notices to mariners with you. Some harbour locks on tidal coasts also open only around high water, see Bridges and locks.

What Sailseer calculates

• North Sea: the tide gauges of the BSH (Germany's Federal Maritime and Hydrographic Agency) with the next high and low waters, astronomical and with storm surge, plus the range. The harbour depth check subtracts half the range of the nearest gauge: a harbour with 3 m at mean water level has only around 1.6 m at low water in Cuxhaven.
• At every point in tidal waters: the water level curve of an ocean model from Copernicus Marine. The Northwest Shelf model covers the North Sea with Ireland and the Faroe Islands, IBI the Bay of Biscay, Iberia, Madeira and the Canaries, plus the Mediterranean model, the models for the Sea of Marmara and the Black Sea, the global model for the Azores, Cape Verde, the Caribbean, the Red Sea and the Indian Ocean, and the Arctic model for northern Norway, Iceland, Greenland and Svalbard. From this come high and low water, tidal range, height above an estimated chart datum and storm surge.
• Moon: springs and neaps with the next dates.
• Tidal stream: it is included in the speed over ground. If it runs against you more strongly than your boat can sail, the plan waits for the turn of the tide. For the whole route, Sailseer sums up how much of the distance runs against the stream, where it waits and where there is wind against tide (from 0.8 kn of stream and 12 kn of wind).
• Tidal gate: at the shallowest point of a leg, the weather analysis checks the water level on arrival and waits if necessary, for one tide at most. In the multi-day trip, “Check the tidal window at each day's destination” checks the shallowest bar in the last 10 nm.
• Observations: tide gauges from, among others, Kartverket in Norway, the Environment Agency in England, the OPW and the Marine Institute in Ireland, Puertos del Estado in Spain and NOAA in Puerto Rico.

Try it on the map with a passage through the Wadden Sea, for example to Norderney.

Limits

Sailseer estimates chart datum from around eleven days of model data. The estimate tends to lie above the official value, so the calculated depth tends to come out too small. The depths over the bars come from a grid of around 115 m (EMODnet), which does not resolve a buoyed channel: “too shallow” is therefore a clear deduction in the rating, not a block. In narrow tidal inlets the model stream is smoother than in reality. In the fjords of Greenland, Iceland and Svalbard, which the model does not resolve, Sailseer takes the nearest water point within 50 km and states its distance: high and low water at the harbour may come earlier or later.

For navigation, the official tide tables, charts and notices to mariners apply, not Sailseer.