A tidal stream is the horizontal flow of water caused by the tide rising and falling: the flood runs in on a rising tide and the ebb runs out on a falling one. It carries your boat with it, changing your speed and course over the ground.
When the tide rises, water has to flow into a harbour or along a coast, and when it falls it flows out again. This movement is the tidal stream. (The word "tide" strictly means the vertical rise and fall; the stream is the horizontal flow.)
- The flood stream runs in, on the rising tide.
- The ebb stream runs out, on the falling tide.
- Slack water is the short period when the stream turns. It often comes near high or low water, but in many places the stream turns some time before or after, so check the local information.
The stream is usually strongest in the middle of the flood or ebb, stronger at springs than at neaps, and strongest where the water is squeezed: narrow entrances, channels between islands, and off headlands. There it can run at several knots, which may be more than a small yacht can sail against in light wind.
Effects on a sailing boat:
- Speed over the ground: a 2-knot stream adds 2 knots going with it and takes 2 knots away going against it. On a 5-knot boat, a trip that takes two hours in still water takes three hours and twenty minutes against it.
- Set: a stream across your course carries you sideways, which can set you onto a hazard or a mark. Watch transits and the chart plotter.
- Apparent wind: sailing with the stream into the wind increases the wind you feel; with the stream and the wind both behind you, the boat may barely seem to move through the water.
- Sea state: wind blowing against the stream raises short, steep waves, often unpleasantly so off headlands and in entrances. Wind with the stream flattens the sea.
Planning: look up the times of high and low water and the stream directions on the chart or a tidal stream atlas, aim to go with the stream on the longest or hardest leg, and avoid narrow places with wind against tide.
More in the book: Chapter 14, Buoys, charts and tides