The tide does two things. It raises and lowers the water — the height of tide — and it moves the water horizontally — the tidal stream (tidal current in U.S. usage). They are driven by the same astronomy, but they are measured differently, predicted from different data, published in different tables, and used for different decisions. This guide separates them and shows where each answer comes from.
Two graphs, height of tide and tidal stream rate, over one tidal cycle with slack water not aligned with high and low water.
Height: will there be enough water?
Height of tide is a vertical distance above chart datum at a moment in time, found from tide tables and the port's tidal curve, or roughly from the rule of twelfths. It answers questions about depth: can we cross the bar, will we be afloat at low water, will we fit under the bridge. Our guides on chart datum and the rule of twelfths cover it.
Stream: where will the water carry us?
Tidal stream is a horizontal flow with a direction (the set, given as the true direction the water flows towards) and a rate in knots. It answers questions about position and course: where an hour's sailing will actually put us, what course to steer to make good a track, whether we will make it round a headland before the stream turns. It is found from:
- Tidal diamonds on the chart: a lettered diamond marks a position; the table gives set and rate for each hour from HW−6 to HW+6 at a named standard port, at springs and neaps.
- A tidal stream atlas: a page per hour relative to HW at the reference port, with arrows showing the set and figures giving neap and spring rates in tenths of a knot (for example 12,24 means 1.2 knots at neaps and 2.4 at springs).
- In U.S. waters, NOAA tidal current predictions for current stations, published separately from the tide (height) predictions, giving times of slack and maximum flood and ebb with rates.
- Local pilotage information: pilot books and almanacs often describe eddies, overfalls and the timing of the turn of the stream at headlands and harbour entrances.
Why the timing is not universal
On a long, open coast where the tide arrives as a progressive wave, the water tends to flow fastest around mid-tide and to be slack near high and low water. Where the tide behaves more like a standing wave — in many bays, estuaries and enclosed seas — the stream can be slack around mid-tide and strongest near high and low water. Most real coastlines are a mixture, and the answer at a given headland or entrance is whatever the diamond or atlas says. Rules of thumb like “strongest at half tide” describe one geometry and are wrong in the other.
The other timing subtlety is the reference port. Tidal diamonds and atlases give times relative to HW at a specific standard port, which may be some distance away. Use that port's HW time, not the nearest harbour's.
Reading a tidal diamond: worked example
Suppose the chart's table for diamond C reads, for HW+2 at the standard port: set 215°, rate 2.6 knots springs, 1.3 knots neaps. HW at the standard port is 13:40 today, and today's range is about halfway between mean neaps and mean springs. Between 15:10 and 16:10 (HW+1½ to HW+2½, the hour centred on HW+2), the stream near diamond C sets towards 215° at roughly 2.0 knots. Over that hour it will move you two miles towards 215° — a vector you draw on the chart for an estimated position or a course to steer. Interpolate rather than rounding to the nearer of springs or neaps; the difference here is 1.3 knots, which over a three-hour passage is nearly four miles.
Using streams in planning
- Tidal gates: headlands and narrows where the stream runs hard. Time your arrival for slack or a fair stream, working backwards from the atlas to your departure time.
- Course to steer: draw the stream vector for the passage time from the start point; the boat's speed arc from its end cuts the ground track and gives the heading (see our course-to-steer guide).
- Estimated position: draw the water track first, then add the stream vector; the end is the EP.
- Wind against tide: a stream running against a fresh wind builds short, steep seas; the same wind with the tide is far more comfortable. The atlas tells you when each applies.
Check yourselfYour pilot book says the stream off a headland turns to the south-west about two hours before local high water and runs at up to three knots at springs. Local HW is 09:00 and it is two days after springs. You want to round the headland with a fair south-westerly stream in a five-knot boat. When should you be there?Show answer
From about 07:00 onward, and as close to the start of the fair stream as you can manage: rates near springs will be close to the three-knot maximum, which is more than half your boat speed, so arriving before the turn would mean making almost no progress against it. Note that the turn is given relative to local HW here, whereas a tidal diamond would be relative to a standard port — always check which reference the source uses.
Key takeaways
- Height is vertical (metres above chart datum); stream is horizontal (set and rate in knots). Different questions, different data.
- The timing of slack and maximum stream relative to HW and LW depends on the place. There is no universal rule; look it up.
- Tidal diamonds and stream atlases are referenced to HW at a named standard port and give spring and neap rates — interpolate for today's range.
- Streams decide course to steer, estimated position and timing at tidal gates. Heights decide depth.
