Quick Facts
| Tidal Diamond Data | Direction and rate for each hour relative to HW at the standard port; springs and neaps columns |
| Interpolation Method | Proportion of mean spring range achieved gives percentage between springs and neaps rates |
| Tide Triangle | Three-vector plot: water track + tidal stream vector + course to steer for required ground track |
| Navionics Currents | Tidal arrow overlays on active chart; update by dragging time slider |
| Surge Effect | Onshore gales can raise actual water level by 1–2 m above predicted, increasing stream rates |
| Eddy Zones | Counter-current eddies form on the lee side of headlands — usable by small vessels and kayaks |
Knowing that a tidal stream exists at a given point is only the start. To navigate efficiently in tidal waters, you need to know its direction and rate at the time of your passage, and then use that information to calculate a course to steer that delivers you to your destination rather than set you sideways of it. This is the craft of tidal stream navigation — and it applies whether you are making a short coastal hop or planning a cross-channel passage.
This guide covers the data sources for tidal streams — diamonds on Admiralty charts, the tidal stream atlas, and electronic chart overlays — the method for interpolating between springs and neaps, vector plotting of the tide triangle, and the important distinction between tidal stream predictions and what you will actually encounter on the water.
Tidal Diamonds on Admiralty Charts
Admiralty nautical charts mark specific positions in tidal waters with a magenta diamond containing a letter — A, B, C, and so on. Each lettered diamond corresponds to a table printed on the chart margin, showing the tidal stream data for that position. The table has rows for each hour from 6 hours before to 6 hours after high water at the reference standard port, and two columns of data: the direction (in degrees True) and rate (in tenths of a knot) at mean spring tides and at mean neap tides.
To use a tidal diamond, first determine the time of high water at the chart's reference standard port for the day of your passage. Then calculate how many hours before or after that HW you will be at or near the diamond's position. Read off the direction and spring/neap rates from the corresponding row of the table. If you are operating between spring and neap conditions, interpolate between the two rate columns.
Diamond data has limitations. Each diamond represents a single point, and tidal streams can vary significantly over short distances in areas of complex bathymetry. Diamonds are relatively sparse in some chart areas. For a broader picture — particularly on passages covering more than one diamond's area — the tidal stream atlas is more useful.
Tidal Stream Atlases: The Hour-by-Hour Picture
Tidal stream atlases are published by the UKHO (Admiralty) and equivalent hydrographic offices for most major tidal waters. Each atlas covers a specific area and contains thirteen chartlets — one for each hour from 6 hours before HW to 6 hours after HW at a reference port. Each chartlet shows tidal stream vectors (arrows) across the area, with numbers beside each arrow giving the mean spring rate and mean neap rate in tenths of a knot.
Using an atlas for passage planning: identify the HW time at the reference port, then work through the hourly chartlets that correspond to your planned passage time, noting the direction and rate of stream across the route. For a long passage, accumulate the stream vectors to find the net set and drift over the full transit — the total displacement from tidal streams that your course to steer must compensate for.
The Admiralty publishes stream atlases for areas including the English Channel, the Irish Sea, the waters around Scotland, and several others. Equivalent publications exist for European waters (SHOM for French waters), and NOAA publishes tidal current charts for key US areas including the San Francisco Bay, Puget Sound, and Long Island Sound.
Interpolating for Intermediate Tidal States
Spring and neap rates represent the maximum and minimum conditions in the lunar cycle. On any given day, the actual tidal stream rate will fall somewhere between these values. Interpolating accurately requires knowing the tidal range for the day in question and comparing it to the published mean spring range and mean neap range for that port.
The standard interpolation method: calculate the range for the day (HW height minus LW height from the tide table). Compare it to the mean spring range and mean neap range. Express the day's range as a proportion between these two. Apply the same proportion to the tidal stream rates — for example, if the day's range is 60% of the way from neaps to springs, the stream rate will be approximately 60% of the way from the neap rate to the spring rate. Many tidal stream atlases include an interpolation graph on the inside cover to simplify this calculation.
Vector Plotting: The Tide Triangle
The fundamental problem of tidal navigation is this: you want to travel in a straight line from A to B over the ground, but the water you are sailing through is itself moving sideways. Your compass course must be angled into the stream to compensate. The tide triangle is the geometric tool that solves this.
On a chart or plotting sheet, draw three vectors:
- The water track: The direction and distance you travel through the water (your course and speed through the water, over the time period being plotted).
- The tidal stream vector: The direction and distance the water itself moves over the same time period (stream direction and rate × hours).
- The ground track: The resultant — the actual track made good over the ground, from start to finish.
In practice, you know the ground track you want to make (the direct line to your destination) and the tidal stream vector (from the atlas or diamond data). The tide triangle lets you solve for the course to steer: draw the tidal stream vector from your starting position, then swing an arc from the far end of the tidal vector equal to your boat's speed through the water. Where that arc intersects your desired ground track is your destination for the time period. The line from the base of the tidal vector to the intersection point is your course to steer.
For longer passages with changing tidal streams, this process is repeated for each hour, accumulating the tidal vectors to find the overall course to steer that delivers the correct ground track across the full passage.
Electronic Chart Tidal Current Overlays
Modern chartplotters from Garmin, Navionics, and C-MAP provide animated tidal current overlays — arrows on the chart showing the predicted tidal stream direction and rate that update as you adjust the time. These overlays draw on the same underlying harmonic data as printed tidal stream atlases, displayed in a format that makes passage planning significantly faster.
Navionics on a tablet or phone allows you to slide a time bar and watch the tidal arrows shift in direction and strength across the chart — a useful tool for quickly identifying the fair tide window for a planned route. Garmin chartplotters with BlueChart g3 charts provide similar current overlay functionality. C-MAP REVEAL and MAX-N+ charts include current and tide data for most tidal areas.
Electronic overlays are derived from models that may have lower spatial resolution than printed atlases in some areas, and they share the same fundamental limitation as all tidal predictions: they represent the astronomical tide, not the actual conditions, which are also affected by wind, river discharge, and barometric pressure. Use electronic overlays as a planning tool and cross-reference against published tidal data for critical passages.
The Difference Between Prediction and Actual Conditions
Tidal stream predictions are computed from harmonic constants derived from years of historical tide gauge measurements. They are accurate representations of the astronomical tide — the component of the tide driven by the Moon and Sun. However, actual water levels and current speeds are also affected by non-astronomical factors that predictions do not include.
Storm surge — the piling up of water against a coast by sustained onshore winds — can raise actual water levels by 1–2 metres above the predicted level. In the North Sea, where the basin shape amplifies surges, a strong northerly gale can produce a surge of this magnitude, significantly increasing stream rates through narrows and altering expected slack water timings. Conversely, prolonged offshore winds can lower water levels below prediction, reducing depth over bars and in approach channels.
River discharge affects tidal streams in estuaries — high river flow after prolonged rainfall strengthens the ebb and weakens the flood, effectively shortening the window during which you can make progress upriver against the combined river current and tidal ebb.
Coastal vs Offshore Streams: Eddies and Acceleration Zones
Tidal stream atlases show average conditions across an area, but the local picture can be far more complex. Around headlands, the stream accelerates as it is compressed by the shoaling water and the land mass — the stream through the narrows is faster than the stream in the open water either side. On the lee side of a headland, a counter-current eddy frequently forms where the flow separates from the land. These eddies can run at 1–2 knots in a direction opposite to the main stream, and are a traditional resource for small vessels working to windward or against the tide: by hugging the coast on the eddy side of a headland, a boat can gain a knot or two of free assistance.
Pilotage in Tidal Waters: Arriving at the Right Time
Approaching a port or anchorage with tidal constraints requires combining tidal height knowledge with tidal stream timing. The ideal arrival is when there is both adequate depth over the bar or in the approach channel and a manageable (ideally fair) current in the entrance. For drying harbours, this typically means arriving two to three hours before high water — enough depth to enter, enough remaining flood to allow time to find a berth and secure, and a rising tide that forgives any navigational error.
Related: Tides and Hazards
Understanding Tides — Physics and Prediction → | Tidal Hazards: Overfalls, Drying Harbours, and Tide Races →
← Tides & Tidal Currents | Tidal Hazards →
Frequently Asked Questions
How do you use a tidal diamond on an Admiralty chart to determine tidal stream conditions? Admiralty charts mark specific positions with a magenta diamond containing a letter — A, B, C — each corresponding to a table in the chart margin showing tidal stream data for that position. The table has rows for each hour from 6 hours before to 6 hours after high water at the reference standard port, and two columns: direction in degrees True and rate in tenths of a knot at mean spring tides and mean neap tides. To use it: determine the time of high water at the chart's reference standard port, calculate how many hours before or after that HW you will be at the diamond's position, then read the direction and spring/neap rates from that row.
What is the tide triangle and how does it solve the problem of tidal set on a passage? The tide triangle is a three-vector geometric tool plotted on a chart that solves the course-to-steer problem when the water you are sailing through is itself moving sideways. The three vectors are: the water track (the direction and distance you travel through the water at your course and speed), the tidal stream vector (the direction and distance the water itself moves over the same time period), and the ground track (the resultant actual track made good over the ground). You know the ground track you want and the tidal stream vector from the atlas or diamond data; the tide triangle lets you solve for the course to steer that delivers you to your destination rather than set sideways of it.
How do you interpolate between spring and neap tidal stream rates for an intermediate day? The interpolation method compares the tidal range for the day in question to the published mean spring and neap ranges for the port. Calculate the day's range from the tide table (HW height minus LW height). Express the day's range as a proportion between mean spring range and mean neap range. Apply the same proportion to the tidal stream rates — if the day's range is 60% of the way from neaps to springs, the stream rate will be approximately 60% of the way from the neap rate to the spring rate. Many tidal stream atlases include an interpolation graph on the inside cover to simplify this calculation.
What are electronic chart tidal current overlays and what are their limitations? Chartplotters from Garmin, Navionics, and C-MAP provide animated tidal current overlays — arrows on the chart showing predicted tidal stream direction and rate that update as you adjust the time. Navionics on a tablet allows you to slide a time bar and watch tidal arrows shift across the chart, useful for quickly identifying fair tide windows. These overlays draw on the same underlying harmonic data as printed tidal stream atlases. Their limitation is that they represent the astronomical tide only, not actual conditions — which are also affected by wind, river discharge, and barometric pressure. Use electronic overlays as a planning tool and cross-reference against published tidal data for critical passages.
Why can actual tidal stream conditions differ significantly from predictions and by how much? Tidal stream predictions are computed from harmonic constants and accurately represent the astronomical tide — the component driven by the Moon and Sun. However, actual water levels and current speeds are also affected by storm surge: onshore gales can raise actual water levels by 1 to 2 metres above predicted in basins like the North Sea, significantly increasing stream rates through narrows and altering slack water timings. River discharge affects tidal streams in estuaries — high river flow after prolonged rainfall strengthens the ebb and weakens the flood. Prolonged offshore winds can lower water levels below prediction, reducing depth over bars and in approach channels.