Set, drift and rate
- Set is the direction the stream flows towards, in degrees true. (Wind is the other way round: a westerly wind blows from the west.)
- Rate is its speed in knots.
- Drift is how far it carries you in a given time, in miles.
What a tidal diamond tells you
A diamond is a magenta lettered symbol on the chart. Its table gives the set and the spring and neap rates for each hour from six hours before to six hours after high water at a named standard port, usually printed in the table heading. Always use high water at that port, not the nearest one.
For our example, the high water reference port's tides today are:
| Time (UT) | Height | |
|---|---|---|
| Low water | 0418 | 1.0m |
| High water | 1042 | 4.6m |
Range = 4.6 − 1.0 = 3.6m. The almanac gives the port's mean spring range as 4.1m and mean neap range as 2.0m.
Step 1: which tidal hour?
Each hour in the table covers the half hour either side of it. So “HW +2” covers from HW +1h30 to HW +2h30, and “HW −1” covers from HW −1h30 to HW −0h30.
Question: what is the stream at diamond C at 1300 UT? 1300 − 1042 = HW +2h18, which falls between HW +1h30 and HW +2h30, so use the HW +2 line.
| Diamond C | Set (°T) | Springs (kn) | Neaps (kn) |
|---|---|---|---|
| HW +2 | 075 | 2.4 | 1.2 |
Step 2: interpolate between springs and neaps
Today's range sits between neaps and springs, so the rate does too:
- How far towards springs = (today's range − neap range) ÷ (spring range − neap range) = (3.6 − 2.0) ÷ (4.1 − 2.0) = 1.6 ÷ 2.1 = 0.76.
- Rate = neap rate + 0.76 × (spring rate − neap rate) = 1.2 + 0.76 × 1.2 = 1.2 + 0.9 = 2.1 knots.
So the stream is setting 075°T at 2.1 knots. Over half an hour that's a drift of about 1.05 miles. Many almanacs include a “computation of rates” graph that does the same interpolation without the arithmetic. If today's range is bigger than the mean spring range, extrapolate: the rate can be higher than the spring figure.
The same stream from an atlas
A tidal stream atlas has one page per hour relative to high water at its reference port. Pick the page the same way (HW +2 here), find your position, and read the arrow nearest to it. The arrow shows the set, and a pair of numbers such as 12,24 gives the neap and spring rates in tenths of a knot: 1.2 knots at neaps, 2.4 at springs. Interpolate exactly as above to get 2.1 knots.
The atlas is easier to read for a passage across a big area. Diamonds give precise figures at one point. Use whichever is nearer and more detailed for where you'll actually be.
Using it on the chart
For an estimated position, plot your water track and distance through the water, then add the tidal stream vector (set and drift) from the end of it. For a course to steer, you lay off the tide first and work back to the heading that cancels it out. See our course to steer worked example.
Common mistakes
- Using high water at the wrong port for the diamond or atlas.
- Mixing UT and BST between the tide tables and your watch.
- Picking the wrong tidal hour: remember the half hour either side.
- Plotting the set as the direction the stream comes from.
- Using the spring rate on a neap day without interpolating.