Tidal hours, explained
Tidal streams are given for each hour of the tide, counted from high water at a reference port: HW−6 to HW+6. Before you can read a tidal stream atlas or a tidal diamond, you need to know which of those tidal hours your time falls in.
The catch is that each tidal hour is centred on its time. HW+2 doesn’t start two hours after high water; it runs from 1½ to 2½ hours after.
What you need
- The time of HW at the reference port named on the atlas or diamond table
- The time you’re interested in
The method, step by step
- Write the HW time in the middle of a table, against “HW”.
- Each row above is one hour earlier (HW−1, HW−2 … HW−6). Each row below is one hour later (HW+1 … HW+6).
- Each tidal hour is centred on its time and runs from 30 minutes before to 30 minutes after it.
- To place a time, work out how long before or after HW it is, and round to the nearest whole hour. Before HW is minus; after HW is plus.
- Use that row of the tidal diamond, or that page of the tidal stream atlas.
Check which port the streams are referenced to. It’s often a different standard port from the one you’re sailing near.
Near a half-hour boundary, work out the exact start and end of the hour rather than rounding by eye.
Worked example
Question 222-222Q · fictional practice data
| Port | Wyvern Quay (standard port) |
|---|---|
| High water | 1121 |
Fill in the tidal hours around high water at Wyvern Quay, then place 1003 in its tidal hour.
| Tidal hour | Centred on | Runs |
|---|---|---|
| HW−6 | 0521 | 0451–0551 |
| HW−5 | 0621 | 0551–0651 |
| HW−4 | 0721 | 0651–0751 |
| HW−3 | 0821 | 0751–0851 |
| HW−2 | 0921 | 0851–0951 |
| HW−1 | 1021 | 0951–1051 |
| HW | 1121 | 1051–1151 |
| HW+1 | 1221 | 1151–1251 |
| HW+2 | 1321 | 1251–1351 |
| HW+3 | 1421 | 1351–1451 |
| HW+4 | 1521 | 1451–1551 |
| HW+5 | 1621 | 1551–1651 |
| HW+6 | 1721 | 1651–1751 |
- Centre of HW−4 1121 − 4 h = 0721
- HW−4 starts 0721 − 30 min = 0651It runs 0651–0751.
- HW+2 ends 1121 + 2 h + 30 min = 1351It runs 1251–1351.
- Place 1003 HW−1 runs 0951–1051, so 1003 is in HW−11003 is 1h 18m before HW; the nearest whole hour is 1.
Answer: HW−4 is centred on 0721 · HW−4 starts at 0651 · HW+2 ends at 1351 · 1003 falls in which tidal hour? HW−1
Common mistakes
These are the slips TideTutor catches in this exact question, in the words it uses to point you back on track:
- HW−4 is before high water, so its time is earlier than HW.
- That's when the hour starts. Where is it centred?
- That's the time HW−4 is centred on. Each tidal hour starts 30 minutes before its centre.
- That's when it ends. It starts 30 minutes before the centre.
- A tidal hour starts half an hour before its centre, not a whole hour.
- That's the time HW+2 is centred on. It ends 30 minutes after.
Questions people ask
What is a tidal hour?
One of the thirteen one-hour periods from HW−6 to HW+6, each centred on a whole number of hours before or after high water at a reference port.
When does HW+1 start?
Half an hour after high water. HW+1 runs from 30 minutes to 1 hour 30 minutes after HW.
Is a time before high water plus or minus?
Minus. HW−3 is three hours before high water, so its clock time is earlier than HW.
Practise until it’s easy
TideTutor gives you endless fresh tidal hours questions like this one, with three-step hints, feedback on the classic mistakes and the full worked solution. Two questions in every topic are free.
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All ports, charts and tidal data in these guides are fictional, for practice only. Never use them for navigation.