Course to steer, and the tidal vector triangle
When there’s a tidal stream, pointing the boat at your destination won’t get you there: the water you’re sailing through is moving. A course to steer allows for that, so the tide carries you along the line you want (the ground track).
You find it by drawing a vector triangle on the chart: the tide, your boat’s movement through the water, and the ground track you want to make good.
What you need
- The ground track from start to destination
- HW at the reference port, and today’s range
- The tidal diamond (or tidal stream atlas) for the area
- Your boat speed through the water
The method, step by step
- Draw the ground track from your start to your destination, and beyond.
- Find the tidal hour for your departure time, then read that row of the tidal diamond: the set (direction the water flows towards) and the spring and neap rates.
- Work out today’s rate, interpolating between the neap and spring rates by today’s range.
- From the start, draw the tide vector: the set, for one hour’s drift.
- From the end of the tide vector, open the dividers to one hour of boat speed (on the latitude scale, 1 minute = 1 nautical mile) and swing them until they cut the ground track.
- The line from the end of the tide vector to that point is the water track. Its bearing is the course to steer. The distance from the start to that point is your speed over the ground.
Steer up into the tide: the course to steer is always on the up-tide side of the ground track.
This gives a true course. On the water you’d then allow for leeway and convert to a compass course.
For passages of more than an hour, draw a tide vector for each hour, end to end, then one water track for the total time.
Worked example
Question 222-222G · fictional practice data
| HW Port Merrow | 1136 |
|---|---|
| Today's range | 2.8 m |
| Springs / neaps | 4.0 m / 1.9 m |
| Passage | G Gannet Rock → L Outer Ledge |
| Ground track | 145°T, 7.5 M |
| Depart | 0923 |
| Boat speed | 5.1 kn |
Using tidal diamond ◇A (referenced to HW Port Merrow), work out the course to steer to stay on the ground track for the first hour, and your speed over the ground. Ignore leeway; bearings are true.
| ◇A | Dir °T | Sp | Np |
|---|---|---|---|
| HW−6 | 226 | 1.2 | 0.6 |
| HW−5 | 052 | 0.1 | 0.1 |
| HW−4 | 054 | 1.2 | 0.6 |
| HW−3 | 056 | 2.1 | 1.1 |
| HW−2 | 058 | 2.4 | 1.2 |
| HW−1 | 060 | 2.1 | 1.1 |
| HW | 062 | 1.2 | 0.6 |
| HW+1 | 064 | 0.1 | 0.1 |
| HW+2 | 242 | 1.2 | 0.6 |
| HW+3 | 244 | 2.1 | 1.1 |
| HW+4 | 246 | 2.4 | 1.2 |
| HW+5 | 248 | 2.1 | 1.1 |
| HW+6 | 250 | 1.2 | 0.6 |
The one-hour vector triangle from G, north up. Tide first, then boat speed swung to cut the ground track.
- Tidal hour 0923 is in HW−2 (HW 1136)
- Read ◇A HW−2: 058°T, springs 2.4 kn, neaps 1.2 kn
- Rate for today 1.2 + (2.4 − 1.2) × 0.43 = 1.7 knRange 2.8 m is 0.43 of the way from neaps to springs.
- Tide vector From G: 058°T, 1.7 M (one hour)
- Water track From the end of the tide, swing 5.1 M to the ground track 145°TSet the dividers to your boat speed on the latitude scale: 1 minute of latitude = 1 M.
- Course to steer Bearing of the water track = 165°T
- Speed over the ground G to where the water track meets the ground track = 4.89 M in 1 h ≈ 4.9 kn
Answer: Tidal hour at 0923 HW−2 · Set at ◇A 058° · Rate at ◇A 1.7 kn · Course to steer 165° · Speed over the ground 4.9 kn
Common mistakes
These are the slips TideTutor catches in this exact question, in the words it uses to point you back on track:
- Is 0923 before or after high water at 1136?
- That's the HW−3 row. Use the row for the tidal hour you found.
- Tidal stream directions are where the water flows to, as printed in the table. No need to reverse it.
- That's the spring rate. Interpolate between the neap and spring rates using today's range.
- That's the neap rate. Interpolate between the neap and spring rates using today's range.
- That's the ground track. Steering it, the tide would push you off it. Allow for the tide.
Questions people ask
What’s the difference between ground track and water track?
The ground track is your path over the seabed. The water track is your path through the water; the difference is the tide.
Which way does a tidal diamond direction point?
The direction the stream flows towards, in degrees true. Draw it as printed.
Why not just add the tide’s rate to my speed?
Only if the tide is dead behind you. Otherwise part of it pushes you sideways, so measure the speed over the ground along the ground track.
Practise until it’s easy
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Try a question nowAll ports, charts and tidal data in these guides are fictional, for practice only. Never use them for navigation.