Module 11 - Tides & Tidal Streams
Tidal Diamonds and Stream Atlases
What you will learn
Read tidal-stream set and rate for the correct hour relative to the named reference port and distinguish spring from neap rates.
Tidal diamonds and stream atlases describe horizontal water movement. Their arrows and tables are indexed to hours before and after high water at a named reference port, which may not be the nearest harbour.
A charted tidal diamond identifies a position whose table gives set and rate, commonly from six hours before to six hours after high water at a named reference port. Set is the direction towards which the water flows, expressed in degrees true; rate is speed in knots. Mean spring and neap rates are normally shown separately.
A tidal-stream atlas shows the pattern over a wider area, usually as one chartlet for each hour relative to the same reference high water. The arrow shows direction and its figures or key show spring and neap rates, often in tenths of a knot.
The reference is a clock for the stream diagram, not a claim that the stream turns at local high water. Confirm the reference port, its predicted HW time, the page's hour label and the applicable rate before using the vector in course-to-steer or estimated-position work.
Build the stream vector from its reference
The atlas page is labelled HW -2, the reference-port HW is 1430, and the passage leg begins at 1230.
1. Align the clock
1230 is two hours before the named reference-port HW, so the HW -2 chartlet is the starting source.
2. Read set and rate
Take the arrow direction and the appropriate spring or neap rate for the route area.
3. Review across the leg
If the leg crosses an hourly boundary, divide it or use the prescribed averaging method rather than keeping one vector indefinitely.
Sense check: A set of 090° pushes the vessel eastward; plotting it westward has reversed the 'towards' convention.
Common mistake or limitation
- Set is the direction the stream goes towards, unlike wind direction, which is named for where it comes from.
- Using local harbour HW instead of the named reference-port HW can select the wrong atlas page.
Recap
- Read the reference port and hour before reading the arrow.
- Plot set towards and rate in knots.
- Choose the correct spring or neap value and review changes during the leg.
Optional quick check
Section 6 of 8
A tidal stream has a set of 090°. In which direction does it flow?
Continue studying Tides & Tidal Streams
This public lesson is part of Revision Module 11. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.