Module 11 - Tides & Tidal Streams
Springs and Neaps
What you will learn
Use the predicted spring-neap range to anticipate depth change and stream strength without assuming a universal lunar delay or equinox maximum.
Spring tides have the larger local range and neaps the smaller range. Alignment explains the cycle, but local spring-tide age and the observed water level are not fixed by the calendar.
Around new and full Moon, the tide-producing effects of the Sun and Moon reinforce one another and spring ranges occur. Around first and third quarter, the effects partly oppose one another and neap ranges occur. The name spring describes the larger range and is unrelated to the season.
A similar cycle repeats roughly every 14 to 15 days, but the largest local range does not occur at one universal delay after alignment. UKHO guidance records local spring-tide ages from zero to four days. Perigee, lunar declination, coastal response and seasonal levels can also influence predicted extremes.
Weather changes observed water without changing the astronomical spring or neap label. For a coastal passage, use the day's predicted HW and LW to calculate range, then use the applicable publication method for stream rate, depth window and clearance.
Turn range into passage limits
A shallow harbour departure coincides with a predicted spring range and a stronger cross-stream outside the entrance.
1. Calculate the range
Use the day's relevant high and low waters rather than naming conditions from the Moon phase alone.
2. Select the stream method
Read the applicable mean spring/neap rate and interpolate only when the source or exercise instructs it.
3. Protect the window
Allow for faster depth change, stronger stream and meteorological uncertainty in the leave-by decision.
Sense check: A spring tide does not guarantee exceptionally high observed water; high pressure or offshore wind may reduce the level.
Common mistake or limitation
- Spring tides are not seasonal and do not occur only near the equinoxes.
- One or two days after new or full Moon is not a universal rule; use the local dated prediction.
Recap
- Springs have larger range; neaps have smaller range.
- The local response controls when the largest range occurs.
- Use predicted heights and the publication's stream method for the passage.
Optional quick check
Section 2 of 11
Which evidence should control a spring-tide harbour departure?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
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