Module 11 - Tides & Tidal Streams
Springs and Neaps
What you will learn
Distinguish spring and neap tidal ranges and use local predictions rather than assuming a universal delay after a Moon phase.
Spring tides have the larger range and neap tides the smaller range. They follow the changing alignment of the Sun and Moon, but the delay between alignment and the largest local range is port-specific.
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. 'Spring' describes the range springing higher and lower; it is unrelated to the season.
A similar spring-neap sequence repeats roughly fortnightly, but the largest range does not occur at one fixed number of days after alignment everywhere. UKHO guidance notes that local spring-tide age can vary from zero to four days. Weather and non-astronomical residuals can also change the observed level without changing the spring or neap label.
For a yacht passage, range matters because it affects available depth, drying height coverage, tidal-stream strength and sometimes the width of a safe time window. Read the day's predicted heights and the applicable spring or neap stream rate instead of inferring them from the calendar alone.
Use range to frame the passage
A shallow harbour is comfortable near neaps, but the planned departure falls close to a locally predicted spring range with a stronger cross-stream outside.
1. Read the heights
Calculate the day's range from the relevant high and low water, rather than naming the tide from the Moon phase alone.
2. Check the stream source
Use the published spring or neap rates, interpolating only with the method supplied by the source when needed.
3. Protect the margins
Allow for the stronger stream, faster depth change and weather uncertainty in the departure window.
Sense check: A spring tide does not guarantee exceptionally high observed water; pressure, wind and surge can raise or lower the astronomical prediction.
Common mistake or limitation
- Spring tides are not caused by the season and do not occur only near the equinoxes.
- 'One or two days after new or full Moon' is not universal; the local prediction governs.
Recap
- Springs have the larger range; neaps have the smaller range.
- Lunar alignment explains the cycle, but local spring-tide age varies.
- Use predicted heights and published stream rates for the actual passage.
Optional quick check
Section 2 of 8
Which statement is safe to use when planning around spring tides?
Continue studying Tides & Tidal Streams
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