Spring tides have the largest range, with the highest high waters and lowest low waters, and come a day or two after full and new moon. Neap tides have the smallest range and come around half moon. Springs have nothing to do with the season.
Tides are caused mainly by the pull of the moon, with a smaller contribution from the sun. Most coasts, including New Zealand's, have two high waters and two low waters a day, about six and a quarter hours apart, so the times move later by roughly 50 minutes each day.
The range of a tide is the difference in height between high water and low water. It changes through the lunar month:
- Spring tides: at full moon and new moon, the sun and moon are in line with the earth and their pulls add together. High waters are higher, low waters are lower, and the range is greatest. Springs usually arrive a day or two after the full or new moon.
- Neap tides: at first and last quarter (half moon), the sun and moon pull at right angles. The range is smallest.
The cycle from springs to neaps and back to springs takes about two weeks.
Why it matters when sailing:
- Depth: at low water springs, banks and rocks that are covered at neaps may dry. Plan any passage over shallows with the actual height of tide, not a guess.
- Tidal streams: with a bigger rise and fall, more water moves in and out, so streams run faster at springs. In narrow entrances this can make the difference between a comfortable passage and not getting through.
- Anchoring and moorings: a bigger range needs more scope at high water, and leaves less water under the keel at low water.
- Leaving a boat on a beach or ramp: a boat left at the top of a spring high water may not float again until the next springs.
Tide tables give the predicted times and heights for standard ports, and the chart or almanac gives corrections for nearby places. Weather also affects the tide: strong winds and changes in barometric pressure can raise or lower it from the prediction.
More in the book: Chapter 14, Buoys, charts and tides