Understanding Tides for Open Water Swimming: Spring Tides, Neap Tides & Currents
In certain parts of the world, like the Mediterranean, the sea remains relatively constant in its location. However, if you're a sea swimmer in the UK, you're well acquainted with the significant tidal variations. Tides can be extensive and ever-changing, ensuring that no two swims are alike.
How does the sea transform?
The sea is impacted by the gravitational pull of the moon and sun in relation to the earth.
Master local tidal behaviour for safer open water swimming. Learn how spring and neap tides affect currents, when to find slack tide, and how to read tide tables.
Spring Tides and Neap Tides
Spring tides happen at full moon and new moon, for 7 days every other week of the month. Neap tides occur only on the half moons for 7 days, so every other week of the month. Spring tides are also known as king tides.
The biggest misconception is that spring tides happen in the season of spring, but they actually occur all year round. They are much more extreme than neap tides -a high spring tide is significantly higher than a high neap tide, and a low spring tide drops a lot lower than a low neap tide. The exact heights, ranges, and timings of tides will vary depending on what beach or stretch of coastline you are swimming on.
Understanding Tidal Currents & The Rule of Twelfths
It’s important to understand that more water moves during the middle two hours of both an incoming (flood) and outgoing (ebb) tide. This means that tidal currents will always be at their absolute strongest during hours 3 and 4 of a 6-hour tidal cycle.
On a spring tide, due to the low tide being lower and the high tide being higher, a vastly greater volume of water moves across the coastline between these two extremes. Consequently, currents move much faster during a spring tide than during a neap tide. When planning a swim, factor in that a spring tide current can easily pull faster than a swimmer's maximum sprint speed.
Slack Tide
In many coastal locations, slack tide -the window when the water stops moving before reversing direction—occurs roughly one hour either side of high or low water. During slack water, lateral currents drop significantly, making it generally safer to swim since you are not fighting an ongoing drift.
However, local geography changes these rules. In some bays or beaches, geographical features create eddies, while shallow shingle shelves or sandbars can make low tide safer or more accessible than high tide. Always check local beach conditions, hazard points, and water depths beforehand.
How to Read Tide Tables & Check Conditions
Learning to read local tide tables is an essential skill for every open water swimmer. Look for:
Tide Heights: Measure the difference between high and low water (tidal range) to identify whether you are on a spring or neap tide.
Tide Times: Note high, low, and estimated slack water times to schedule your entry.
Reliable Tools: Websites and apps like Surfline (formerly Magic Seaweed), tide forecasting tools, and coastal meteorological reports are great resources for general sea conditions.
Beyond digital data, always speak with experienced local swimmers, sailors, coastguards, and knowledgeable locals to learn how the water behaves around specific headlands, piers, and beaches.
Kirsty’s Top Tips for Swimming in Tidal Waters
1. Know Your Moon Phase & Tide Type
Check the Cycle: Always identify whether you are swimming on a Spring tide (Full or New Moon) or a Neap tide (Half Moon).
Respect Spring Tide Speed: Remember that Spring tides move significantly more water over the same time frame. The lateral drift during a Spring tide can easily exceed your swimming pace, so plan a route where you aren't fighting a head-current to get back to shore.
2. Time Your Swim Around the "Rule of Twelfths"
Avoid the Middle Hours: Tidal movement isn't constant throughout the cycle. The currents peak during hours 3 and 4 (the middle two hours) of both incoming (flood) and outgoing (ebb) tides.
Target Slack Water: Aim to enter the water during slack tide—roughly one hour before to one hour after high or low water - when the water slows down before changing direction, giving you the calmest swimming conditions.
3. Plan Your Route with the Current, Not Against It
Swim Into the Current First: If you are swimming an out-and-back route, always swim into the current on your way out while your arms are fresh, allowing the flow to assist you on your return journey when you're fatigued.
Identify Shoreline Sighting Marks: Pick fixed landmarks on the beach (buildings, groynes, distinct hills) to monitor your position. If you notice you are moving sideways faster than you are moving forward, aim your body diagonally into the current to compensate for drift.
4. Consult Local Knowledge & Check Tide Tables
Verify Local Slack Times: Never assume slack water happens exactly at high or low tide for every beach—geographical features, headlands, and narrow channels can shift slack water times by up to an hour.
Ask the Experts: Before diving into an unfamiliar tidal spot, cross-reference tide tables with local swimmers, coastguards, or sailors who know how the water behaves around specific beach features.
For more information on understanding tides, sea conditions, and water safety, you can book a session in with me where we go through all of the local sea theory. Read HERE.