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Tide Information

Spring & Neap Tides




 It is very important to ensure that your plans fit with the tidal predictions for the day of your trip.   Most slipways and launch sites are tidal. Ceck the times of high and low water and assess how they will affect your trip when you launch and later head for home.   If the tide turns to a wind-against tide direction, the sea may become much rougher.  Check whether it will be a neap or spring tide.  Beware of harbour entrances where tidal currents can be quite severe.   For a comprehensive tidal prediction service for Swansea visit Easy Tide link below. 


The world's oceans are in constant flux. Winds and currents move the surface water causing waves. Ocean levels fluctuate daily as the sun, moon and earth interact. As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world's oceans to rise and fall. 
Imagine the earth covered completely by water. As the earth spins, this water is balanced evenly on all sides by centrifugal force. The moon has a gravitational pull on this layer of water as it orbits the earth. This pull causes the water to bulge toward the moon. Because the earth is spinning there will be a bulge on the opposite side of the earth as well.   As the earth rotates on its axis, each location on the earth will experience both tidal bulges. The areas of high water levels are high tides and the areas of low levels are low tides.  Since the earth and the moon rotate around the sun, there is an added modifying factor. When the sun and moon are aligned, there are exceptionally strong gravitational forces, causing very high and very low tides which are called spring tides, though they have nothing to do with the season. When the sun and moon are not aligned, the gravitational forces cancel each other out, and the tides are not as dramatically high and low. These are called neap tides.   Tides vary from day to day. As the earth, moon, and sun orbit, their positions constantly shift, causing slightly different gravitational effects. This causes the tides to occur at slightly different times. Tides also vary from place to place. Geographical position determines the level of tide.

Swansea Tide Predictions


Click here for Accuweather tide timetables @ the port of Swansea

Admiralty Chart Datum


At sea, Admiralty Chart Datum (ACD) is used in relation to water height.  This is based on the Lowest Astronomical Tide (LAT) and is the level below which the predicted tide will never fall, although in practice wind and high pressure may occasionally force the actual water level below this.  Values are given in metres and are used as the basis for sounding on all UK navigation charts.  This means that any sounding immediately shows the minimum depth of water available - with any known tide level being a 'plus' which may be added with care.

Port of SwanseaThe ACD zero is 2.82 metres above the cill guage zero.  Typical tide readings will be 2.8 metres above the previous Ordnance Datumn of Predictions.


Swansea Channel Maintenance Level (ML) is 4.2m below ACD. Swansea Ferryport Maintenance Level (ML) is 4.9m below ACD.

Enclosed docks and marinas in South Wales have various datums, some based on lock inner cills, some on constructed steps.  In some places the dock is dredged to datum, in others a Maintenance Level above or below the datum is dredged.

Note: ACD is also referred to as Chart Datum (CD).  This is safe enough if there is only one chart datum about.

Pilotage Notes: In Swansea Bay tidal streams flow anti-clockwise for 91/2 hrs (Swansea HW–31/2 to +6), with at times a race off Mumbles Head. From HW–6 to–3 the stream reverses, setting N past Mumbles Head towards Swansea. Keep seaward of Mixon Shoal. When N of SW Inner Green Grounds (SWIGG) SCM lt buoy, Q (6) + L Fl 15s keep to W of the dredged channel and clear of commercial ships. Yachts must motor in harbour and approach, maximum speed 4 kn.

Tidal Action



Tidal Variations

It is helpful, but not essential, to understand the reason why the tides vary in height and time on different days.  Twice daily, the tide comes in, and each day it will be approximately 50 minutes later than the previous day. Astronomically, this is because as the Earth moves around on its axis every 24 hours, the Moon during the day and night moves through one-thirtieth of its orbit around the Earth. This means that it will be approximately one-thirtieth in a different position  24 hours later.

 This, of course, influences the tides because of the importance of its gravitational pull on the world's oceans and seas. The water pull is best visualised as a horizontal movement, not as a vertical up and down movement as viewed from the shore.  There is a considerable variation in the height of the tide from week to week.

 Because of its nearness to the Earth, the gravitational pull of the Moon is the stronger influence than that of the Sun. As the Moon orbits around the Earth it exerts its gravitational pull as illustrated (diagram to follow):

(1)  Highest Spring Tides - Moon and Sun in conjunction, with combined gravitational pull producing the highest tides. High tides on the opposite side are due to the centrifugal force of the Earth's rotation.
The highest tides in England occur about two days after the New Moon.
(2)  Neap Tides - Moon's orbit moves towards 90° to that of the Sun, when they are at quadrature, and their relative pulls, about 7 to 3 in favour of the Moon, result in Neap tides, with smaller variations.
(3)  Spring Tides - Moon and Sun in opposition, with combined gravitational pull.
(4)  Neap Tides - Moon's orbit moves toward the quadrature.

 The highest spring tides of the year occur after the equinoxes in March and September.  However, the weather may not be so good, and rain and gales are frequent. The Tide Tables cannot be relied on for precise highs and lows of the tides. A storm surge out in the Atlantic can put at a metre on a high tide in exceptional circumstances, and the wind can also prevent the tide from receding as much as it should.

 It is interesting to note that high spring tides occur at approximately the same time of the day every year in each location on the coast. This means that high springs occur at dawn and dusk in around Mumbles and Swansea.

 The configuration of the shore has a large bearing on tide heights and range, tending to increase in bays and funnel-shape inlets like the Bristol Channel and St. Malo, near the Channel Islands, in France.

UK Hydrographic Office Tide Predictions

National Ocean Service

BBC Tides


Tides (UK) (a few locations only)   INSTANT ACCESS

Tide Predictions: Neptune (fee required to unlock the download)

Tide Prediction Program*** WXTide 32


Equinox Times & Dates  (Eric W. Weisstein's Astronomical pages)