Glossary of terms:
- Chart Datum: water depths that are displayed on a nautical chart such as Lowest Astronomical Tide (L.A.T.)
- Current - a horizontal movement of water
- Drying Height: vertical distance of the seabed that is exposed by the tide, above the sea water level at the lowest astronomical tide.
- Ebb: Tidal current moving away from a shore / into a harbour/river etc
- Flood: Tidal current moving towards a shore / into a harbour/ river etc
- High Water (H.W.) / High Tide - The maximum height reached by a rising Tide.
- Low Water (L.W.) / Low Tide - The minimum height reached by a falling tide.
- Lowest Astronomical Tide (LAT): The lowest tide level that can be predicated to occur.
- Neap Tide: Tides of decreased range / currents that occur twice month when the moon is at 90 degrees to the earth / you see a 1/4 moon.
- Range: The difference between HW and LW
- Spring Tide: A tide that occurs just after a new or full moon with the greatest difference between HW & LW
- Tide Table - Tables which give daily predictions of the heights of HW's and LW's
Where to find tidal information?
Internet i.e. EasyTide
How to Interpret a Tide Table - Examples:
Using EasyTide Website Data:
Date: 14th April 2020:
Tidal Range = 5m
Tidal Range = 4.4m
Tidal info taken from EasyTide - + 1 hour for 'Summer Time' added via website
Using Almanac Data:
- Low Water Time: 01:39 / Low Water Height: 1.2m
- High Water Time: 08:08 / High Water Height: 3.1m
Tidal Range = 1.9m
- Low Water Time: 14:22 / Low Water Height: 1.1m
- High Water Time: 20:54 / High Water Height: 3.3m
Tidal Range = 2.2m
+ 1 hour has been added to adjust for 'Summer Time' as 16th September is in a 'non-shaded' area.
To calculate heights of tide between LW and HW see the tidal curve on EasyTide website or complete a Tidal Curve (see our Tidal Curves Tutorial)
Calculating 'actual' depth:
Chart Datum (L.A.T.) is found on a chart of the area you're planning on going afloat. See Charts Tutorial for more info.
Chart Datum + Tide Height = Depth of Water
7.2m @ 10:14
0.5m @ 10:14
Spring & Neap Tides:
Occur approx every 14 days near a full or new moon.
Spring tides are the highest high tides and lowest low tides, where the tidal range is at its largest which also means the tidal flow will be at its strongest..
Occur approx 7 days after springs and around the first and last quarter moon phases.
Neap tides are the lowest high tides and highest low tides, where the tidal range is at it's smallest.
How to find out if it's a Spring or Neap Tide from a Tide Table?
Many (though not all) tide tables will have small symbols showing the moon phases (see adjacent image).
If your tide table does not show the Moon Phases you can still figure out whether it is Spring or Neap tides by looking at and comparing the tidal ranges across a 4 week period on a tide table.
Tidal Range on 16th May = 1.3m (Neap Tide)
Tidal Range on 6th June = 3.7m (Spring Tide)
UT = Universal Time
GMT = Greenwich Mean Time
BST = British Summer Time (i.e. + 1 hour)
Effect of Barometric Pressure on Tidal Heights:
Extremely high pressure will make the tide heights less
Extreme low pressure will make the high tides more
Rule of Twelfths:
A way of estimating the expected water level 'a rule of thumb'
If the Tidal Range was 6m and if LW was at 12:00
- 1st Hour (12:00 - 13:00): Rise of 0.5m
- 2nd Hour (13:00 - 14:00): Rise of 1m
- 3rd Hour (14:00 - 15:00): Rise of 1.5m
- 4th Hour (15:00 - 16:00): Rise of 1.5m
- 5th Hour (16:00 - 17:00): Rise of 1m
- 6th Hour (17:00 - 18:00): Rise of 0.5m