The moon rotates around the earth every 28 days, and also rotates on its axis every 24 hours. Due to the effects of the gravitational pull of both the sun and the moon onto the earth the water level in the seas and oceans is pulled – which creates tides. Larger tides (called
Springs) occur when the moon and the sun are aligned, and smaller tides (called
Neaps) in between.
http://commons.wikimedia.org/w/index.php?title=File:How_tides_are_created.jpgBecause the moon is closer to the earth it has a stronger gravitational pull and therefore a greater effect than the sun.
As the earth rotates on its axis the water on the earths surface is pulled towards the moon which creates
High Tides. The following trough is called
Low Tide.
The earth is also rotating around the sun in an elliptical orbit, and at certain times of the year is closer than others. This causes a larger range (the difference between high and low water) in March and September which is known as the
Spring and
Autumn Equinox. The associated Low Water is known as the
Lowest Astronomical Tide (LAT).
The last important term is known as the
Range of Tide which is the difference between High and Low Water.
Tides are predictable and the height and time change according to where you are in the world. For major ports (known as
Primary Ports such as our own at
Granadilla) these are published as
Tide Tables, for other ports (known as
Secondary Ports) only the height and time difference between high and low water is published for Springs and Neaps.
http://commons.wikimedia.org/w/index.php?title=File:Definitions_of_Spring_and_Neap_tides.jpgIf you need to know the height of the tide at a particular time (i.e. can you get over a sand bar) then you use a
Tidal Curve and the information from the Primary Port.
With a yacht drawing 2.4m what is the latest time on the morning of the 6th July we could pass over a sand bar 1.7m below chart datum?
Looking at 6th July the range is 0.65m which is closest to Neaps on the Mean Ranges on the Tidal Curve.
The depth required for the yacht is 2.4m + 0.5m clearance = 2.9m
The minimum depth required in the channel is 2.9m – 1.7m = 1.2m
The latest time we can leave is
09:51.
http://commons.wikimedia.org/wiki/File:Use_of_Tide_Tables.jpgSmaller harbours do not have published tide tables, just difference information to the nearest standard port published in Tidal Almanacs. They are known as
Secondary Ports i.e. Las Galletas.
http://commons.wikimedia.org/wiki/File:Extract_from_Las_Galletas_Almanac_Entry.jpgIt must be remembered that the weather does have an affect on tidal heights. The tide tables are written for standard pressures of 1013. High pressure can alter this by as much as 0.1m per 11mB.
