1. POLKA sailing TIDAL THEORY YACHT RACING AND RYA TRAINING CENTRE
Where to find them?
Local Notice to Mariner
Admiratly webite
(local associattion)
© Polka Sailing 2017

2. Tidal Theory Gravitational pull
The moon orbits the Earth over a 28 day cycle creating Spring and Neap tides
The gravitational pull from the moon and sun is the main cause of tides
Ferrous metal and magnetic fields will affect the compass, for instance...
Neaps
Springs
Springs
Neaps

3. Tidal Theory Spring tides
...we experience higher high waters and lower low waters
When the gravitational pull from the moon and sun are in line...
Ferrous metal and magnetic fields will affect the compass, for instance...
Greater range of tide

4. Tidal Theory Neap tides
...we experience lower high waters and higher low waters
When the gravitational pull from the moon and the sun are at right angles to each other...
Ferrous metal and magnetic fields will affect the compass, for instance...
Smaller range of tide

5. Tidal Theory World tidal patterns
Ranges can differ enormously within a country
The second biggest is in the UK
Ferrous metal and magnetic fields will affect the compass, for instance...
Tidal patterns around the world are very complex
The biggest tidal range in the world is in Canada
Bay of Fundy – Nova Scotia, Canada – Range 16.3m
Bay of Fundy (mean range 13m)
Bristol Channel (mean range 12m)
Collier Bay NW coast of Australia 10m
SE coast 1 – 3m

6. Tidal Theory Effects of weather
Strong winds for prolonged periods can affect the tide
Hold the water back making the tidal height greater than predicted
Push the water out making the tidal height less than predicted

7. Tidal Theory Effects of weather
Barometric pressure can affect the tide
High pressure can make the tidal height less than predicted
Low pressure can make the tidal height greater than predicted

8. Tidal Theory Tidal terms Mean High Water Springs Mean Low Water Neaps
Mean Low Water Springs
Highest astronomical tide
Height of light
Clearances
Mean High Water Neaps
Spring range
Neap range
Tidal terms
Charted depth
Chart datum
Height of tide
Depth of water
Height of light (above MHWS)
Clearance of bridge (above HAT)
HAT
MHWS
MHWN
Neap range
Spring range
Height of tide
Depth of water
MLWN
MLWS
Charted depth (below CD)
Chart datum

9. Standard Ports VICTORIA Tidal heights
Tide tables are produced for large (standard) ports such as Port Victoria
Ferrous metal and magnetic fields will affect the compass, for instance...
VICTORIA
VICTORIA

10. Standard Ports Tidal heights ...for example Thursday 3rd January
The tide tables contain daily information about the times and heights of high and low water...
And at 1920 the prediction is 0.8 of a metre
At 0643 it’s predicted to be 1.0 metre
At 0043 the height of tide is predicted to be 5.5 metres
At 1301 the prediction is 5.7 metres

11. Standard Ports Tidal heights
...it’s possible to find the height of tide for a time of our choosing by using the Tidal Curve
By selecting the appropriate time of HW and the heights of HW and LW for Victoria...

12. Standard Ports Tidal heights
...it’s possible to find the height of tide for a time of our choosing, by using the Tidal Curve
By selecting the appropriate time of HW and the heights of HW and LW for Victoria...

13. Standard Ports Tidal heights
What will be the height of tide at Victoria on Thursday 3rd January at 1540 UT?

14. Standard Ports Tidal heights
Enter HW time and fill in the boxes for each hour after HW
1301
1401
1501
1601
1701
1801
1901

15. Standard Ports Tidal heights ...and draw a line between them
Mark in the heights of HW and LW...
...and draw a line between them
1301
1401
1501
1601
1701
1801
1901

16. Standard Ports Tidal heights
...then draw across to meet the HW/LW line...
...continue the line up to the HW scale
In this example the height of tide will be approximately 3.7m at 1540 UT
Draw a line upwards until it meets the curve...
Find 1540 UT on the bottom scale
(1540 UT approx)
1301
1401
1501
1601
1701
1801
1901

17. Standard Ports Tidal heights
It’s also possible to find out when a specific height of tide will occur...
...when will the height of tide at Victoria fall to 2.5 metres?
1301
1401
1501
1601
1701
1801
1901

18. Standard Ports Tidal heights ...then draw across to meet the curve...
...continue down to the bottom scale
In this example the height of tide will fall to 2.5m at approximately 1640 UT
Draw a line down from the HW scale until it meets the HW/LW line...
1301
1401
1501
1601
1701
1801
1901

19. Standard Ports Tidal heights
A decision had to be made regarding which part of the curve to use...
...the unbroken red line representing Springs, or the broken blue line for Neaps
1301
1401
1501
1601
1701
1801
1901

20. Standard Ports Tidal heights
The Spring line was used on this occasion, because the range for this time period...
...5.7m – 0.8m = 4.9m range...
...is the mean Spring range
1301
1401
1501
1601
1701
1801
1901