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

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

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

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

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

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

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

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

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

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

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...

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...

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

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

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

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

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

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

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

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