How to Sail - Understanding Tidal Flows

Tidal flows are an important part of navigation at sea and have to be considered when plotting your course to steer.

The gravitational forces created by the sun and moon on the rotation of the earth, affect the rise and fall of ocean water, this is called tide and happens twice each day.
Such rise and fall of water levels causes changes in the depth of the water level and produces oscillating currents. These are known as tidal streams. Wind can further influence water levels and intensity of tidal stream.

Learning to sail requires in-depth knowledge of seawater levels at all times. You should therefore remain aware of high and low water affects your yachts course to ensure safe navigation.

The information you require can be found in yachting nautical almanacs issued annually, shown on charts, and booklets issued by local yacht clubs for its members. The tables show the tidal flows direction, speed and depth of the tidal stream, at a given date and time of the year of every standard port. This is then plotted on your chart from your passage plan start datum, this will create what is called a tidal vector and closing that vector off with a third line will give you to a given time to travel the degrees of correction, plus or minus, you will have to make to your course to steer.
Other considerations to your course to steer are;

Lee way. The affect the wind will have on your sailing yacht, which will vary to the wind speed and direction.

Deviation. The influence the yachts infrastructure has on the yachts compass. This is obtained by spinning the yacht through 360 degrees and checking the differences on the yachts compass reading to an actual calibrated compass. This process is normally undertaken by a qualified marine surveyor.

Wave action. Similar to Lee way the wave can affect the yachts course to steer and should be considered.

Charts
When using your chart to obtain tidal flow information the table can normally be found in a convenient clear area of the chart. It will refer to an alphabetical list given the information in respect to the letters of the alphabet. On the chart its self you will find what are called “tidal diamonds” these are diamond shaped marks on the chart, with their specific letter of the alphabet shown inside the diamond shape. The location on the chart of each tidal diamond gives you the information required from the table for the specific area of ocean. When plotting your initial course you can then see which diamonds information can be used along the path of your course for the correction to your course.

You must check the date on your chart being used and if necessary apply the correction for the year the chart is being used.

Tides
The tidal differences caused through the cycle of the sun and moons influence are called “Spring and Neap” tides. Spring tides are the highest and Neap the lowest in the tide cycle given in feet or metres. It is a gentle progression from Neap to Spring tides where you will see the gradual increase in tidal flow as you get closer to the Spring tide. The highest and fastest tides are normally two days after a full moon.

For more information and tips on learning how to sail, please go to;
http://www.learntosailnow.com

Happy Sailing