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Understanding Stages of Tide – Learning to Read Tidal Flows



Tides and Sailing – How to Read Tidal Flows and Stages of a Tide

New York (USA), October 28, 2012

Learning to Read Tidal Flows

Tide is the daily rise and fall of ocean waters occurring at periodic intervals due to gravitational pull of sun and moon on the rotating earth. Tides occur twice each day. Tide could be high tide or low tide. Tides cause changes in depth of marine and estuarine water. This produces tidal streams. Such streams play an important role in coastal navigation.

Stages of a Tide

Seawater rises and falls in repetitive cycles. A cycle occurs as:

1. Water rises or flows into beach area for many hours.

2. Once water reaches the maximum height, it does not increase any further. This is high water. Tidal currents stop at this point. It is also known as slack tide or slack water as water level does not increase beyond this level.

3. Next, tide changes direction and this is referred as ‘turning’.

4. Now seawater level continues to fall over many hours. This is referred as ‘ebb tide.’

5. Finally, water stops falling at the lowest level. This point is also referred as slack as water level does not fall further.

6. Next, water starts rising again and this is turning.

This is one tidal cycle. Such cycles could be semidiurnal or diurnal. Semidiurnal is two high waters and two low waters each day. Diurnal is a single tidal cycle in a day. However, in most places tides are semidiurnal. Tidal heights are different in diurnal and semidiurnal cycles. This inequality in tidal heights is referred to as higher high water and lower high water or higher low water and lower low water in tide tables.

Relation between Tides and Sailing

1. Tidal streams or currents affect movement and direction of sailing boats. Sometimes they change course of a sailing boat or yacht. A tidal stream is the horizontal flow of water. While sailing, remain aware of strength and direction of tidal current at any particular time. If direction of wind and tide are same, sea waters would remain calm. If wind direction and tidal flow are in opposite directions, waves develop and seawater could be choppy.

2. Tidal range is distance between highest sea level during high tide and lowest sea level during low tide. This is normally an annual average value to indicate tidal intensity in that specific area. You cannot always depend on tidal range when you run aground. If the tide is rising when you run aground, you can benefit when water starts rising. However, if tide is falling right when you run aground, it poses difficulties and you should take proper measures to free your yacht.

3. Gather information about tidal height. This is important for safe navigation and anchoring. Tide atlas, Tidal calculator, nautical almanac, and similar others provide all essential information about tidal movements and heights. Tide atlas shows direction of tidal stream and rate of flow. A Tidal calculator calculates height and time of tide for any specific location at any given time. Nautical almanac provides day after day information about tides for each standard port for a year. It also gives information about tidal heights and timings. Collect all basic information about tides along the path you intend sailing before setting sail. Further, different areas have different tidal ranges. Never underestimate any tidal range and always remain prepared for every type of tidal range.

Guide to Sailing and Ocean Cruising in a Medium Sized Yacht
The Complete Reference Guide to Sailing and Ocean Cruising in a Medium Sized Yacht


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