Tides & tidal

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1 - Information from the chart

Position of tabulated tidal levels data with designation 'a' - chart symbol Most often the chart presents succinct tide tables for certain positions. These positions are marked with the “square”. The table below shows us an example for two different positions. The first refers to Cowes (UK), the second to a position south of Cowes.
 Position Heights above LAT
Mean HW Mean LW
Spring Neap Spring Neap
 Cowes 1,7 m 1,5 m 0,2 m 0,4 m
Position of tabulated tidal levels data with designation 'a' - chart symbol 5,2 m 4,3 m 0,4 m 1,2 m
This data only provides us with average high and low waters heights. Moreover, it is merely valid at spring or neap tides. To use it we need to first find out how many hours we are from high water. Secondly, we need to know if it is spring or neap or sometime in between at that particular moment. We shall use this table to solve two types of problems. Finding height of tide at a particular location at a particular time:

Almanacs and many other nautical publications contain predictions of the times of high and low tides at many major standard ports. Also listed are differences in times of tides from these ports for additional secondary ports. To work with this succinct data we need two extra tools:

Shoal problem:
Our shoal near Cowes has a charted depth of 1 meter and we would like to cross it at about 15:00 hours with our yacht (draft 1,5 m).

From any nautical almanac we find that HW occurs at 03:18 15:53 and LW occurs at 09:45 22:03 at a standard port nearby. We also find that at our location HW occurs one hour later and that spring tide is due in two days. Hence, we have a HW around 17:00.

So, after three interpolations we derive the water height at 1500 hours. Considering the charted depth leads to an observed depth of 4,9 meters, enough for our draft of 1,5 meters.

Bridge problem:
An overhanging rock, power lines or bridges have their clearances charted with respect to another chart datum than LAT. Normally, 'high water' or 'MHW spring' are used as reference planes.

An example:
Above our shoal hangs the 'Cowes bridge'. At 15:00 hours we would like to pass this bridge, which has a charted height of 20 meters to HW. Our mast is 23 meters high. In the example above we found that the water height was 1,1 meters below HW level at that time. Obviously, we will have to wait!
So, at what time will we be able to pass under this bridge?
The water height must be 3 meters lower than HW level (5,0 m). That is almost 9/12 of the range (4,3 m) indicating four hours after HW. Conclusion, we will have to wait at least six hours in total.

2 - Information from tide tables

Instead of mere averages, a tide tableDetailed Tide Table. provides us each day with the times of high and low water for a particular place. Basically, it is same table like the one we found in the chart, but is extended for every day in a year. By using this method we get more accurate water heights since it involves less interpolation. The example shows us a part of a very detailed tide table, which even includes heights for every hour.

3 - Information from tidal curves

   © sailingissues.com   
In most tables the tides can also be characterized by a tidal curve. This method substitutes the rule of twelve providing more accurate heights. The left side contains the water height information with the lowest heights to the left where also the chart datum is indicated. The low water height will be marked at the bottom and the high water height will be marked at the top.
Tidal Curve.
The area under the curve will be marked with the time information.
To find the water height at a specific time we need to know first how many hours before or after the HW this is. Then

Tidal Curve:
Finding Heights.

Tidal Curve:
Finding Time with Height.
Often this is done when the curve is not sinusoid and the rule of twelve is rendered useless.



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21 June 2016
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