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These movements of water are known in North America as currents and in Britain as tidal streams. Currents to the English are the non-periodic movements of water caused by the major wind systems of the world including the Gulf Stream, and the North Atlantic Drift.
In tidal waters, knowing the height of the tide and the direction of the tidal stream or current is important for safe navigation. Some areas have enormous tidal ranges of 11 metres or more, whereas others have very small ones. Areas with large [ tidal ranges ] may seem daunting, but they are not difficult to cope with once you know the relatively simple procedures for calculating tidal heights and stream or currents.
Tides are the vertical rise and fall of the surface of the sea, caused by the [ gravitational attraction ] of the moon and, to a lesser degree, the sun. Rotation of the earth causes a semi-diurnal tide in most parts of the world. In other words, there is a tide with two high waters and two low waters every 24 hours, 50 minutes.
Other parts of the world experience a different effect, due to the moon’s path and other geographical influences, and have only one high and low water each day. Called a diurnal tide, it occurs mostly in the tropics. Diurnal tides usually have small tidal ranges. Other places have a combination of diurnal and semi-diurnal tides, two high and low waters every day of various heights and are called mixed tides.
In some coastal areas, the shape and alignment of the coastline causes large tidal ranges creating powerful tidal streams or tidal currents. During [ neap tides,] the sun and moon are at 90° to each other relative to the earth i.e. the first and last quarters of the moon and hence their individual gravitational pull works against each other, producing the lowest high tides and the least amount of movement of water.
Tide tables for local areas are issued by harbour authorities and document tidal time and height information for each day of the year plus the information found in nautical almanacs. It is impractical for the almanac to include full details for all the ports within its area, so a system of "standard" and "secondary" ports is used.
The nautical almanac provides a complete tide table for each standard port, showing the times and heights of high and low water for every day of the year. A diagrammatic tidal curve allows calculation of tide height for any time between high and low water.
Tidal information for smaller, secondary ports is provided as corrections, known as tidal differences. These are applied to the time and height data from the standard port to find time and height at the secondary port. If the tide being calculated falls between springs and neaps, interpolate between the figures given to achieve the best accuracy.
Electronic calculators and software to computate tidal times and heights at thousands of ports around the world are available. This allows the navigator to find the time and height of tide at any time and place without referring to tables and doing calculations and are accurate enough for most purposes.
Tide tables give the times of high and low water in the port's zone time or standard time, but some local tables show times as clock time corrected for daylight saving time. Be aware of which system your chosen nautical almanac uses, and convert to clock time if necessary. When sailing between two countries be aware they may be in different time zones and amounts of daylight-saving time.
Tide tables show their base time zone on each page and show the correction that must be applied to convert to local clock time. When calculating secondary port information from standard port data, calculate in zone time and convert the answer to local time to avoid errors.
On charts, soundings or depths and drying heights are measured from [ chart datum ]. The chart datum is the level from which measurements are made to determine the heights of tides and is normally the level of low water at the time of the lowest astronomical tide (LAT). Everything above this line is shown on coastal charts as drying heights and below this line is shown as charted depths in metres. Nautical almanacs provide heights of an average spring tide's high and low waters called the Mean High Water Springs (MHWS) and Mean Low Water Springs (MLWS). The equivalent heights for an average neap or small tide (MHWN and MLWN) are given and terrestrial objects heights are gauged from the [ MHWS level ] .
Tide tables display the height of the tide simply at high and low water but the need to know its height at other times requires the use of the nautical almanac's tidal curve. An alternate method is to calculate the height using the Rule of Twelfths formula.
To explain the rule of twelfths assumes that the tide rises and falls proportionally and that the duration between rise and fall is six hours and is suitable for areas where high accuracy is not required. The rule of twelfths formula is based on the fact that the change in water depth is not constant, but increases its pace until reaching maximum ebb or flow, then decreasing until slack water.
The rule of twelfths divides the tidal range (from high water to low water and vice versa) into twelve and assumes that the tide rises or falls as follows:
To find the [ height of the tide ] at two hours after high water when the tide table gives HW at certain hour, calculate the range of the tide 6 ft and then divide it into twelfths which equals .5 ft. Multiply .5 by 3 (twelfths) = 1.5ft. Subtract 1.5ft from 6ft to arrive at a height of 4.5ft above low water.
Finding out the depth of water at a time other than high water and low water when crossing a shallow water, there is a requirement to know exactly when the tide’s height is enough to allow the boat to pass over.
The [ tidal curve ] is represented as a graph which plots the movement of a tide over a complete tidal cycle. With the cycle varying between spring and neap tides then two curves are shown. Spring tides are represented by a solid line, and neap tides by a dotted one. Use the appropriate tidal curve to estimate what time the tide will reach a specific height or the tide's height at a specific time.
The nautical almanac offers tidal curves for each standard port and these can be used to obtain the time or height of tide at the secondary ports. This method is the way to do secondary port tide calculations. Simply enter the tidal data given for the secondary port by calculating the time of high water then enter these figures into the tidal curve, and proceed as for a standard port.
The rising and falling of the tide causes a horizontal flow of water known as tidal streams or tidal currents. Tidal streams or tidal currents are strong during spring tides and weak during neap tides. The tidal stream or current’s direction at any time is called its set, and the strength is called its drift.
When shaping a course or plotting a position, you need to know both the tidal set and tidal drift for the area. Tidal stream or current set and drift information is obtained from the chart, an almanac, or a tidal atlas that covers the area of sailing.
Positions where tidal streams or tidal currents have been measured are [ marked ] by a letter on the chart within a diamond shape, and are referred to as tidal diamonds or tide symbols. For every tidal diamond, a [ table ] on the chart shows the tide set and drift for spring and neap tides for each hour before and after high water at the standard port.
You need to know the time of high water at the standard port for the day and the tidal diamond nearest your position. High tide is represented as zero on the table. Select the required hour either before or after high water and from the table select the column headed by your tide symbol letter.
The strength of the drift in knots and the set of the stream or current, in degrees true are tabulated for the spring or neap rate, or if required, interpolate between the two. If between two tidal diamonds, plot the results for both on the chart and interpolate between the two.
The [ tidal atlas ] is the document which provides information about times of tides as well as direction of water flow. A tidal atlas gives tidal stream or current information for specific areas as a separate page for each hour before and after high water at a standard port. Every page shows the set of the stream or current by arrows and the drift at spring and neap tides in figures. Drift may be shown in knots and tenths of a knot, or it may simply be shown in tenths
The tidal atlas has the high water page marked up with the time of high water for the day at the standard port, with each page before and after marked up with the appropriate time. Choose the appropriate page to find the tidal arrow nearest your position.
Direction is measured with a plotter and be sure to note the spring and neap rate shown by the arrow. If the day's tidal range lies between the spring and neap range, interpolate between the two or as an alternative use a computation of tidal rates table provided in the tidal atlas.
To find the rate of drift during periods between spring and neap tides, use the tide height calculation formula:
Tide range for the day = High Water - L.W heights divided spring range of tide multiplied by spring rate of drift.