Back to Nautical Definition Charts Rotation of the Earth Distances Position Lines Sailings Chart Work Exercises Information from Charts Tides Sextant

Tides

Tides

Tides are the periodic motion of the waters of the sea due to changes in the attractive forces of the moon and sun upon the rotating earth.

Tides can either help or hinder a mariner. A high tide may provide enough depth to clear a bar, while a low tide may prevent entering or leaving a harbor.

Tidal current may help progress or hinder it, may set the ship toward dangers or away from them.

By understanding tides, and by making intelligent use of predictions published in tide and tidal current tables and of descriptions in sailing directions, the navigator can plan an expeditious and safe passage.

Tide And Current

The rise and fall of tide is accompanied by horizontal movement of the water called tidal current. It is necessary to distinguish clearly between tide and tidal current, for the relation between them is complex and variable.

For the sake of clarity mariners have adopted the following definitions: Tide is the vertical rise and fall of the water, and tidal current is the horizontal flow. The tide rises and falls, the tidal current floods and ebbs. The navigator is concerned with the amount and time of the tide, as it affects access to shallow ports. The navigator is concerned with the time, speed, and direction of the tidal current, as it will affect his ship’s position, speed, and course.

Causes Of Tides

The principal tidal forces are generated by the moon and sun. The moon is the main tide-generating body. Due to its greater distance, the sun’s effect is only 46 percent of the moon’s. Observed tides will differ considerably from the tides predicted by equilibrium theory since size, depth, and configuration of the basin or waterway, friction, land masses, inertia of water masses, Coriolis acceleration, and other factors are neglected in this theory. Nevertheless, equilibrium theory is sufficient to describe the magnitude and distribution of the main tide-generating forces across the surface of the earth.

General Features

At most places the tidal change occurs twice daily. The tide rises until it reaches a maximum height, called high tide or high water, and then falls to a minimum level called low tide or low water.

The rate of rise and fall is not uniform. From low water, the tide begins to rise slowly at first, but at an increasing rate until it is about halfway to high water. The rate of rise then decreases until high water is reached, and the rise ceases.

The falling tide behaves in a similar manner. The period at high or low water during which there is no apparent change of level is called slack water. The difference in height between consecutive high and low waters is the range.

All the oceans are made up of several separate oscillating basins. As such basins are acted upon by the tide-producing forces, some respond more readily to daily or diurnal forces, others to semidiurnal forces, and others almost equally to both.

Hence, tides are classified as one of three types, semidiurnal, diurnal, or mixed, according to the characteristics of the tidal pattern.

In the semidiurnal tide, there are two high and two low waters each tidal day, with relatively small differences in the respective highs and lows. Tides on the Atlantic coast of the United States are of the semidiurnal type.

In the diurnal tide, only a single high and single low water occur each tidal day. Tides of the diurnal type occur along the northern shore of the Gulf of Mexico, in the Java Sea, the Gulf of Tonkin, and in a few other localities.

In the mixed tide, the diurnal and semidiurnal oscillations are both important factors and the tide is characterized by a large inequality in the high water heights, low water heights, or in both. There are usually two high and two low waters each day, but occasionally the tide may become diurnal.

Such tides are prevalent along the Pacific coast of the United States and in many other parts of the world.

Special Tidal Effects

As a wave enters shallow water, its speed is decreased. Since the trough is shallower than the crest, it is retarded more, resulting in a steepening of the wave front. In a few estuaries, the advance of the low water trough is so much retarded that the crest of the rising tide overtakes the low, and advances upstream as a breaking wave called a bore.

Bores that are large and dangerous at times of large tidal ranges may be mere ripples at those times of the month when the range is small. Examples occur in Calcutta, India and Haining, China, and in the Tsientang Kaing.

Other special features are the double low water (as at Hoek Van Holland) and the double high water (as at Southampton, England). At such places there is often a slight fall or rise in the middle of the high or low water period.

The practical effect is to create a longer period of slack water at high or low tide.

Tides and Tidal Streams

The sun and moon exert gravitational forces on all parts of the earth.  One of the results of these attractive forces is the production of Tide Generating Forces.

As the differential attractive force due to the moon is approximately twice that of the sun it should be apparent that tides are more influenced by the moon.  For this reason the interval between two successive high waters is approximately half a Lunar Day (12hours and 25 m).

Spring Tides

The configuration, when the sun and moon are in line occurs at New Moon when the two bodies are in Conjunction.  A High Water (HW) is produced at that point on the earth’s surface, which is nearest to the sun and moon and also at the opposite point.  Two Low Waters (LW) lie approximately mid-way between the two high waters.

When the sun and moon are in line, but on opposite sides of the earth, they are said to be in Opposition.  This occurs at Full Moon and produces a similar situation to that which occurs at New Moon when the sun and moon are in Conjunction.

When the sun and moon are in line either at New Moon or Full Moon, the maximum tide raising forces occur.  These maximum forces produce the Highest High Waters and the Lowest Low Waters, which are referred to as Spring Tides.

Neap Tides

When the sun and moon are at right angles to each other they are said to be in Quadrature.  In this configuration the smaller attractive forces of the sun oppose the larger attractive forces of the Moon.  This results in Lower High Waters and Higher Low Waters, which are referred to as Neap Tides.

Range of Tide and Duration of Tide

The Range of Tide is the height difference between a HW height and the preceding or following LW height at a particular place.  The largest range occurs at Spring Tides and the smallest range at Neap Tides.

Duration of Tide is the time interval (time difference) between a HW time and the preceding or following LW time at a particular place.

DEPTH AND SOUNDINGS

It is frequently necessary to know the precise depth of water beneath the keel or over some charted danger.

Depth of water can be measured by hand using the Hand Lead Line, or by use of the Patent Sounding Machine, or by some type of Echo Sounding Device.

In the case of the hand lead line the depth so measured is from the surface water level whereas with an echo sounding device depth is measured from a transducer situated in the bottom of the vessel.

In both cases it is necessary to apply a correction to the measured depth in order that comparison can be made with the depth shown on the chart.

Charted depth

The depth shown on a chart is measured from a level known as Chart Datum (CD).  On a modern chart, Chart Datum is normally placed at the level of the Lowest Astronomical Tide (LAT), which is the lowest level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.  It follows that the depth shown on a modern chart is the least depth, which can be expected under normal conditions.

Datum

A tidal datum is a level from which tides are measured.

There are a number of such levels of reference that are important to the mariner.

The most important level of reference to the mariner is the sounding datum shown on charts. Since the tide rises and falls continually while soundings are being taken during a hydrographic survey, the tide is recorded during the survey so that soundings taken at all stages of the tide can be reduced to a common sounding datum. Soundings on charts show depths below a selected low water datum (occasionally mean sea level), and tide predictions in tide tables show heights above and below the same level. The depth of water available at any time is obtained by adding algebraically the height of the tide at the time in question to the charted depth.

By international agreement, the level used as chart datum should be low enough so that low waters do not fall very far below it. At most places, the level used is one determined from a mean of a number of low waters (usually over a 19 year period); therefore, some low waters can be expected to fall below it. The following are some of the datum in general use.

Mean low water (MLW) is the average height of all low waters at a given place. About half of the low waters fall below it, and half above.

Mean low water springs (MLWS), usually shortened to low water springs, is the average level of the low waters that occur at the times of spring tides.

Mean lower low water (MLLW) is the average height of the lower low waters of each tidal day.

Tropic lower low water (TcLLW) is the average height of the lower low waters (or of the single daily low waters if the tide becomes diurnal) that occur when the moon is near maximum declination and the diurnal effect is most pronounced. This datum is not in common use as a tidal reference.

Indian spring low water (ISLW), sometimes called Indian tide plane or harmonic tide plane, is a low water datum that includes the spring effect of the semi-diurnal portion of the tide and the tropic effect of the diurnal portion.

It is about the level of lower low water of mixed tides at the time that the moon’s maximum declination coincides with the time of new or full moon.

Mean lower low water springs (MLLWS) is the average level of the lower of the two low waters on the days of spring tides.

Some still lower datum used on charts are determined from tide observations and some are determined arbitrarily and later referred to the tide. Most of them fall close to one or the other of the following two datum.

Lowest normal low water is a datum that approximates the average height of monthly lowest low waters, discarding any tides disturbed by storms. Lowest low water is an extremely low datum. It conforms generally to the lowest tide observed, or even somewhat lower.

Once a tidal datum is established, it is sometimes retained for an indefinite period, even though it might differ slightly from later observations.

In some areas where there is little or no tide, such as the Baltic Sea, mean sea level (MSL) is used as chart datum. This is the average height of the surface of the sea for all High Water Datum

Heights of terrestrial features are usually referred on nautical charts to a high water datum. This gives the mariner a margin of error when passing under bridges, overhead cables, and other obstructions. The one used on charts of the United States, its territories and possessions, and widely used elsewhere, is mean high water (MHW), which is the average height of all high waters over a 19 year period. Any other high water datum in use on charts is likely to be higher than this. Other high water datum are mean high water springs (MHWS), which is the average level of the high waters that occur at the time of spring tides; mean higher high water (MHHW), which is the average height of the higher high waters of each tidal day; and tropic higher high water (TcHHW), which is the average height of the higher high waters (or the single daily high waters if the tide becomes diurnal) that occur when the moon is near maximum declination and the diurnal effect is most pronounced. A reference merely to “high water” leaves some doubt as to the specific level referred to, for the height of high water varies from day to day. Where the range is large, the variation during a 2 week period may be considerable.

Because there are periodic and apparent secular trends in sea level, a specific 19 year cycle (the National Tidal Datum Epoch) is issued for all United States datum. The National Tidal Datum Epoch officially adopted by the National Ocean Service is presently 1960 through 1978. The

Epoch is periodically reviewed for revision.

LW and HW Heights

LW and HW heights are always measured from Chart Datum.  Unless preceded by a minus sign, LW heights are always additive to the charted depth.  LW heights preceded by a minus sign indicate that LW falls below datum and that the charted depth is reduced at LW.

Example

(a) Charted Depth                   10.0 m             (b) Charted Depth        10.0 m

LW from ATT                          1.0 m               LW from ATT             -1.0 m

Predicted Depth at LW             11.0 m                                                 9.0 m

Rise of Tide

Rise of Tide is the vertical distance measured from LW to the actual water level.

Height of Tide

Height of Tide is the vertical distance measured from Chart Datum to the actual water level.

Height of Tide = LW Height + Rise of Tide.

Charted Heights

The charted height of lighthouses, hills and other high objects is given above the level of Mean HW Springs (MHWS).

In the case of a lighthouse the charted height is measured from MHWS to the centre of the focal plane of the light.

Example

A lighthouse near Portsmouth has a charted height of 19 m. find the actual height of the lighthouse (p.m.) on July 29th.  Rise of Tide for the required time was calculated as 2 m.

Charted Height of Lighthouse (above MHWS)                          19 m

MHWS (from ATT Table V) Level above CD              4.7 m

LW (from ATT)           1.7

Rise of Tide                  2.0

Height of Tide                                                               (-) 3.7

Water Level BELOW MHWS                                                 1

Actual Height of Lighthouse above water level                           20 metres

TIDAL LEVELS

Tidal levels for Standard Ports are listed in Table V of the Admiralty Tide Tables.

STANDARD PORTS AND SECONDARY PORTS

There are two types of ports, Standard Ports and Secondary Ports.  Secondary Ports are based on Standard Ports which have a similar tidal curve.

The Admiralty Tide Tables include the Tidal Curve and full details of day predictions for Standard Ports.  They also list the Time and Height Differences of Secondary Ports from the chosen Standard Port.  To give full daily predictions for each Secondary Port would clearly require a very large volume indeed and the above method makes for a more convenient size.

Admiralty Tide Table (ATT) are published in three volumes:

Vol. 1. European Waters (including Mediterranean Sea).

Vol. 2. Atlantic and Indian Oceans (including tidal stream predictions)

Vol. 3. Pacific Ocean and Adjacent Seas (including tidal stream predictions)

Each of these volumes is divided into two parts:

Part 1. Standard Ports.

Part II.  Secondary Ports.

Standard Ports are listed in the Index to Standard Ports printed on the inside Front Cover of each volume.

Secondary Ports are listed in the Geographical Index at the rear of each volume and which also includes the Standard Ports.

Each port is assigned an Index Number.

TYPES OF TIDAL PROBLEM

Tidal problems fall into 4 main types in which it is necessary to find:

1.         Time of HW and LW on a particular day.

2.         Height of Tide at an Intermediate Time between HW and LW.

OR

Reduction to Soundings (Correction to Leadline) at an Intermediate Time

3.         Time at which m Required Height of Tide (or Depth of Water) is reached.

4.         Correction to Apply to the Charted Height of a Lighthouse or other shore object.

Tide Calculations

Accuracy and Interpolation

It must be borne in mind that meteorological conditions which differ from average may cause differences between the predicted and actual tide.  Despite these limitations, calculations should be worked as accurately as possible within the limits of the tables and, particularly in examinations, all interpolation shown.

Time Zones, Zone Time, Standard Time and Time Differences

The Time Zone for the port in question is clearly shown in the top, left hand comer of prediction pages in Admiralty Tide Tables.  Daily predictions are given in the normal Standard Time of the port.  Before attempting calculations, time zones, zonetime and standard time should be understood.

The world is divided into 24 Time Zones.  Each Time Zone is 15’ of longitude in width.  The ‘Zero Time Zone’ extends from 7.5˚W to 7.5˚E this zone keeps GMT.  In each of the remaining 23 zones the time differs from GMT by a whole number of hours and is numbered, in sequence, 1-12: East of Greenwich with a Negative (-) prefix, West of Greenwich with a positive (+) prefix.

e.g.       60˚E lies in the 52.5˚E-67.5˚E Zone (Zone -4)

10˚W lies in the 7.5˚W-22.5˚W Zone (Zone +1)

To obtain the Zone Time for a position or place, subtract the Zone number algebraically from GMT.

Example-Given GMT 1200, what is the correct Zone Time to keep in Longitude 60˚E (Zone - 4)?

GMT -(Zone) = Zone Time 1200 - (- 4) = 1600         i.e. 4 hours ahead of GMT

or,

Zone Time+ (Zone) = GMT

1600    +         (-4) = 1200

For convenience on land, a Standard Time is adopted throughout a given country.  In most cases Standard Time or Legal Time is that of the Zone in which the country mainly lies.  Countries like USA and Australia, which extend over several Time Zones, generally adopt several Standard Times.

To prolong daylight hours, many countries also adopt (for part of the year) a form of Daylight Saving Time or Summer Time.  For such periods, the time of the Eastward zone is usually adopted, e.g. BST (British Summer Time), which is kept in the UK from a date in March to a date in October, is the time for Zone - 1, i.e. GMT, -(-)1 = GMT + 1 hour., Time Differences for Secondary Ports, when applied to the printed times of HW and LW at Standard Ports will give the times of HW and LW at the Secondary Port in the Zone Time tabulated in the tables for the Secondary Port.

Finding the Times of HW and LW

Examples:

Find the Standard Times of HW and LW at SHEERNESS on January 7th.

1.         Turn to “Index of Standard Ports” in ATT Volume I.  Find that Sheerness is a Standard Port for which daily predictions are given.

2.         Turn to the - daily predictions for Sheerness.

Four times are given with the height of the tide at each instant. The High Waters are the higher figures, 5.3 m and 5.5 m. The Low Waters are the lower figures, 1.2 m and 0.9 m.

LW      HW

Times:  0325    0952

1609    2237

Note that the times are Standard Times for the Time Zone indicated.  In this case the Time Zone is GMT

Find the BST (British Summer Time) of HW and LW at Greenock on August 18th and also the Duration and Range of the AM rising tide.

1.         Check that Greenock is a Standard Port.

2.         Turn to the daily predictions for Greenock.

3.         Note and apply the time difference: GMT + 1 hr = BST.

4          Subtract the AM LW times and heights from the following HW times and heights to obtain the Duration and Range of Tide.

High Waters are 3.1 m and 3.4 m and Low Waters are 0.3 m and 0.4 m.

LW                  HW                  LW                  HW

GMT                0442                1145                1704                2345 (Aug 18th)

Time Diff.         +01                  +01                 +01                  +01

BST     0542 (18th)       1245                1804                0045 (19th)

HW                  12 45   3.1m

LW                  05 42   0.3 m

Duration           7h 03m 2.8 m Range

Note the time difference has made the BST of the second High Water occur on the following day, which is August 19th.

Admiralty Tidal Prediction Form - NP 204

NP 204.  The form is included in the back pages of Admiralty Tide Tables

Note that the form contains boxes numbered 1-16.  Boxes 1-5 are completed for Standard Ports.  Boxes 1-16 are completed for Secondary Ports.

To Find the Height of Tide at an Intermediate Time between HW and LW (European Standard Port)

Checklist 1

1.         Check that the port is in the ATT “Index to Standard Ports”.  Turn to the appropriate page.  Complete heading in NP 204.  In boxes 1-4, write down the Times and Heights of HW and LW which lie either side of the required time.  Subtract LW height from HW height to obtain Predicted Range. Enter in box 5.

2.         Turn to the Tidal Diagram for the port (first page of predictions for that port).  Plot the heights of HW (top LH scale) and LW (bottom LH scale), which occur either side of the required time.  Join by a sloping fine.

3.         Write down the HW time in the HW box below the curve.  In adjacent boxes, enter other times (differing by one hour intervals from HW time) to “embrace” the required time.  It may be helpful to write down the required Time Interval from HW (i.e. HW Time - Required Time).

4.         Plot the required time on the time scale (note, 10 minute divisions).  Through this point draw a vertical line to intersect the appropriate curve- see below instructions.

5.         Note Spring and Neap Ranges shown in the box (top right).

Predicted Range equal to or greater than Spring Range - Use Spring Curve only.

Predicted Range equal to or less than Neap Range - Use Neap Curve only.

Other Ranges - Interpolate (and draw in the appropriate part of the curve as explained below.)

6.         From vertical intersection with curve, proceed horizontally to sloping line (2) then vertically to the Height Scale.

7.         Read off the Height of Tide (also known as Reduction to Soundings or Correction to Leadline).

Interpolating between Spring and Neap Curves

To interpolate between curves (i.e. where the Predicted Range lies between the Spring and Neap Ranges):

Express: {Spring Range - Predicted Range} / {Spring RangeNeap Range} as a fraction.

Using this fraction, sketch in an intermediate curve, parallel to the Spring curve this fractional distance away from the Spring curve towards the Neap curve, e.g. Fraction Ľ; draw curve Ľ of the distance away from the Spring curve towards the Neap curve.

Where there is an appreciable change in duration between Spring and Neap tides, interpolating between curves may result in a slight error. The error is greatest near LW.

Do not extrapolate.

Example

(European Standard Port)

Find the Height of Tide at Londonderry at 1300 Standard Time on August 22nd.

Use Checklist 1.

Standard Port   Londonderry   Time / Height Required 1300

Secondary Port            NA       Date 22 Aug    Time Zone GMT

 TIME HEIGHT STANDARD PORT HW LW HW LW RANGE 1012 1614 2.6 0.3 2.3 Seasonal Change Standard Port DIFFERENCES Seasonal Change Secondary Port SECONDARY PORT Duration

Interpolation (Step 5)

Predicted Range equals Spring Range - use Spring Curve.

NB At this port the Spring and Neap curves coincide throughout the tidal cycle - so interpolation “between curves” is, in any case, not possible.

Plotting (Steps 2, 3, 4, 6 and 7)

Answer           1-6 metres is the Ht of Tide above Chart Datum.

Example

(European Standard Port)

Find the Height of Tide at Avonmouth at 1500 Standard Time on November 28th.

Checklist 1. (Step 1)

Standard Port   Avonmouth     Time / Height Required 1500

Secondary Port            NA       Date 28 Nov    Time Zone GMT

 TIME HEIGHT STANDARD PORT HW LW HW LW RANGE 1658 1104 12.2 2.7 9.5 Seasonal Change Standard Port DIFFERENCES Seasonal Change Secondary Port SECONDARY PORT Duration

Interpolation (Step 5)

Spring Range    12.3                 Spring Range                12.3

Neap Range     6.5                   Predicted Range           9.5

Spring – Neap  5.8                   Spring - Predicted        2.8

Fraction – distance from Spring Curve = {Spring – Predicted} / {Spring – Neap}

= 2.8 / 5.8 = 0.48 or about ˝

Plotting (Steps 2, 3, 4, 6 and 7)

Answer:  9.8 metres is the Height of Tide at 1500 GMT

To Find the Time at which a Required Depth is reached

(European Standard Port)

Checklist 2

1.         Check that the port is in the ATT “Index to Standard Ports”.  Turn to the appropriate page.  Complete heading in NP 204.  In boxes 1-4, write down the Times and Heights of HW and LW which lie either side of the required time.  Subtract LW height from HW height to obtain Predicted Range-enter in box 5.

2.         Using a Sketch to assist, find the Height of Tide necessary for the required depth.

3.         Turn to the Tidal Diagram for the port.  On the diagram, plot heights of HW (top LH scale) and LW (bottom LH scale) which occur either side of the required time.  Join by a sloping line.

4.         Write down the HW time in the HW box below the curve.  In adjacent boxes, enter other times (differing by one hour intervals from HW time) to “embrace” the required time.  If helpful, write down the required Time Interval from HW (i.e. HW time - Required Time).

5.         From the required Height of Tide on the HW height scale, proceed vertically down to the sloping line, then horizontally to the appropriate curve (see point 6).

6.         Note Spring and Neap Ranges shown in the box, (top right).

Predicted Range equal to or greater than Spring Range-Use Spring Curve only.

Predicted Range equal to or less than Neap Range-Use Neap Curve only.

Other Ranges – Interpolate as previously explained, and draw in the appropriate part of the curve.

7.         From the intersection at the appropriate curve, proceed vertically down to the time scale.

8.         Read off the required time (or time interval) using the 10-minute divisions to assist.

Example

(European Standard Port)

Find the Standard Time when a ship of draft 5.5 m will have 2 m clearance under the keel over a 4.5 m shoal off Portsmouth on the a.m. ebb tide of June 29th.

Follow the instructions in Checklist 2. (Step 1)

Standard Port   Portsmouth     Time / Height Required 3.0 m

Secondary Port            NA       Date 29 June   Time Zone GMT

 TIME HEIGHT STANDARD PORT HW LW HW LW RANGE 0530 1058 4.0 1.5 2.5 Seasonal Change Standard Port DIFFERENCES Seasonal Change Secondary Port SECONDARY PORT Duration

Sketch and Preliminary Calculations (Step 2)

Draft                            5.5 m Clearance                       2.0 m

Required Depth            7.5 Charted Depth                    4.5

Required

Height of Tide               3.0 m

Interpolation (Step 6)

Spring Range                4.1 m                           Spring Range                4.1 m

Neap Range                 2.0 m                           Predicted Range           2.5 in

Spring – Neap              2.1 m                           Spring – Predicted        1.6 m

(Fractional Distance from Spring Curve) = {Spring – Predicted} / {Spring – Neap}

= 1.6 / 2.1 = 0.76 = ľ

Plotting (Steps 3, 4, 5, 7 and 8)

To Find the Corrected Height of a Lighthouse

(European Standard Port)

Use earlier instructions (Checklist 1), ‘To Find the Height of Tide at an Intermediate Time between HW and LW.  Note that the charted height of a lighthouse or other shore object, is  given above MHWS.  This level can be found from the Table of Tidal Levels on the chart, if included, or, for Standard Ports, from ATT, Table V.  For Secondary Ports, MHWS is obtained by applying the height difference in Part II to the Standard Port MHWS level given there.

Example

A lighthouse off Sheerness has a charted height of 15 m. Find its height above water level at 0930 GMT on April 22nd for use with a vertical sextant angle.

Follow the - instructions in Checklist 1. (Step 1)

Standard Port   Sheerness        Time / Height Required 0930

Secondary Port            NA       Date 22 April   Time Zone GMT

 TIME HEIGHT STANDARD PORT HW LW HW LW RANGE 1140 0523 5.7 0.6 5.1 Seasonal Change Standard Port DIFFERENCES Seasonal Change Secondary Port SECONDARY PORT Duration

Interpolation (Step 5)

Predicted Range is equal to Spring Range          -           use Spring Curve only.

Plotting           (Steps 2, 3, 4, 6 and 7)

Sketch and Final Calculation

Charted Height Lt. Ho. 15.0 m

MHWS                                    5.7 (from Table V)

Height above CD                      20.7

Height of Tide                           4.1

Corrected Height Lt. Ho.          16.6m

Answer 16-6 metres is the corrected height.

SECONDARY PORT TIDE CALCULATIONS

European Secondary Ports-ATT Volume 1

The method of finding the time and heights of HW and LW at any Secondary Port given in the Admiralty Tide Tables is by means of simple differences in times and heights from predictions for the Standard Port indicated, which is the most suitable, not necessarily the closest, Standard Port.

Finding HW and LW Times and Heights

(European Secondary Ports)

Checklist 3

1.         Find -the Secondary Port in the Geographical Index at the back of ATT.  Note its number.

2.         Refer to ATT Part 11 with Secondary Port number and note the relevant Standard Port, its number and page number in Part 1.

3.         Refer to Part 1. Find HW and LW times and heights at the Standard Port for the required day.  Enter in NP 204, boxes 1-4, and complete box 5 with the predicted range.

4.         Refer back to Part II, bottom RH page “Seasonal Change in Mean Level”.  Using its number to assist, locate the correct line and find the Standard Port’s Seasonal Change (for the month).  Enter this change in boxes 6, with its sign changed (see note).

5.         Note the time and height differences for the Secondary Port and the related reference times and reference heights at the Standard Port.

6.         Interpolate between Secondary Port time differences and height differences using graph paper or by calculation.

7.         Enter the interpolated Secondary Port time and height differences in boxes 7-10.

8.         Using its number to locate the right line, find and enter the Secondary Port’s Seasonal Change for the month in boxes 11.

9.         Apply differences to Standard Port times and heights to find times and heights at the Secondary Port (boxes 12-15).

Note The Standard Port height quoted in ATT already includes the Seasonal Change which must be subtracted algebraically so that subsequent interpolation is carried out on the unadjusted tidal heights.

Example

(European Secondary Port)

Find the times and heights of HW and LW at Brodick Bay for the AM rising tide on August 1.

Follow title instructions in Checklist 3.

Standard Port   Greenock (404)           Time / Height Required AM rising

Secondary Port            Brodick Bay (408) Date 01 Aug          Time Zone GMT

 TIME HEIGHT STANDARD PORT HW LW HW LW RANGE 0939 0329 2.8 0.8 2.0 Seasonal Change Standard Port (404) + 0.1 + 0.1 Sign changes DIFFERENCES 0000 + 0005 – 0.2 0.0 Seasonal Change Secondary Port (408) 0.0 0.0 SECONDARY PORT 0939 0334 2.7 0.9 Duration -

Note that in this case (i.e. at Brodick Bay No. 408) the Secondary Port time and height differences at HW and LW remain constant for different Standard Port times and heights compare these differences with those at Lamlash (409) where differences are not constant.

Answer HW 0939 GMT 2.7m LW 0334 GMT 0.9m at BrodickBay.

(SEE ATTACHED SHEETS NEXT)

Extracts for above: