Knowledge of tidal movement is essential in undertaking safe navigation, especially in coastal waters. Simply, if there is insufficient depth of water, you will go aground.
Tides are the periodic rise and fall, or vertical movement of the levels of the worlds oceans. The difference between tides and tidal flows or currents must be appreciated. Tides affect only the depth of water. Tidal currents are the horizontal movement caused by tides, and effect the track and SOG of a vessel.
Tides are generated by the effect on the Earth's oceans by gravitational forces between the earth, the moon and the sun, by centrifugal force due to the Earth's rotation, and by centrifugal force due to the Earth's solar orbit. It should be appreciated that to scale, the oceans are no more than a layer of gladwrap on a soccer ball. | Earth | The moon has the greater gravitational effect, some 2.2 times greater than the sun at the Earth's surface. Because of it's fluidity, water tends to accumulate on the parts of the Earth's surface directly toward the moon and on the surfaces directly opposite the moon where centrifugal force of rotation exceeds the force of lunar gravity. Consequently, over all other areas the water levels are depleted. The regions of accumulation and depletion move over the surface as the position of the Moon varies relative to the Earth, mainly because of the Earth's daily rotation but also because of the Moon's orbital motion around the Earth, which is a period of 27.3 days, or a sidereal month. A lunar or synodic month, the period between one full moon and the next, is 29.5 days.The moon also rotates on its axis once every 29.5 days in the opposite direction to its orbit, giving the appearance from Earth that it does not rotate. The plane of the moon orbit around the Earth is generally in alignment with the plane of the Earth orbit around the Sun, i.e. it is tilted at 23.5° to the Earth polar axis.

Theoretically, if the Earth was completely covered in open deep water, two tidal waves 180 deg. apart and approx. 2 metres high would continuously circle the globe. However, the inertia of the water, the existence of continents, and effects associated with the water depth result in much more complicated behaviour. For the open ocean areas, tidal levels vary between zero and approx. 1 metre range. e.g. tidal range on mid Pacific islands is very small.

Because of the Earth's period of rotation, there are generally two high and two low tides per day at any given place, (semidiurnal tides),but they occur at times that change from day to day. The average interval between consecutive high tides is 12 hours 25 minutes. Tides occurring once daily are called diurnal. The gravitational effect of the Sun is similar and additive to that of the Moon. The tides of largest range or amplitude are called spring tides, and occur at New Moon, when the Moon and the Sun are in the same direction relative to Earth, ( in conjunction), and at Full Moon, when they are in opposite directions, ( in opposition). The tides of smallest range are called neap tides, and occur at intermediate phases of the Moon, at seven and a quarter days after new or full moon, in the first and last quarters, when the moon and sun are separated at 90 deg., ( in quadrature), and the gravitational effect of the sun diminishes that of the moon. | Phases of the moon | are caused by the varying amount of moon surface exposed to sunlight as viewed from a particular point on Earth.

Tides are most easily observed, and of greatest practical importance, along coastlines, where the amplitudes are exaggerated. When tidal motions run into the shallow waters of the continental shelf, their rate of advance is reduced, energy accumulates in a smaller volume, and the rise and fall is amplified. The details of tidal motions in coastal waters, particularly in channels, gulfs, and estuaries, depend on the details of coastal geometry and water-depth variation. Tidal amplitudes ( the contrast between spring and neap tides), and the variation of times of high and low tide all vary widely from place to place.
For these reasons, purely theoretical calculation of the times and heights of tides at a particular station is quite impossible. Tides are successfully predicted on the basis of accumulated observations of the tides at the place concerned. The periodical motions of the Moon and the Sun relative to the Earth contribute a range of variables at all locations, and occupy a cyclic period of 18.6 years, called the Saros cycle It is during this cyclic period that lowest astronomical tides, LAT, are recorded as the datum for tide predictions.
Other terrestrial occurrences affect tide levels. Strong onshore winds may increase tides in localised areas causing a tide to 'pile up'. Storm surge during cyclones is due to the combined effect of high wind velocity and low atmospheric pressure. High atmospheric pressure can suppress tide levels. Seismic activity can have an effect over large areas. The effect of a deep cyclone in the Coral Sea will generate abnormal local coastal swells up to a week later.
The study of tidal predictions is part of the science of hydrography, and comprehensive data is published annually and on a web site by the | National Tidal Facility | based at Flinders University, Adelaide. State authorities each have hydrographic departments, and publish annual tidal data books. | Tidal Predictions. |   See also | Tides. |

Tidal Terminology.   Refer to pages 201 - 203 of the 2007 tide book.

Reading of Tide Tables.    Gladstone. Refer to pages 135 to 137 of the 2007 tide book.

Tidal Datum. This is the lowest astronomical tide recorded over a solar cycle of 18.6 years, and is the datum or reference point for all soundings on charts and tidal heights.This fact is clearly noted in the tide tables. The height of a tide must be added to the charted depth below this datum to obtain the actual depth at a particular time.
Standard ports are those which have the tide time and heights tabulated for every day of the year.
See p. 204 of the 2007 tide book.
Secondary ports are those whose tide time and heights are obtained by applying corrections to the associated standard port. Refer to p. 205 - 210 of the 2007 tide book. Note that some Qld. coastal places experience diurnal tides.
Intermediate tide heights for all ports can be calculated using the Standard Tidal Curve tables.
Refer p. 211 - p. 212 of the 2007 tide book. Note that the vertical scale of the curve tables is tidal range, not tide height, and the previous or next low tide height must be added to obtain depth of water. Tidal ranges between the standard curves can be approximated.
Alternatively, heights can be calculated using the ' Rule of 12ths.' Tidal range covers a period of approx. six hours. Determine the range of the tide and divide by 12. Assuming a low tide, in the first hour, the tide would rise 1/12th of the range, in the second hour 2/12ths, in the third hour 3/12ths, in the fourth hour 3/12ths, in the fifth hour 2/12ths, and in the last hour 1/12th. Total 12/12ths.
This method may be demonstrated diagramatically. On a circle, the diameter of which represents tidal range, mark off 30° angles and construct horizontal chords. The intercepts on the vertical diameter will be in the ratio of 1:2:3:3:2:1.

Compare results using this method and using the tidal curve tables.
Remember that the above methods are approximations and that the height of the tide is always taken as being above chart datum. Do not confuse this with tidal range or height above low water.

Applications of this Data. To determine under keel clearance, ukc, for crossing a shoal area at a particular time, add the chart sounding to the current height of the tide. This will give the actual depth of water. Subtract the vessel draft. Treat drying heights as negative mathematical quantities. e.g. Middle Bank and the Narrows.
To determine a safe anchorage on a falling tide, the fall of the tide must be less than the ukc. Subtract the tide height above the next low tide height at the time of anchoring , from ukc. In both cases, allow sufficient ukc at low tide for the effect of wave height.
Do not confuse under keel clearance with depth of water.
Gladstone Harbour and the Narrows provide some exceptional navigation hazards by way of reefs, sandbars, and general lack of water depth at low tides. It is essential that the boat skipper is able to determine the depth of water over these hazards, and to ensure sufficient under keel clearance (ukc) for passage and for anchorage in a falling tide.        

| Narrows Tides. |
Until 2003, chart soundings and tidal predictions for the Narrows had been based on data up to 25 years old. Recent hydrographic surveys and more accurate tidal observations have resulted in a revision of the official data.
A new Narrows chart issued by Qld. Transport in December 2003 shows the minimum depth in the Narrows channel as a drying height of 2.1 m. above the chart datum. This is at a point in the channel adjacent to the entrance to Monte Christo Creek, between beacons 12 and 14.

The 2007 Tidal Prediction Tables p. 207 gives tides in this section, Boat Creek as (Gl. x 1.17) - 0.05 m.
and Ramsey Crossing (Glad. x 1.26) + 0.09 m.

It is therefore prudent to calculate ukc for a Narrows passage using Boat Creek data.

As an example, for a vessel drawing 1.8 m. and a 3.61 m. Gladstone tide, the ukc on prediction would be
( 3.61 x 1.17) - 0.05 - 2.1 - 1.8 = 0.27 m.

Mariners navigating these waters are strongly advised to calculate the clearance as above. By reference to the official tide prediction tables, and a chart of the correct scale, depth and ukc for a shoal passage can be quickly calculated.

Return to Navigation Quiz Answer 9 of the Quiz.

Awareness of tidal flows is of importance when navigating in areas of restricted manoeuvring. Note the strong tidal flow conditions in the main channel on the Gladstone Boating Safety chart. A strongly running tidal flow can quickly force a vessel off course onto a hazard or against a channel marker beacon.

Safe boating.         For further information on tides, visit | Qld. Tidal Information | Tide Predictions. |

Return to Coastal Navigation.