Tides, Currents and Chart Datum in Australia

Depth is the one navigation quantity where being slightly wrong has immediate physical consequences. It is also the one most often reasoned about loosely, because a chartplotter shows a number and the number looks definite.

The number on the chart and the water under the keel are related by arithmetic that every skipper should be able to do without a phone.

#Chart datum: the reference nobody sees

Charted depths are not depths at some average sea level. They are depths below a defined vertical reference — the chart datum.

Modern Australian charts commonly use Lowest Astronomical Tide (LAT): the lowest level the tide is predicted to reach under average meteorological conditions over a long period. The consequence is important and frequently misunderstood:

• charted depths are deliberately conservative
• the predicted height of tide is normally added to the charted depth
• the water will rarely be shallower than charted, but weather can push it below prediction

Drying heights — the figures on sandbanks and rocks that dry — are expressed as heights above chart datum, usually underlined on the chart. Those you subtract the height of tide from, not add to.

Older charts and older cruising notes may use different datums, and heights of lights and bridges are referenced to a different datum again (commonly a high-water level). Read the chart's own notes rather than assuming.

This is the same class of problem as coordinate datum, covered in the GNSS guide: the number is only meaningful once you know what it is measured from.

#The under-keel clearance calculation

Available water  =  charted depth
                 +  height of tide at that place and time
                 -  vessel draft
                 -  safety margin

Worked example, a yacht drawing 1.9 m:

Charted depth ................  2.4 m
Height of tide (1420) .......  +1.8 m
                              -------
Water depth .................   4.2 m
Draft .......................  -1.9 m
                              -------
Under-keel clearance ........   2.3 m

That looks comfortable. Now account for the error terms:

The margin is what absorbs all of that. A fixed 0.5 m margin is reasonable in a well-surveyed marina approach in calm water. It is not reasonable crossing a bar in swell, or approaching a reef in an area with Zone of Confidence C or D — see marine charts Australia.

And confirm what your own display is telling you. A depth sounder may show depth below transducer, depth below keel or total water depth depending on the configured offset. Know which, and verify it — this is one of the most common commissioning errors, covered in depth sounders and transducers.

#Working a tide prediction

For most recreational purposes, official tide predictions for the nearest standard port, adjusted for your secondary port, are sufficient — and modern navigation software will do the arithmetic.

The judgement remains yours:

• Springs and neaps. Range is greatest near new and full moon, least at the quarters. The same anchorage can be comfortable at neaps and untenable at springs.
• Time offsets. A secondary port's high water can differ from the standard port by hours, not minutes
• Meteorological effects. Low barometric pressure raises actual sea level above prediction; high pressure and strong offshore wind can lower it. Prolonged onshore wind can raise it substantially.
• Rule of twelfths remains a useful mental approximation between high and low water, but it assumes a roughly sinusoidal tide. In places with double high waters or strongly distorted curves it will mislead.

Always use current official predictions for the specific location rather than a generalisation.

#Tidal streams matter more than tidal height

For passage planning, the horizontal movement of water usually dominates the vertical.

A 3-knot stream against a 6-knot boat halves the speed over ground. The same stream with you turns an 18-hour leg into a 12-hour one. And the interaction between wind and stream determines sea state far more than wind speed alone: wind against a strong current builds short, steep, breaking seas that are genuinely dangerous in conditions that would otherwise be unremarkable.

Practical consequences:

• plan constrictions, entrances, bars and straits around slack water or a favourable stream — a tidal gate
• build the passage timing backwards from the gate
• treat wind-against-current areas as no-go rather than uncomfortable
• expect eddies and counter-currents close inshore that the tidal stream atlas does not resolve

#Major Australian current systems

Beyond tidal streams, Australia has persistent ocean currents that affect passage planning materially.

The East Australian Current runs generally south along the New South Wales coast and can be strong, with large warm-core eddies that break off and persist. It is a substantial help southbound and a substantial hindrance northbound, and the wind-against-current sea state problem on this coast is well known and has produced serious incidents.

The Leeuwin Current flows generally south along the Western Australian coast, warm and strongest in the southern autumn and winter.

The Indonesian Throughflow feeds the north-west, and tropical circulation dominates the Top End.

Sea-surface temperature and current data is available as model output and via satellite delivery, and is one of the more useful GRIB layers for coastal passage planning — see weather routing.

#The S-100 connection

The Australian Hydrographic Office says it is focused on selected S-100 products from 2026, including S-104 Water Level Information and S-111 Surface Currents.

That matters here specifically. Today, chart, tide and current are three separate mental layers a skipper integrates by hand. The S-100 framework is designed so a navigation system can combine dynamic water-level and current datasets with the chart directly.

As the charts guide notes, the sensible position for a buyer in 2026 is still to buy equipment that solves today's problem — but on a large premium refit, ask the manufacturer about its S-100 roadmap.

#Anchoring implications

Tide drives the two numbers that matter at anchor: the depth you set in, and the depth you will have at high water.

• scope is calculated on maximum expected depth, not the depth on arrival
• swinging room must allow for the tide turning the boat through 180°
• a rising tide can float you over the obstruction you cleared on the way in
• a falling spring tide can leave a keel boat aground where it was comfortable at midnight

The anchoring electronics guide covers anchor alarms and depth alarms in this context.

#Bottom line

Learn the arithmetic, know your chart's datum, configure your sounder offset correctly, and size the safety margin to the survey quality and the sea state rather than to habit. Then plan the passage around the streams, because the water moving sideways will affect your day more than the water moving up and down.