Tidal theory in the Yachtmaster syllabus was written for a country with big, regular, semi-diurnal tides. Australia has every kind of tide, sometimes in the same state. This guide explains the mechanics, then works through the calculations the exam expects, using Australian ports.
The regimes
- Semi-diurnal (two roughly equal highs and lows a day): the east coast from Torres Strait to Tasmania, and the Northern Territory. Sydney's range is 1 to 2 m; the Whitsundays 3 to 4 m; Broad Sound near Mackay up to 9 m; Darwin up to 8 m.
- Diurnal (one high and one low a day): much of the west and south-west coast. Fremantle's range is often under a metre and the pattern is dominated by a single daily cycle with strong seasonal and weather effects.
- Mixed: South Australia's gulfs and much of the west, where semi-diurnal and diurnal components combine. At neaps in the gulfs the two nearly cancel and the water barely moves for a day or more: the dodge tide, a favourite examiner's question.
Whatever the regime, the tables give you the times and heights of high and low water for standard ports, and the method for finding heights in between is the same.
Datum and what the numbers mean
Australian charts and tide tables use Lowest Astronomical Tide (LAT) as chart datum. A charted depth of 2.5 m means 2.5 m of water at LAT, the lowest tide predicted under average weather. The height of tide from the tables is added to charted depth to give the depth now. Drying heights (underlined on the chart) are subtracted. Weather can push actual levels below datum: a strong offshore wind and high pressure in a shallow bay like Moreton Bay or Port Phillip can take 0.3 m off the prediction.
Worked example: secondary port and anchoring
The figures below are illustrative, chosen to show the method, and are not real predictions.
Situation. You plan to anchor in a bay that is a secondary port on Sydney (Fort Denison). You will arrive at 1600 and stay until 0800. Your yacht draws 1.9 m. You want at least 1 m under the keel at the lowest point of the night. The chart shows 3.0 m at the spot you like.
Step 1: standard port predictions. The tables for Fort Denison give high water 1330, 1.7 m; low water 1950, 0.4 m; high water 0210, 1.5 m.
Step 2: secondary port differences. The tables give the secondary port as high water +0010, low water +0015, height differences high water 0.0 m, low water +0.1 m. So at the secondary port: high water 1340, 1.7 m; low water 2005, 0.5 m; high water 0220, 1.5 m.
Step 3: the lowest water while you are there. Low water at 2005 is 0.5 m. Depth at low water = charted depth 3.0 m + height 0.5 m = 3.5 m. Under the keel: 3.5 m minus 1.9 m draught = 1.6 m. Comfortable.
Step 4: the range for the anchor. From low water 0.5 m to the next high water 1.5 m the water rises 1.0 m. Scope is calculated on the maximum depth: 3.0 m + 1.5 m = 4.5 m, plus the bow roller height of, say, 1.0 m gives 5.5 m. At 4:1 on chain that is 22 m; in a forecast 25 knots you would lay 30 m or more and check the swinging circle against the next boat and the shore.
Step 5: the height at a given time. If you need the height at 1800, use the tidal curve for the standard port (or, roughly, the rule of twelfths). From 1340 high (1.7 m) to 2005 low (0.5 m) is a range of 1.2 m over 6 hours 25 minutes. At 1800, about 4 hours 20 minutes after high water, the tide has fallen roughly 9/12 of its range: about 0.9 m, so the height is about 0.8 m. On the curve you would read this more precisely.
That is the whole method. The examiner may ask it as "can we get over the bar at 1500?", "how much chain?", "when can we leave the marina?" or "what will the sounder read at 0300?", but the steps do not change.
Tidal streams
Heights tell you if you will float; streams tell you where you will end up. On the east coast, streams are weak offshore and strong in entrances and island passages: the Whitsunday passages run at 3 to 5 knots, Port Phillip Heads at up to 8. Tidal stream information comes from atlases (hourly chartlets referenced to high water at a standard port) and from the diamonds on the chart. For a course to steer, take the stream for each hour of the leg from the atlas, interpolate for springs or neaps, and draw the vectors. For a tidal gate, find the time of slack water from the atlas or the local tables and plan to arrive then.
Australian specifics worth knowing
- Northern ranges are enormous. Darwin, Broome and the Kimberley demand tidal planning for every anchorage and every departure; marinas there sit behind locks.
- Broad Sound in Queensland has the largest range on the east coast, close to 9 m, because the funnel shape of the coast amplifies it.
- Weather matters more where tides are small. In Perth and Adelaide a barometric or wind-driven surge can exceed the astronomical tide.
- The Whitsunday bullets are wind, not tide, but strong streams between islands combine with them to produce standing waves at spring tides.
- Bars (Wide Bay, Noosa, the Gold Coast Seaway, Swansea, Yamba) are crossed on the flood and never on an ebb against swell.
Questions people ask
Are Australian tide tables in the same format as UK almanacs?
The Australian National Tide Tables and the state tide tables give standard-port predictions and secondary-port differences in a familiar format, though the layout differs from the UK almanacs used on many courses. Training centres in Australia usually teach with an RYA training almanac and then show you the local tables.
What is the rule of twelfths and is it accurate enough?
It estimates the tide rising or falling by 1/12, 2/12, 3/12, 3/12, 2/12 and 1/12 of its range in successive hours after high or low water. It assumes a regular six-hour semi-diurnal tide, so it is a rough check on the east coast and unreliable where tides are diurnal or mixed. Use the curve for anything that matters.