How to understand tides in Australia

Tides, like the weather, are another part of boating that anyone who takes to the helm has to factor into the trip.

Not only are there the effects of tides rising and falling, but environmental factors such as distant low-pressure systems sucking all the water away, wind pushing against tides to create uncomfortable conditions, and currents as water flows from shallow sections to deep.

Understanding tides is particularly important if you plan on navigating coral reefs such as that dream trip up to the Great Barrier Reef where coral-fringed safe anchorages are generally inaccessible at low tide.

Tides are related to the gravitational pull of the moon, with the water’s height peaking on the side of the Earth facing the moon, and on the side directly opposite it. 

Where it peaks is high tide, and where it is drawn from creates low tides. The moon, which doesn’t rotate but circles the Earth in the same direction, takes a little over 27 days to circle the Earth.

However, a lunar or tidal day – the time it takes for a point on the Earth to rotate back to the same point under the moon – takes around 12 hours and 50 minutes, which equates to the time difference between high tides.

The best way to get some water under your hull is to wait for the right tide. The difference between high and low tides can range widely.

According to the Bureau of Meteorology, Australia’s largest tidal range is at Derby in the Kimberly region of Western Australia, largely thanks to the large continental shelf around it that amplifies the effects of tides, swell and waves. Tides there can vary by as much as 11.0 metres, or the equivalent of the height of a five-storey building.

That statistic is odd, because in the same state, Mandurah, just south of Perth, is renowned for its lack of tidal movements. These little-to-no tide movements occur at around about a dozen locations worldwide called amphidromic points, one of which happens to be off the south-west coast of Western Australia.

Another part of Australia that experiences large tide differences is the northern coast of Tasmania.

However, the rule of thumb is that the western side of Australia has a tidal difference of about a metre – the spin of the Earth tends to push water west against the African shoreline – to around two metres on the south-eastern coast as the spin of the Earth pushes water up against the Australian coastline.

South Australia can sometimes experience something called a “dodge” tide where the tide level stays unchanged for up to two days at a time. The name has a pretty cool historical link; explorer Matthew Flinders, the first person to circumnavigate Australia and frustrated by not knowing what the tide was doing when he visited the area, referred to the phenomenon after a Charles Dickens book character called The Dodger. It stuck.

Tides can also create viscous, swirling currents, particularly where offshore and coastal currents meet.

The rule of 12ths gives scope to how much tidal movement you can expect over the course of a tide.

We’ve already established that the time from one high tide to the next takes 12 hours and 50 minutes, so a low tide to a high tide will take around six hours and 25 minutes.

The flow of the tide is not linear, meaning it won’t rise or fall by the same amount. You can sort of summarise the change in height as “weak” at low tide, “strong” at mid-tide and “medium” at high tide.

The rule divides each low-to-high-to-low into 12 segments. Let’s assume that we’re going to experience a 1.2-metre difference between tides, and we need at least 0.8 metres of water under the hull to get over a reef with a bit of a safety margin.

Using the rule of 12ths and assuming a six-hour cycle from low tide to the next low tide, we divide the tidal movements into 12 parts. In the first part of the cycle, the first 1/12th and the first hour, we can expect the tide to rise by 1/12 of its full height, or about 0.1 metres, although in reality as this is a "weak" tidal movement this will be slightly less than 0.1 metres.

On the second part of the cycle, or at 2/12ths, in two hours we can expect the tide to rise another 0.2 metres to give us a height at the end of the second hour of around 0.3 metres.

The period between the second and third hours is the period where the tide will rise the fastest.

Moving to the third and fourth part of the cycle gives us our back-to-back 3/12ths, with the change in tide equal to around 0.3 metres at the end of the third and fourth hours, with two rises of 0.3 metres giving us 0.6 metres for a total rise of 0.9 metres. 

We then go back to 2/12ths to the end of the fifth cycle with a rise of around 0.2 metres to give us a total rise of 1.1 metres.

In the final hour, our final 1/12 of the rising tide, we can expect a "medium" rise of around 0.1 metres for a total of 1.2 metres.

This last point is important. If you ground at high tide, it means you only have around an hour of time to refloat your vessel, assuming the next high tide that is 12 hours and 50 minutes away is higher, not lower, than the one on which you ran aground.

There are other things you can do to keep on top of tides and how they are likely to affect you.

Learn how to read charts and relay them to the real world so that the symbols become physical landmarks you can use.

Ask everyone you can about suggested routes and passages, learn the cardinal marks and look out for lighthouses that can all help keep you in deeper waters.

A good pair of polarised sunglasses makes a huge difference out on the water. On a sunny day, the water acts like a mirror, reflecting light and making you blind to what’s below the surface.

Polarised sunglasses filter out this glare, letting you see through the water. The difference, even with a cheap pair of polarised safety glasses used on worksites, is huge.

Or forward and high. Having a spotter looking over the bow of the boat will help the skipper find elusive passages through shallows and coral that will be largely invisible from the helm.

If you’re unsure about the entrance to an anchorage because it’s uncharted and you’re pretty sure there is ground around, it’s time to launch the dinghy.

The idea is that the dinghy can find you a path through shallow water, and you follow several hundred metres behind it.

A dinghy fitted with an electronic sounder is the ideal solution, but remember that for centuries skippers used string lines to sound the depth and follow channels.

The multifunction chartplotters that are slowly becoming more powerful and affordable can lull boaters into a false sense of security. 

Dynamic mapping that factors in tidal changes can be a great tool for boaters, but if you follow the chartplotter’s same breadcrumb trail that you laid out while heading out at high tide and it’s now low tide, you could be inviting trouble.

The Bureau of Meteorology publishes tide predictions for Australia, the South Pacific and Antarctica. You can narrow down the search by state or territory, or click on a map to jump to a location or tidal stream.

The BOM search function will give you times for the next high and low tides for the area you're focussing on, as well as links for a downloadable PDF document with a year's worth of tide data for the previous, current and next year.