What makes Bass Strait a ‘gnarly’ part of the Sydney to Hobart yacht race?

Will Oxley expected the 1998 Sydney to Hobart yacht race to be a rough one. The veteran sailor, who had completed the race many times before, had seen the forecasts for strong winds on the journey south. But when he and his crew started getting slammed by 12-metre waves and 80km/h winds as they journeyed towards Tasmania, Oxley knew something wasn’t right.

Then another wave hit.

“We got hit by a fairly decent breaking wave, rolled upside down and stayed there,” he said back in 2006, recalling the race’s worst-ever year that saw six people die, 55 more rescued by helicopter, and 60 yachts forced to retire.

“We had eight people inside [the hull of the boat] with water up to our knees discussing what the next plan would be,” he said. “Somewhere between three and five minutes later another monster wave hit the keel and the boat righted itself, but the mast was broken.”

Oxley and his crew had just felt the full force of Bass Strait, a body of water with ferocious waves, harsh winds, and an unpredictable nature that has occupied the hearts and minds of sailors for years.

Entrants in the annual Sydney to Hobart Yacht Race regularly say Bass Strait is the hardest part of the entire race, which is now in its 77th year. After the 1998 race, organisers made changes to safety regulations, including a more detailed compulsory safety briefing. More than 100 yachts, from small two-person boats to 30-metre “super maxis”, will compete this year, battling it out from Sydney down to Constitution Dock in Hobart – via Bass Strait.

So, just what makes this stretch of water so tumultuous?

What is Bass Strait?

Stretching 250 kilometres from west to east and 200 kilometres north to south at its narrowest point is the Bass Strait (or just Bass Strait, as many people call it) that separates Victoria and Tasmania. It might look like any other stretch of ocean – big, blue and mostly empty – but hidden just below the surface is another world of mountains and canyons, lakes and cliffs. And they all affect what happens on its surface.

Bass Strait was once not a body of water. For much of the world’s ancient history, Tasmania and Australia’s mainland were connected by a long strip of land called the Bassian Plain.

Indigenous Australians crossed into Tasmania via the Bassian Plain tens of thousands of years ago. But about 18,000 years ago, during the end of the last global ice age, the melting of ice caps around the world resulted in the flooding of the lowest sections of the plain.

Scott Nichol, the director of national seabed mapping at Geoscience Australia, has spent years exploring what lies beneath the surface of Bass Strait. “You can see evidence of old sand dunes that should be on the coastline that are now 50 or 60 metres underwater,” he says.

For a few thousand years, the sea was 60 metres below its current levels. “There was enough time for coastal sand dunes to form, and in some cases they’ve actually been preserved,” Nichol says.

In other words, what were once dune-lined areas of coast are now all underwater. The upshot? Bass Strait is shallow. It has an average depth of just 60 metres, presenting risks to less experienced sailors who are used to navigating in deep ocean waters. Arthur Lane, a Sydney to Hobart veteran who leads the race’s organising committee, says the shallow water means “a bit of a twist on the stomach”.

But the most important thing to notice about the depth of the strait is at its edge …

What makes Bass Strait treacherous?

There are two things to know about this body of water. The first is that Bass Strait’s geomorphology – the shape of the sea floor – changes rapidly over a very short area. There’s no better example of this than at the eastern edge of the strait, right where the Sydney to Hobart adventurers pass: the Bass Canyon.

The Bass Canyon – the largest undersea canyon in Australian waters – plunges to depths of 3.5 kilometres yet within “a really short distance”, Nichol says, the seafloor rises to be only 120 metres below sea level. You can see the drop-off represented in the image below.

The second thing to know is that this very spot is where the East Australian Current stops. (The East Australian Current is the one from the movie Finding Nemo.) As this heat map below from CSIRO shows, it moves vast amounts of warm water from the Coral Sea south down the coast. And after flowing for hundreds of kilometres, it suddenly appears to stop moving south.

It doesn’t take a geoscientist to work out what might happen here, when tonnes and tonnes of water, sometimes flowing at 90 centimetres a second, hit kilometre-high walls: a lot of movement – or, in technical parlance, upwelling – where deep water is forced up the canyon walls and meets the surface.

“You’ve got waves colliding with your currents,” says Nichol, “and that’s where you get very steep, very dangerous waves forming. And that creates the hazard for yachts.”

“In terms of treacherous water, it’s got to be up there with the most treacherous in the world.”

Sydney to Hobart veteran Peter Langman

It’s why Peter Langman, who will lead one of the smallest boats to Hobart, gets nervous thinking about it. “I’ve seen it pretty gnarly,” he says. He has noticed that as the water becomes more shallow, “the waves tend to stand up quite a lot”. “It can be extremely rough. In terms of treacherous water, it’s got to be up there with the most treacherous in the world.”

Geoscience Australia’s Nichol is quick to add that this is not a constant; it’s actually “the worst-case scenario” with upwelling less likely during summer – but the East Australian Current is always there.

Then there are local tides, which generally flow west to east in the strait. “Bass Strait is relatively narrow,” says Nichol. “So the tides need to squeeze through the Bass Strait every day, going backwards and forwards – and that may be in a different direction to all the other currents rushing around.”

Even this map, of the shortest possible route, shows how the race passes along the eastern edge of Bass Strait (yes, that bit with the big drop-off).

Any more potential curveballs in Bass Strait?

It’s not just what’s happening under the water that makes it difficult to cross Bass Strait. Simon Louis, from the Bureau of Meteorology in Tasmania, says cold fronts around southeast Australia can split up and become westerly and southerly winds that later meet around Bass Strait.

“Sometimes then, you’ll have the westerlies and southerlies interacting in and around Bass Strait in ways that can be quite tricky to predict exactly,” he says. “The timing and interaction of those winds can vary a lot depending on the nature of the front.”

And low-pressure systems appear to have a liking for southeast Australia. “It can be a preferential area for them to develop and intensify,” Louis says. “That has caused issues in past races. So if that happens, obviously you can get a fairly sudden deterioration in conditions and quite strong winds and heavy seas.”

These systems can move very quickly and are notoriously difficult to predict. That’s why, while Oxley’s crew were expecting strong winds, they were surprised by just how strong they were.

Hopefully, this year will be “smooth” sailing. Current weather forecasts from the Bureau of Meteorology suggest light northeasterly winds tracking south for the first two days of the race, when the super maxis will travel through the strait. But just as smaller boats get closer to Tasmania, those winds will adjust to be south-easterly on Wednesday, which will present them with challenges as they attempt to make it down the Tasmanian coast.

“I’d love to say that I’m expecting north-easterly winds and a nice run down to Hobart,” says Langman. “But literally anything could happen … We’ve got to be prepared for whatever.”

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