A guide to crossing Cook Strait: What it's really like crossing one of the 'most dangerous' stretches of water, and how operators decide if it's too rough

Thursday, 24 November 2022

As a quartermaster on ferries crossing Cook Strait, Mick Williams can remember times when waves would surge over the bow of a ferry as the ship crashed down on to the water.

But those days were rare, Williams said. “The chances are you’re going to have a good trip.”

He noted he was talking from the perspective of a seafarer, who had worked on ships around the world, so his perception could be different to that of someone who spent little time on the water.

People were aware conditions in Cook Strait could change quickly, he said, and that was partly why the stretch of water had an undeserved bad name.

**READ MORE:

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**

“I don’t want to dramatise it. I love it, it’s a nice stretch of water,” Williams said.

“I certainly wouldn’t put anyone off crossing Cook Strait, but you choose your time,” he said. “It’s beautiful on a nice day.”

Big waves and fierce winds do cause the cancellation of some Cook Strait ferry crossings, and on Wednesday MetService was warning of a chance of heavy swells through until Thursday evening along the Porirua and Kāpiti coastline.

Occasionally crossings that go ahead cause a bit of a stir.

For example, passengers on the ferry Kaiarahi in winter 2017 reported they nearly ran out of vomit bags. People on board who regularly crossed the strait said it was the worst sailing they had experienced. Interislander later cancelled sailings because of deteriorating weather conditions that had whipped up 7m swells.

On Wednesday, Interislander executive general manager Walter Rushbrook said guidelines for deciding to cancel a sailing include a wave height of 5m for passenger sailings and 5.5m for freight sailings.

The master of a ferry had over-riding authority to cancel sailings at lower heights should they feel it was justified, Rushbrook said.

”These standards ensure both the comfort of our passengers and the safety of the freight we carry onboard our ferries.”

Williams said he could remember times when no one was able to stand still on the bridge. “In my case, you hang onto the wheel. I was also trying to steer the vessel through the weather,” he said.

He wasn’t sure of the height of the biggest swells he had encountered in the strait, but said they were “quite high”.

“The bow goes right up in the air, then it comes down and bangs on the water. It’s quite powerful.”

On a reassuring note, Williams said the ferries had enough personnel on board to deal with a worst case scenario, and the crew members were experienced seafarers.

He pointed out that about half the trip from Wellington to Picton was also inside the calmer waters of the Marlborough Sounds and Wellington Harbour.

Even on a calm day there would be a bit of a swell going through the entrance to Wellington Harbour, and the ship would start to move. “There will be some kind of swell, then the ship just drives itself through it,” Williams said.

Sailing through the entrance to the Sounds required concentration. “At the entrance you have all the tides meeting each other,” although it wasn’t something passengers would notice.

“I never get fed up with steering through the Marlborough Sounds. You always see something different,” he said.

Williams said it was not unknown for a passenger to get sick even on a calm crossing, but that was rare. Even in more normal conditions it was unusual for anyone to get sick.

On its website, Interislander said typically, strong winds from the north were nothing to worry about. The water would remain calm, but outdoor decks might be closed.

“However, a strong wind from the south, known as a southerly, can cause significant swells,” the company said.

“Our Interislander Cook Strait ferries have stabilisers, which help keep our ships steady in moderate swells. But there will be days when we sail in choppy water that is a little uncomfortable for some.

“There are also times when we delay or cancel sailings when it’s too unsafe to go.”

Passengers could delay travel to a day with better weather. Depending on the type of fare, they could do that free of charge, or might have some charges to pay.

A Niwa study said Cook Strait was noted for a high frequency of strong winds.

“Northerly winds occur more frequently, but are generally not associated with high waves in the strait,” the study said.

High waves did affect the strait when winds were from the south, because of the relatively open exposure to that direction.

A Niwa report on Wellington’s climate said that in an average year, Baring Head – on the North Island south coast near the entrance to Wellington Harbour – had 255 days with wind gusts above 63kph, and 72 with gusts above 96kph.

Cook Strait acted as a “giant wind tunnel”, the Niwa report said. That was because it was the only gap between the mountainous North and South Islands, and located in the westerly wind belt known as the Roaring Forties.

“The swells on Cook Strait can increase rapidly with a southerly storm, sometimes reaching heights of 15m or more.

“Because of the wind funnelling and strong tidal flows, Cook Strait is regarded as one of the most dangerous and unpredictable stretches of water in the world.”