A second generation Emerald-class ferry undergoes testing near the entrance to Sydney Harbour in early March.

A second generation Emerald-class ferry undergoes testing near the entrance to Sydney Harbour in early March.

The document by ferry operator Transdev warns masters that trials in early March showed that the new Emerald-class ferries became “airborne”,resulting in “tunnel slamming”,when the vessels sailed directly into waves or on a 45-degree angle to them at about 10 knots.

“This can be detrimental to the vessel’s integrity and the safety and comfort of the crew and passengers,” it states.

Tunnel slamming refers to water rising between the two hulls of the catamaran ferries,jolting the bridge deck which structurally joins the hulls together.

“Running ahead of the swell must be avoided,as this may cause ‘trapping’,which results in the vessel bow-diving or broaching,and a loss of control,” the memo warns.

The document – obtained by theHerald – advises masters that the safest and most comfortable course for passengers when crossing Sydney Heads is to have the swell directly on the beam of the Emerald-class ferries. The beam is the width of a vessel at its widest point.

Last weekend a 19-year-old teenager was rescued from the harbour after falling overboard from the Clontarf ferry several hundred metres from North Head. The Clontarf is one of the three Emerald-class ferries now used on the Circular Quay-Manly route.

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Graeme Taylor,from community group Action for Public Transport,said the internal Transdev memo highlighted the fragility of the new Emerald-class ferries which had a “very jerky movement” and slammed into swells even in relatively mild conditions.

Mr Taylor said the government needed to acknowledge that the new ferries were unsuitable for the Manly route because they handled swells poorly,leading to cancelled services,and were too small for large crowds over summer.

“The best solution for these problems is to reinstate the four Freshwater-class ferries seven days a week and transfer the generation-two Emeralds to the Inner Harbour,” he said.

A Freshwater ferry endures large swells on the Manly route.

A Freshwater ferry endures large swells on the Manly route.Credit:Nick Moir

Despite opposition from locals,last October the governmentretired the Queenscliff,one of four Freshwater vessels,almost 40 years after its maiden voyage on the Manly-Circular Quay route. The new Emerald-class ferries are replacing two of the four Freshwater vessels.

Transdev,which operates Sydney’s government-owned ferries,said the new Emerald-class ferries were now clear to operate to their “full survey swell height certification” for the harbour after completing sea trials early last month.

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While Sydney has been hit by “unseasonable weather”,Transdev said historical wave data suggested that the new vessels would be able to operate to Manly 99 per cent of the time. Both the new ferries and the Freshwater vessels were suspended when it was deemed unsuitable for passenger services due to sea conditions,despite their “capacity to withstand it”,the company said.

Northern Beaches Council deputy mayor Candy Bingham said she was concerned that Transdev was expecting the masters of the new ferries to “do the impossible with the vessels they were supplied with”.

“They are just not suitable for the route due to the size of the swells at the heads,” she said. “They are just going to get totally trashed on this route,and we are already seeing evidence of that.”

Labor transport spokeswoman Jo Haylen said it was clear that the Emerald-class ferries would continue to have difficulty handling heavy swells. “For many passengers on the Manly route,the replacement bus has taken the place of a regular and reliable ferry service,” she said.

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