KiwiRail's plan to dock its new mega ferries in Wellington's inner harbour has a major problem - the city is too windy.
Wellington's harbourmaster is also warning the new site could mean recreational boaties are no longer able to use the water.
KiwiRail has committed to buying two new huge Interislander ferries which are more than 200 metres long and 30 metres wide.
A resized Wellington wharf is needed to dock them and KiwiRail wants to move closer to town, right beside competitor Bluebridge at Kings Wharf.
KiwiRail has commissioned a series of simulations of how the new wharf would work with ferry movement in the harbour.
The three ships they used were similar to, but not an exact match, to the two current Bluebridge ships and the ones KiwiRail wanted to buy.
Wellington regional harbourmaster Grant Nalder likened the simulation to a giant video game.
"They have got the port loaded in, so you're looking at a screen and you're looking at Wellington city. And you're on a ship's bridge. What the ship is depends on what you purchase and plug into the computer. So you get the manoeuvring characteristics of whatever you would like to put in there and you run exercises, " Nalder said.
During the two-day simulation, 28 exercises were conducted - 14 had a risk score of low to moderate and eight exercises were found to have a high very high risk.
The simulation revealed that KiwiRail's proposed new wharf, adjacent to Kings Wharf would reduce the ship's turning area by about 200 metres.
Nalder said in Wellington's prevailing northerly wind the ships would need to turn as far north into the inner harbour as they could before backing in.
And as the wind increased, so did the likelihood of needing a tug. And tugs were not always available.
"The tugs can work any hour of the day, there's a resourcing issue there. If they've got a tanker at Seaview, that's 40 minutes for the tugs to cross the harbour, so increased tug use could lead to delays, because if a ferry needs a tug to get into a birth and the tug's not available they need to wait," he said.
The proposal sites a working berth on the port's east, while on the western side will sit a lay out berth for a ferry either in maintenance or not sailing due to bad weather.
But the report found that separation between the two ships is at one point only 50 metres.
The fear, outlined in KiwiRail's own report, is that in a strong southerly wind this gap could soon be lost, resulting in a possible collision.
"When Strait Shipping are backing in they are to the west of the new wharf and if the ship is there, if there are any problems, wind, mechanical etc, you've got the potential of making contact with a KiwiRail ship," Nalder said.
As the harbourmaster, he is responsible for the safety of everyone - from paddle boarders right up to cruise ships and tankers.
At the moment in the inner harbour, Bluebridge operates four arrivals and departures each day, successfully avoiding recreational harbour users.
With the Interislander using the same space, that figure jumped to at least eight, and Nalder saw the possibility of queues forming.
"I'm sure the companies would work to stagger their timetables, but they don't always run to timetable, you get delay, you get other issues. So, potentially for quite a large part of the day there will be a large ship moving in Lambton harbour," he said.
The end of recreation in the inner harbour?
Port Nicholson Yacht Club commodore Pedro Morgan said it would end racing in the inner harbour and that others were affected.
"The harbourmaster has made it clear that KiwiRail's plan will end the recreational use of Lambton Harbour. This will be the end of sailing, rowing, ocean swimming and waka ama, in the central city," he said.
Even Wellington's annual fireworks display would be at risk under KiwiRail's plan.
"KiwiRail needs to know that Wellingtonians won't accept that," he said.
Morgan said he would like some of the capital's MPs, including Grant Robertson, to speak out against people losing access to the harbour.
Proposal feasible - KiwiRail
In a statement, KiwiRail said the simulation showed the proposal was feasible and the risks appeared manageable.
It pointed to Picton where the Cook Strait ferry berths worked adjacent to a large marina.
"In the busy summer high season, Interislander and Bluebridge are safely running extra services in and out of Picton several times a day when recreational craft are also at a peak. Subject to the right harbour management controls being in place, we can operate safely and successfully in Wellington too," it said in a statement.
And it said the fireworks took place further out in the main harbour, not adjacent to the Kings Wharf area.
KiwiRail said it would work with the port, regional council, harbourmaster and others to undertake more detailed work.