A shipping specialist says Interislander's replacement ferry may not be ideal for the rough unpredictable seas of Cook Strait.
Stena Alegra, which has replaced the damaged Aratere, was brought into service only last Monday but broke down in Wellington Harbour on Friday because of a faulty valve in its cooling system.
It sailed later in the day but spent Friday night at the dock in Picton because of poor conditions in the Strait, returning to Wellington Saturday morning.
Saturday night it remained at the dock in Wellington, because of four metre swells in the Strait, and cargo and passengers had to be shifted to the Kaitaki ferry.
John Riding from Marico Marine says Stena Alegra uses water-filled tanks known as flume tanks to keep it stable in heavy swells, unlike Interislander's other ferries which use special fins called stabilisers.
Mr Riding says flume tanks work best in regular swells, which are uncommon in Cook Strait, and crew can take several months to learn how to use them properly.
He says the ship can sail safely in heavy seas but is less comfortable for passengers.
Interislander's general manager Thomas Davis says although the ship is less comfortable in high seas, swells of four metres are not common and he does not think there will much disturbance to Stena Alegra's timetable.
Meanwhile, Interislander general manager Thomas Davis said the Stena Alegra, would sail from Wellington to Picton at 8pm on Sunday carrying both passengers and freight.
He said it would make a return trip from Picton to Wellington with freight only about 1am on Monday.