Miami-based SunStone Ships has signed a deal for two more Infinity-class expedition cruiseships at China Merchants Heavy Industry.

The vessels are the fourth and fifth in the series. Sunstone said it is in talks for additional vessels and it is expected that the remaining five options will be exercised within 2019.

The first of the potential 10-ship series, Greg Mortimer, is scheduled for delivery in August 2019 for charter to Aurora Expeditions which has taken a 50% stake in the vessel.

Also currently under construction are the Ocean Victory for delivery in September 2020 and Ocean Explorer in March 2021.Ocean Odyssey is pencilled in for delivery in May 2022.

Aurora Expeditions is taking two vessels on charter and Victory Cruise Lines a third. It has just been announced that SunStone has reached agreement to sell Victory to American Queen Steamboat Co which will take over the charter as part of the deal.

SunStone is unlikely to have ordered any of the ships without firm charters lined up.

The Infinity series vary in passenger capacity between 130 and 200, with a crew capacity of between 85 and 115.

The vessels will all be built to Ice Class 1A, Polar Code 6, with Safe Return to Port, dynamic positioning and zero speed stabilisers.

“We are very pleased with the development and speed with which we are signing these newbuild contracts,” said SunStone’s president and chief executive Niels Erik Lund.

SunStone is owned by Lund and three other Danish investors, including Clipper Group.