A group of environmental activists have set sail from Amsterdam on a seven-week voyage to South America to attend the UN climate conference.

It had been raining on the Amsterdam waterfront on Wednesday but the sun came out in the early afternoon as a small crowd waved farewell to the 36 activists and five crew on the boat.

Over the next seven weeks, they will face long days on the churning Atlantic ocean with no land in sight. But their challenge on arrival is no less daunting: to convince the world’s governments meeting at COP25 in Chile that emissions from flying – the quickest, cheapest way from the Netherlands to Santiago – must be curbed urgently to prevent global heating reaching calamitous levels.

“We really want to make a statement that we have to stop subsidising the aviation sector and we have to start investing in these alternatives,” said Anuna De Wever, a founder of Belgium’s school strike movement, who aged 18, is one of the youngest on board.

As well as learning how to sail the Regina Maris, a three-mast schooner propelled by wind, activists will brainstorm on a plan for greener transport, which they plan to present to the world’s governments in Santiago.

“We will mostly depart with a question, [which is] what can the sustainable and fair future of flying look like,” said Jeppe Bijker, one of four initiators of the Sail to the COP project.

As well as asking government officials to tax aviation fuel, there is also a message to the public that chimes with the no-fly movement epitomised by Greta Thunberg, who recently completed her own transatlantic voyage to New York in a zero-carbon boat. The Swedish teenager, who inspired the global school strike movement, is due to meet the group in Santiago.

But Bijker said he is not calling for people to give up flying completely: “But we do ask them to think about why are you flying. Where do you need to go? Do you really need to go? Maybe it’s a business flight … and there is an e-conference solution. Do you need to take short-haul trips four times a year or could you fly once for a longer trip?”

The group are urging climate policymakers to take action following reports that emissions from aviation are rising even faster than predicted, prompting alarm that flying will derail meaningful action on the climate emergency.

The plan to sail to the COP began life as a WhatsApp group between Bijker and three fellow Dutch climate activists, who got to know each other through the green transport cause.

By the middle of this year the project had snowballed into a project uniting climate activists from nearly a dozen European countries, as well as NGOs and universities, which have offered financial and research support.

Most were strangers to each other until a few days before leaving Amsterdam. Now they will be thrown together 24 hours a day on the 45 metre-long ship, sharing cramped four-person dorms and learning the ropes of sailing.

Vegan meals will be prepared by one of the crew in the tiny kitchen and they will stop along the way at Casablanca, Tenerife, Cabo Verde and Recife to pick up fresh fruit and vegetables and touch dry land for a few hours.

Unlike Thunberg’s zero-emissions boat, there is an engine to bring the boat into port and avoid days becalmed at sea.

“We will try to use it as little as possible but we do want to arrive on time,” said Bijker. The Regina Maris is due to dock at Rio de Janeiro in mid November. From the Brazilian coastal city, they will take a bus through the Andes to complete their journey to Santiago, to attend the COP starting on 2 December.

“We are a lot of people and it’s a small space,” said Adélaïde Charlier, another co-leader of Belgium’s school strike movement, who like De Wever and Thunberg has put her university studies on hold for climate activism. Drawing a parallel with the world’s limited resources, she said: “I think it’s part of the symbolic message to be on a small boat for this long with that many people.”

As the wind whips over Amsterdam’s IJ river and the boat bobs in the sway, she admitted worries about sea sickness and the “scary and exciting” journey ahead. “I think it’s going to be a hard journey but it’s necessary for the message. We have this climate urgency and for almost a year we have been trying to find as many as possible actions that would bring back the topic on the table.”

Participants do not know yet how or when they will return to Europe. Some are looking at a return journey on the Regina Maris next spring, while others are thinking of cargo ships. “That’s also part of the message,” Bijker said. “It’s more that we want to show how hard it is to get to the other side of the world without flying and basically how strange it is that it is this hard and why are there not more alternatives.”