In July and August the population of Oinoussa, a small island lying between Chios in the north-east Aegean and the coast of Turkey, jumps from around 500 people to more than 3,000.

With fewer than 30 hotel beds, the island offers few facilities for ordinary tourists. Its summer visitors are Greek shipowners with roots there, their extended families and guests.

Some open up refurbished family mansions built during Oinoussa’s glory days early in the 20th century. Others stay on yachts in a picture-postcard harbour protected by two offshore islets, each capped with a whitewashed church.

Oinoussa is unique among the 40-odd remote Greek islands that are still inhabited year-round, thanks to years of carefully targeted philanthropy.

“If the shipping families hadn’t stepped in, the island would have been abandoned – everyone would have moved either to Chios or Athens,” says John Pateras, an Oinoussan ship operator.

Situated on a main Mediterranean trading route, within easy sailing distance from Chios and the coast of Anatolia, Oinoussa was once a showcase for the Greek shipping industry.

From the 1860s, Oinoussan families pooled their resources to build ships that were crewed and managed by family members. In its heyday, the Oinoussa-controlled fleet numbered about 300 ocean-going ships and its leading families opened offices in Piraeus and London.

The island fell into decline after the second world war. To rebuild their fleets, shipowners needed access to modern communications, banks and brokerages.

The rescue effort was led by the Pateras and Lemos families, both among the island’s wealthiest. They funded much of the infrastructure that makes it possible for families to live on Oinoussa year-round – and encouraged other owners to participate.

The donations include two high schools, a full-time doctor’s position, a desalination plant, and an island-based car ferry that makes a daily return journey to Chios. The government paid for undersea cable links with Chios providing electricity and, recently, broadband internet access.

“If you have children, it really matters to have a doctor on call and a regular water supply,” says Mariana Iliopoulou, a graduate in hotel management who runs the citizens’ advice bureau and mayor’s office.

Education has traditionally been a priority for Greek philanthropists. The island’s high schools – one, a vocational school for training merchant navy officers – have 65 students and 25 teachers, the country’s highest pupil-teacher ratio.

Facilities include luxuries unknown in the state education system, such as “smart” classroom whiteboards linked with personal computers, and optional tennis lessons.

Students who win a university place receive grants equal to a junior civil servant’s salary from the island’s benefactors. Graduates of the merchant navy school are guaranteed well-paid jobs, given that the fleet under the Greek flag is short of officers.

Panayotis Kounis, head-teacher at the high school and curator of a generously endowed naval museum, has the delicate task of matching the philanthropists’ proposals with the island’s needs.

“It’s good to have a football stadium, but we also appreciate having an amphitheatre with excellent acoustics for music and theatre performances,” he says.

Evangelos Angelakos, the mayor, is trying to reduce Oinoussa’s reliance on hand-outs from wealthy expatriates. He wants to see a limited amount of high-end tourism – such as university summer schools and small-scale conferences – that would create new jobs and attract more permanent residents.

A shipowner who divides his time between Piraeus and Oinoussa – a six-hour commute by motor-yacht – he pursues funding from European Union programmes and the government budget.

“My job is to make sure the island doesn’t miss out on money it’s entitled to. Because of Greece’s administrative weakness, it’s a time-consuming process that has to be done from Athens,” he says.

Under his stewardship, the harbour has been enlarged to make room for larger vessels.

EU funds have been secured to help finance new water distribution and waste collection systems, and a waste treatment plant. Completing a ring road that would open up more beaches to tourists is also a priority. A luxury resort project is under discussion. Making Oinoussa a regular stop on the ferry route from Piraeus would put it properly on the map, he says.

"The philanthropists have done a tremendous job for the island but, inevitably, that’s created a culture of dependency. We need to revive a spirit of entrepreneurship," he says.