Prime Minister Andreas Papandreou is learning that there is a big difference between managing an economy and theorizing about it as he used to do at the University of California at Berkeley when he was the chairman of its economics department.

A year into office as head of Greece's first Socialist government, Papandreou has been unable to work any magic on his nation's recession-numbed economy despite his previous eminence as an economist. The severe economic crisis he inherited upon coming to power last year shows few signs of going away.

Greece, like so many other nations today, has fallen on hard times because of the global recession that has slowed much economic activity to a crawl.

A visit to Eleusis Bay, 15 minutes from the traffic-snarled capital of Athens, shows how hard the times are for Greece. Lying at anchor, tied in rows of 10 or 20, bobs a vast fleet of hundreds of idled freighters and oil tankers that once were the pride of the Greek merchant marine, traditionally one of the biggest money earners in the Greek economy.

The world economic slump has put more than 600 Greek-registered ships -- almost 20 percent of the merchant fleet -- out of work, according to shipping officials here, leaving thousands of sailors unemployed.

In the first six months of this year, the shrinking demand for Greek ships in world trade has cost the economy $666 million, according to Western economists. Greek Central Bank officials estimate that by year's end the earnings from shipping will be $1.2 billion instead of the $2 billion for which the government had planned.

Add to this a 5.4 percent decline in foreign worker remittances and a 6.2 percent drop in tourist revenues in the same period -- the other two dominant foreign-exchange earners in Greece -- and the country's problems become obvious.

As an economist, Papandreou analyzes the economic problems facing him for visitors with the cold detachment of an academic. But as a politician he is aware that his dreams of "socializing" Greece's antiquated economic order are limited by multiple factors beyond his control.

Having been swept into office with optimistic promises of bringing dramatic change, Papandreou has found himself forced to temper the expectations he raised because of the economic realities he inherited and the grim world recession that offers little prospect of any economic stimulation from abroad.

"It could be that the prime minister's experiment in socializing the nation may never even be given a fair chance because of conditions that he cannot control," said one senior European diplomat sympathetic to him. "Socialism takes money, and that is something that Greece has precious little of today."

Papandreou these days increasingly talks of the art of the possible, and not of the visions of the future that once dominated his rhetoric.

While there used to be boasts of irrevocably setting in motion Greece's transformation during his first four-year term of office, aides now talk of a need for two or three terms to get the job done.

"We are convinced we can in fact modernize and transform Greece as we hoped," said Gerasimos Arsenis, the former governor of the Bank of Greece whom Papandreou elevated to the important post of minister of national economy in a Cabinet shuffle last summer. "But what we hoped could be done in four or five years could take much, much longer, especially if the world economic situation remains as it is. We have had to adjust to the fact we will have to go slower than we had hoped."

But in that slowness lies political trouble. Already there has been grumbling that the government has delivered much less than it promised. And it may give even less in the future.

In the government's efforts to reduce the 25 percent inflation and cut back on the huge $2.5 billion balance of payments deficit it inherited from the government of the New Democracy Party, Papandreou is planning new belt-tightening measures for next year.

In a recent interview, the prime minister said that because of a need to try to stabilize the economy, "we shall shape up an incomes policy for 1983 that will be substantially more conservative than the one this year."

That was necessitated, he said, in part by the partial failure this year of a policy that sought to raise wages quickly for lower income workers by about 35 percent, while holding back increases for higher earners in an effort to slow inflation.

Papandreou said, however, that perhaps because of the recession more than his own policies the balance of payment deficit this year may be down to $2 billion, and the inflation shaved a few points to 22 percent.

"It is not something to write home about," he said, "but it is still something positive." With money tight, with receipts down and the nation's foreign currency holdings under $1 billion, he has been forced to concentrate his social program on what economists here term "costless" reforms that nevertheless are expected to have a significant social impact.

These include such reforms as the legalization of civil marriage, the decriminalization of adultery, the restructuring of the university system, changes in the electoral laws and a plan for the eventual decentralization of authority that Papandreou hopes, when the money is available, will democratize society by placing much decision-making in the hands of local officials.

Nothing, however, can happen until more money can be found to fund the other social and economic programs the government wants to implement.

A new investment law passed in the spring was supposed to attract both foreign and local capital. So far, it has not.

Arsenis, the minister of the economy, is working on a repeatedly postponed five-year plan that, he insists, will seek to revive the economy through an aggressive investments policy, export reorientation and competitive import substitutions. He readily admits, however, that success still hinges on the world economy.

Musing over his problems recently, Papandreou recently sounded almost nostalgic for his old job at Berkeley.

"A friend recently asked me how it was I was having so much trouble making changes here when I was able to make so many changes at Berkeley in my department," he said, "and I was forced to realize how simple things were there as compared to here."