TO MOST people, a slow boat to China is the name of an old Hit Parade number. But to one type of dedicated traveler it is the formula for a perfect trip.

For the freighter travel buff, happiness is a cargo ship meandering from one exotic port to another at a pace guaranteed to unwind and a price to make cruiseship passengers green with envy.

"Once you've taken a freighter trip you're addicted," says Merrian E. Clark, editor of the seminnual Ford's Freighter Travel Guide. Freighter addicts are as numerous as ever, but the number of passenger-carrying cargo ships has declined in recent years, and it often takes a long wait to catch that slow boat.

Freighter fans don't seem to mind. "They are usually retired people with time and money who love the sea," said Alexander Bock, a Boston travel agent who has specialized in freighter travel for the past 25 years. Midwinter cruises down the coasts of South America are a particular favorite, Bock said.

What is the attraction that makes people wait for months -- even years -- for the chance to share ship with a load of lumber or a cargo of canned hams on voyages where the ports of call are uncertain and the duration of the trip open-ended?

Price is part of it, enthusiasts agree, but even more important is the very special, almost undefinable, atmosphere of a sea-going cargo ship.

A freighter is both comparatively cheap and relatively expensive, depending on your point of view and what you want it to do for you. "It's definitely not the cheapest way to get from one place to another," said Clark. "I have to tell youngsters that all the time."

The cost of a freighter trip averages $40 to $80 a day and a typical voyages lasts a mimimum of 30 days, but often as long as 90 days -- all of which adds up to sveral thousand dollars paid out at one whack. So, as cheap transportation, a freighter can't compare with a budget airfare package, but it does compare favorably with the cost of a cruise. Cargo ship prices can be less than half those of a first-class cruisehip for comparable -- or even better -- accommodations.

Regarded as a cruise, a freighter trip can be a remarkable bargain. Consider the United Yugoslav Line's around-the-world sailings, on which berths are so south after by freighter travelers that there is currently a two-year waiting list.

The Yugoslavian company operates three cargo ships, two of which carry eight passengers each and a third accommodating 12 people. All sail regularly from California -- usually Long Beach -- and return to the West Coast, Seattle as a rule, after circumnavigating the globe with stops in Central and South America, the Mediterranean, India, Southeast Asia and Japan. The trip takes anywhere from five to seven months since the ships spend about five or six days in each port. The price ranges from $5,500 per person for a double cabin on an eight-passenger vessel up to $7,000 for a suite. A similar voyage on a luxury liner would be at least double or triple that.

Like most postwar passenger-carrying cargo ships, the Yugoslav vessels are extremely comfortable. They have air conditioning, spacious cabins, a lounge with a bar selling liquor and cigarettes at duty-free prices, and a private dining room for passengers. There are no pools, although many other freighters do have them, but passengers have deck chairs and their own lounging areas. Typically, meals are from a fixed menu with dishes reflecting the national taste of the crew -- Serbo-Croatian cuisine in the case of the Yugoslav line.

Typical also is the two-month leeway in the length of the voyage.

"We always stress that departures and arrivals are 'on or about' and that you have to have the time and be flexible," said Mary Leblanc, vice president of Freighter World Cruises of Pasadena, Calif., which acts as an agent for a dozen or so of the principal cargo lines that carry passengers.

But if you're not obsessive about punctuality or on a tight schedule, a freighter's ramblings can work to your advantage. Usually, passengers pay in advance for a voyage of a certain number of days. If the voyage proves to be longer, most lines don't charge for the extra time on board.

Leland Pledger of Salem, Ore., editor of the Freighter Travel News newsletter of the Freighter Travel Club of America, recalled one member couple who recently sailed on a freighter to Greece. The ship was so often delayed in route, by everything from engine trouble and striking stevedores to political disturbances, that it fell weeks behind schedule. The passengers, for whom the delays meant longer shoretime in Mediterranean ports, were not unhappy -- and the original price covered the entire long voyage. "When they got home, they figured it cost them about $18 a day," Pledger said.

The Freighter Travel Club was founded in 1958 as a clearing house for freighter information. Many of the longest freighter voyages -- the ones to the Orient -- begin on the West Coast, and interest in freighter travel is quite strong there. But there is no shortage of enthusiasts on the East Coast, either.

Although the average freighter voyage is a month or more, there are also trips that can be squeezed into the traditional two-week vacation. And, since the tramp steamer of legend and lore is pretty much a thing of the past, many freighters now sail on regular schedules. One of the most popular freighter trips out of the Northeast is the United Brands banana boat which shuttles weekly between Albany, N.Y., and Panama or Honduras. The roundtrip takes about two weeks, usually with a short layover in Central America, and the cost is $400 for the Honduras trip, $430 for the Panama voyage. The vessels on the run vary, as do their national origins, but generally carry two to seven passengers.

One of the shortest -- and cheapest -- East Coast freighter trips sails every 10 days, year round, from North Sydney, Nova Scotia, to the tiny, foggy but thoroughly French Island of St. Pierre. The trip takes 15 hours and the price is $40. The freighter, "Ile de St. Pierre," carries automobiles as well as general cargo.

A few cargo ships, such as those of the San Francisco-based Delta Lines, carry as many as 100 passengers and have an on-board ambience much like that of a cruiseship. But most freighters carry only from two to a dozen passengers. This makes freighter travel an intimate experience, an attraction for many travelers but a put-off for others. One of the commonest questions asked by would-be freighter travelers is: "What if I'm stuck on a long trip with people I don't like?" It does happen, but apparently not very often.

"People who travel by freighter are really very special and very nice -- they usually get along," said Clark. They also tend to be of similar ages and backgrounds, sharing interest -- such as a love of freighter travel. And they are almost invariably vigorous, with papers to prove it.

Since most freighters don't carry a doctor, shipping lines general require passengers over 65 to get a certificate of good health from their own physician. Some companies won't carry passengers over a certain age, but the limit varies from line to line and can be as high as 79. Freighters are wider, and because of their cargo, generally heavier and lower in the water than cruise ships, which makes for less rolling -- a feature often appreciated by older passengers.

As a rule, freighter passengers are veteran travelers -- most were turned on to sea travel by cruise ships -- and well able to entertain themselves without the guidance of a cruise director. They usually have to, since recreation at sea consists mainly of reading, playing cards, exploring the ship, chatting with crew and fellow passengers, or just reclining in a deck chair. Meals are family style, usually taken with the ships's officers and without the dressy formality of cruiseship dining. It's a regime that many travelers find habit forming.

"I prefer a real freighter," said Mrs. David Fisher, 73, of Falmouth, Mass., a cruising buff who has gone around the world on luxury liners and sailed on larger passenger-cargo ships. Her favorite voyage, however, was one that took her around the horn of Africa on a 12-passenger freighter. "It took 2 1/2 months just to reach Capetown, but the captain was gracious and the passengers very congenial," she recalled. "I'd love to do that again. Freighter travel is an excellent value -- and you meet interesting people."

For some travelers, the romance of the sea is a real lure. Peter Domker of Worchester, Mass., who has made several freighter trips between the United States and his native Netherlands, said: "I find it very exciting to go across the ocean: The eerie feeling when you move through the fog, the roll of the ship, and being able to talk to the crew and walk through the engine room. It's not like a cruise ship. It's adventuresome -- and you're part of it."

Most cargo ships being built now do not have room for passengers. "It seems to have leveled off now, but in the '70s we saw a lot of passenger freighters disappear," said Clark. Her freighter itineraries -- about 150 less than it once did.

Because of the widespread use of containers, freighters are also spending less and less time in port. "It's getting to be mainly a boat ride," Pledger said. "What people like is break bulk cargo that takes several days to unload."

Most major ports in the United States and North America are containerized, but progress comes slowly in other parts of the world. So, for many years to come, old-fashion freighters will still make their leisurly way into sleepy, sunbaked ports where cargo will be laboriously hauled from the hold on the backs of native laborers while crew and passengers explore the shore at their leisure.

They will be anachronisms, these ships, but there will be waiting lists to sail on them.