AT 11:15 A.M. MONDAY, the 220-foot general cargo tramp steamer "Mini-Lace" left a wharf in New Bedford, Mass., took a harbor pilot aboard, and headed out into the afternoon southwesterly on Buzzard's Bay.

The tough fishermen left behind on the quayside, busy welding their rigs and preparing to take on fuel oil, hardly followed her with their eyes. Ships have come and gone from New Bedford for 300 years. And they go for profit, or they come to scrap.

The unglamorous "Mini-Lace" was going for profit too -- but when she got beyond the breakwater, she she did something a merchantman hasn't done there in many generations.

She spread a sail to the wind.

On her foredeck, a single 100-foot high unstayed mast turned slowly, unrolling the 3,000 square feet of Dacron sail furled around it. On the bridge, Capt. Stylianos Vlitas judged the angle of the breeze by eye, touched a few buttons on the console before him, and trimmed his sail to the 12-knot breeze.

Under power, the Mini-Lace was making her service speed of 8 knots. But when Capt. Vlitas cut her engines back to idle, the 3,000-dead weight-ton ship slowed, but she did not stop. The silent press of wind on sail kept her moving at a steady 3 to 4 knots.

The ship tacked a few times, trying out her new rig, and by 6 o'clock was back at her pier again. New Bedford was pretty much the same, except that the fishermen had gone home.

"I would characterize that voyage as a major milestone in maritime his tory and the rational use of energy in bottom-line economic terms," said Lloyd Bergeson the next day from his office in the Windship Development Corp. Bergeson's company designed the Mini-Lace rig. She is the first commercial cargo ship modified to take advantage of the wind since the age of sail ended.

For the first time in this century, a cold-hearted look at the economics of the merchant marine indicates that sail power can turn a better profit that diesel power alone.

The United States Maritime Administration (MarAd) took an earlier look in 1974 -- after the oil crisis of the year before had sent prices from $2 to $11 dollars a barrel. That study, conducted by the University of Michigan, concluded that sailpower for the merchant fleet was feasible -- but marginally unprofitable.

Now a barrel of marine bunker costs $33, and the picture has changed. Today, a 100,000-dead weight-ton foreign flag tanker pays $11,000 a day for fuel alone -- 65 percent of its total operating expenses. For American flag vessels, whose labor costs are higher, the figure is 40 per cent. The 25,000 principal commercial ships operating today now account for more than 8 percent of the total oil consumption of the non-communist world.

This spring, MarAd issued a new report -- the result of l4 months of study by Windship. It declares that small to medium-sized ships, equipped with automated sail-systems backed up by conventional power plants, offer fuel savings of 20 to 30 percent, are competitive in building or retrofit costs, and promise total operational savings of 5 to 15 percent.

"There is no romanticism about this," Bergeson says. "This is the bottom line. Eventually, I believe, 20 to 40 per cent of the world's fleet will be sail assisted."

Ceres Hellenic shipping company, which operates a fleet of 100 vessels in its Mini Line, has also contracted to have a second vessel fitted this fall with a more advanced sail plan -- a vertical airfoil called the wing-mast, which computer studies suggest will be more efficient still.

The MarAd report selected the wingsail over a varied of other commercial sail plans, from the Dynaship rig -- a modern version of the square rig of old -- to such exotic designs as the wind turbine, a deck-mounterd windmill which would drive the ship's propellor. Windship, which is selling sail to shipowners tailored to specific cargo needs and sea routes, is pushing the wingsail.

Hard data, however, is not yet available from the sea trials of the Mini-Lace, and Bergeson, a former shipyard manager with a Yankee shrewdness, declines to report even her fuel capacity at this stage.

He feels that misplaced romanticism has already hurt the cause of sail power in the U.S. merchant fleet, and that one sarcastic newspaper editorial in Washington set his own research program back a year, giving the Japanese a head start in development.

The head start was the 1,600-ton Shin-Aitoku Maru, the world's first sail-equipped oil tanker. Launched last summer, the ship has folding wing sails and a computer that automatically adjusts engine speed to maintain a given timetable. With the wind at 33 knots at a 90-degree angle to the vessels course, The Shin-Aitoku Maru theoretically cruises at 15 knots by sail alone. Her builders report that the sailing rig was installed for a cost of $260,000. It has been predicted that the sails will cut the ship's annual fuel cost of $442,000 by 50 per cent.

These figures have not yet been supported by evidence, but the lure of a free energy source has attracted interest from the U.S. Navy to individual maritime businesses--all of whom see the potential of the wind as a free energy source.

Kenneth C. Morisseau, a mechanical engineer with the Naval Sea Systems Command, has proposed that the Navy use sail power systems for fleet tugs, tenders, and other support ships.

Meanwhile, in Norfolk, Va., a colorful tugboat skipper has fitted his boat with two masts, dubbed her a "tugantine," and set out to cut his fuel bills in half -- which could save him $500 on the round trip to Baltimore. Several West Coast tuna fishers have rigged sails to their boats. An oil rig, being towed from the Gulf of Mexico to her station in the North Sea, was fitted with a single Dacron sail. The ungainly tow was said to have gained a half-knot of speed for some of the trip.

Perhaps the grandest sail-power proposal of all, and the one that comes closest to reviving the glamour of the great square-riggers, is the much-discussed Dynaship concept.

The Dynaship is a modern square sail plan for a vessel as much as 500 feet long, with six 200-foot masts, each with five sails which could be set and furled mechanically without the need for the crew to go aloft. Such a ship would have potential average speed of 11 knots over appropriate trade routes, under sail power alone. The design was developed in the Shipbuilding Institute at Hamburg, Germany, in the 1950s, and United States license is held by Dynaship Corp. of Palo Alto, Calif.

So far, no Dynaship has been built. To Bergeson, it is an outmoded concept. He belives that the complicated rig would be unacceptably expensive to build and maintain, wind resistance from its towering spars would make motor-sailing impractical, and that it would be unable to pass under the bridges of waterways such as the Cape Cod Canal, which leads to Boston harbor.

William Warner, president of Dynaship, has not come to that conclusion.

The one vessel his firm has at sea is a 53-foot sailing fishing boat with a 20-ton refrigerated hold. The "Feresa," named after a pygmy killer whale common to her operating area off Hawaii, is rigged with automatically furling triangular sails not unlike those of a yacht. She has a small diesel engine, but can make 8 knots steadily under sail alone.

"The Feresa sailed from Seattle to Honolulu, and she's been operating there for two months, and she still hasn't had to refuel," said Warner. "The important thing is that she's already making money for her owners."

Warner is now talking to maritime interests in Chile about a 70-fsto 100- foot version of the Feresa. "There are plenty of fisheries applications," he says. "For example, there used to be a mother ship to store the catch at Midway, which is a thousand miles from Hawaii. The price of fuel made that trip unprofitable, but sail would work. And among the small islands of the Pacific trust territories, a lot of the trading has just stopped because of fuel prices. You wouldn't even have competition anymore."

"The point is that the Japanese are going after sail, and we have to, too. We have found a new way to make money, and that's the capitalist way. We're out there hustling."

There was a time, of course, when all ships were sail-powered, and in their day the wooden ships and their iron men set many admirable records of seamanship and endurance. If wind power is such a good idea, why did it die out in the first place?

First, in the mid-19th century it took crews of 100 or more to man the sailing ships, which required constant repair under way. When pressure built to improve their working conditions and increase their salaries, especially after the publication of Richard Henry Dana's "Two Years Before the Mast," reporting on the exceptional rigors of their life, the profitability of the vessels declined.

The mid-19th century was also the era of the clippers, the fastest sailing ships in history, with captains burned out at age 25 and vessels sprung and ruined after three years in service.

Ships like the James E. Baines, a square rigger which once averaged 21 knots across the North Atlantic, were less extreme in design than the clippers, but set records which yachts men still chase today and are hard pressed to match. But records are deceiving: they do not reflect the many voyages beset by headwinds, or calms, and the inevitable disruption of trade caused by overdue cargoes. An excellent average speed for a traditional five-masted bark of the period was 8.5 knots.

Ironically, by the time steam took over at the turn of the century, commercial sail had progressed to a remarkable efficiency. The 8,000-ton bark Preussen, built in Germany in 1902, required only 41 men to handle its 33 sails. That same year, the seven-masted gaff schooner Thomas W. Lawson was launched in Quincy, Mass. She set 40,000 square feet of sail which could be hoisted and reefed by only 12 men, assisted by steam winches.

Commercial sailing vessels thus be came obsolete at the moment of their first real efficiency, a fate similar to that which later befell the steam locomotive. Now, with modern technology, the efficiency of sail power can redouble again. Computers are available to show not only what shape sails work best, but to permit the entire rig -- formerly worked by scores of men swarming aloft under the most abominable conditions -- to be adjusted from a bridge consol at the touch of a button. Ships' crews need not increase in size, nor even study sailing: the ancient art of sail trim can be analyzed automatically.

The sailing ships of the 19th cen tury accomplished their missions in spite of problems easily solved today. Navigation was then as much an art as a science, whereas today ship's positions are tracked by satellite. Other satellites monitor the weather, allowing careful route selection to catch the best wind, as well as to avoid major storms. The square riggers of old had rigs of spruce and iron, lines of hemp and sails of flax or canvas, requiring constant replace ment and repair. Today's metallurgy, synthetic fibers and hydraulics would remove innumerable inefficiencies and expenses.

In the future the Windship Corp. envisions, tankers and freighters sprout wingsails, but continue to be fitted with substantial power plants. This development takes place among small to medium vessels, on routes with predictable winds such as the North Pacific and North Atlantic, and areas of heavy trading such as the Caribbean. Existing ships are retrofitted with sail plans, at costs they can make back in only a year or two by fuel savings of 20 to 30 per cent. When new keelples are laid, the expense of the sailing rig is saved by the in stallation of less powerful, and costly engine systems. Each application of sail to commerce is the result of careful computer analysis -- not trial and error.

The result is more profit for the merchant marine -- a profit which increases relatively as the cost of oil goes up -- and a reduction in the expenditure of irreplaceable fossil fuels.

"You know," said William Warner, talking about the 1,000-mile run from Oahu to Midway, where he would like to place a sail-powered mother ship for tuna fishermen. "It doesn't matter if you get there a little slower, if right now there's nobody there at all."

Change is already in the air. The Liberty Ships of World War II had a design speed of 10 knots, which came to be seen as impossibly slow. In years since, ship design pressed steadily toward more speed, until the Sea-land corporation's SL 7 contain ership came off the drawing boards capable of a full 33 knots.

The SL 7 was a state-of-the-art commercial thoroughbred, all right. But she gulped ever-more-costly fuel accordingly. So, this epitome of the oil-powered freighter has been with drawn from service. Just like the clipper ships almost 100 years ago.