ST. MARY'S CITY, MD. -- Sailing close to the wind, the 60-foot Cayenne cruised down the St. Mary's River last week, its student crew of eight exhilarated by the experience of operating a world-class racing boat for college credit.

The flagship of the fleet at St. Mary's College, where more than half the students take up sailing and the boat-to-student ratio is among the best in the country, was the tax-deductible gift last December of Don Tate, a builder of oceangoing yachts who lives near Annapolis.

"Last-minute tax planning," Tate termed it then.

"The gift that keeps on giving" is what St. Mary's College and its 1,200 students call it.

Either view is equally correct. For the donor, it is a tax bonanza. For the college, the boat is a prized acquisition, usable immediately to enhance a sailing program and later, at sale time, as a source of cash.

Indeed, it was the second time around for the spartan Cayenne, a venerable racing boat donated in 1976 by its prior owner to the Naval Academy, which sold the boat to Tate. When Tate couldn't get his $120,000 asking price last year, he had it appraised at $200,000 and instead gave it to St. Mary's College, in a rural corner of Southern Maryland where the Potomac empties into the Chesapeake Bay.

The gift of the Cayenne capped a banner year of yacht donations that increased the riverside school's fleet by half, to 20 good-sized sailboats. In fact, the value of boat donations, $530,000, far exceeded the $322,000 in cash contributed to the state school's nonprofit fund-raising foundation.

Last year brought record boat donations as well to the U.S. Naval Academy in Annapolis, which received 15 boats valued at $4.6 million, and to several other schools, including Stanford University, which accepted 37 boats of over 200 offered. Washington College, a small Eastern Shore school in Chestertown, received nine boats last year, of a total of 16 given to the school over the last few years.

But the boat windfall may be over: This fall, as the sailing season draws to a close and tax season approaches, college officials are expecting no similar largess from donors. The shifting tides of tax law, they say, mean that yacht donations are ebbing, and that those who give boats in 1987 must be motivated more by philanthropy than by tax breaks.

With the top income tax bracket reduced from 50 percent last year to 38.5 percent this year and to 28 percent in 1988, the effective cash value of a tax-deductible gift has declined sharply.

"The goose that laid the golden egg may be dead," said George Curran, executive director of the Naval Academy Sailing Foundation, which to date this year has received no boats. "I'm not very hopeful," he said.

"The goose is very feeble right now," agreed Alan Hamerstrom, an Annapolis yacht broker whose firm handled the donation last year of a 39-foot boat to St. Mary's College. "It's been declining as the tax structure has changed. As the tax rates come down, the donations have also come down."

An Internal Revenue Service spokesman said he could find no specific rulings regarding the donation of boats, but school officials, yacht brokers and others say the IRS began tightening up on such gifts in 1985. At that time, it started requiring institutions that sold gifts in less than two years after receiving them to report the transaction. That acted as a check on inflated appraisals used as the original basis for tax deductions.

School officials say last year's frenzy of yacht giving was fed by the imminent tax law change but also by other factors. They cited a continuing soft market in used boat sales due to rapid design changes that rendered even recently built racing boats obsolete. Depressed fuel prices also affected the market as yachtsmen sought to unload sailboats to buy power boats.

In a pamphlet prepared for yacht brokers in 1984, the University of Delaware said bluntly: "A donation of property is usually assumed to be a philanthropic gesture by the donor. However, when dealing with vessels, this is rarely true. A potential contributor usually wishes to receive as much as possible in monetary consideration, whether in green cash or tax benefits."

For cash-poor boat owners, the University of Delaware also offered a "bargain sale-tax deduction" combination, in which the donor received some cash and a reduced write-off. Delaware, which no longer has such a program, sold the boats and used the proceeds to support its marine research vessel.

So many boats were offered that schools with waterfront programs routinely turned many of them away. Several of the St. Mary's rejects wound up at Washington College, which owns 13. "We put them in business," said Mike Ironmonger, director of St. Mary's waterfront program.

St. Mary's and Washington College have received but one boat each this year. Unlike the Southern Maryland school, however, Washington College, on the Chester River, has yet to establish a student sailing program.

"Most are drydocked," said David Fraboni, associate development director. "A couple are used by faculty," and, occasionally, to entertain financial contributors. The primary purpose of the college's fleet, Fraboni said, is "for investment." But the school plans to sell them after two years and use the proceeds to buy boats "more appropriate" for student use.

Argued Fraboni, "It's not what it was, but it's still the best tax break you can get." Among last year's donors, he said, was a man unable to sell his boat for $25,000 who reaped the book value of $50,000 as a deduction.

Stanford, located near San Francisco Bay, one of the premier sailing areas of the West Coast, has 16 donated boats in use and another 15 or so for sale. The school is using the sale proceeds to replace its entire fleet of 17 small racing boats, said head sailing coach Joe Petrucci. Donors of vessels that are sold can have a small sailboat named after them. "We offer donors a namesake. It's quite effective," Petrucci said.

"The type of donor we're expecting from now on is a strong supporter of the Stanford sailing program, or a distress situation, where a family member passed on and the heirs don't know what to do with the boat."

When the wife of Dr. Raymond Brown, a retired Annapolis surgeon, died in 1983, he had to decide what to do with their boat, the Hunky-Dory, built in 1938 for offshore fishing. "I was looking for some place where it would be taken care of, appreciated. I wasn't interested in having someone sell it."

Through a friend who is an Annapolis yacht broker, he found St. Mary's, donated the boat and realized a tax deduction of $22,000. But Brown gave more than he got. Since handing over the boat in 1984, he also has given the college about $25,000 for upkeep, including $10,000 for a new engine.

The sailing program at the school, which boasts the highest SAT scores in the Maryland state system, has grown extensively under Edward Lewis, president of the college since 1983. Aside from formal classes in sailing and windsurfing, there is a varsity sailing team that is nationally ranked, and students and faculty may take the boats out for recreation.

In addition to its fleet of 20 large sailboats, the school keeps 20 smaller ones and 14 sailboards for windsurfing. "We have a nice little fleet," beams Ironmonger, gazing at the boats docked at the school's "minimarina."

Each August, St. Mary's College sponsors the Governor's Cup, which has grown into one of the largest regattas on the East Coast. It also competes in the Chesapeake Bay Yacht Racing Association and keeps four boats docked in Solomons, where Wednesday night races are regularly held throughout the season.

The school also uses its fleet for public relations, the waterborne equivalent of a competitive football program in its ability to create good will among political and alumni leaders. This summer, for example, the Maryland legislative black caucus was taken for a sail.

Yachting visitors to the college, who tie up free at the pier, are given a "waterfront packet" that includes passes for the campus pool and showers and a "Dear Fellow Yachters" letter from waterfront director Ironmonger noting the school's "very active boat donation program."

Lewis said the program provides a unique opportunity for young landlubbers to become old salts. "We're on the water. It would be a shame not to take advantage of it," he said. "We are a sailing school."

There was no mistaking this fact on the water last week, as the advanced sailing class took out the Cayenne for the first time.

"I'm majoring in biology here with a strong interest in sailing," said freshman Ted Sensenbrenner, 18, of Parkton, Md. Exclaimed Stephanie Colen, an 18-year old sophomore along for the ride, "God, I love this school!"