"A lot of lonely hearts want to sail." Captain Giles Kelly smiles, his blue eyes twinkling. "Sailing is a romantic, beautiful and challenging sport. It uses the forces of nature, and with the emphasis we're putting on back-to-nature these days, sailing fits right in."

Captain Kelly, a retired U.S. Navy officer and adjunct professor in American University's Physical Education department, teaches the sport for both AU and Open University, and says he's delighted to see that interest in sailing, even among contented hearts, is growing each year. Even better, people are doing their sailing right here in town.

"Annapolis is so crowded," Kelly says, "why go all the way out there to learn? The Potomac is lovely and as clean as it's ever been, there's a large area to sail in, the winds are fine, it's not too choppy, and we have a first-rate harbor patrol -- it's an ideal place to learn and it;s terrific for day sailing."

Along with other harbingers of spring like the daffodils along Rock Creek Parkway, white sails are already popping up on weekends, moving briskly along through the waters of the Potomac, into the Washington Channel and up the Anacostia. Marinas on both the D.C./Maryland and Virginia sides of the Potomac are now taking signups for hands-on sailing classes, which usually start mid-April. Early birds are already in the middle of hands-off lessons (taught entirely in the classroom) so they'll be ready to take advantage of balmy spring winds.

Dave Carvey, who teaches a hands-off class two evenings a week for the U.S. Coast Guard Auxiliary, echoes Kelly on the surge in sailing interest: "Last year we could hardly give this class away, but this year 48 people showed up! I guess it's a sport that almost everyone can do now, easily and relatively inexpensively." Local marinas offer day-sailers for rent at a reasonable cost and several have sailing associations that certified sailors can join. (The season fee entitles members to unlimited sailboat use.)

And so, landlubbers, you can cast off without leaving the area. If you want to set sail this summer, or even if you just want to crew for friends, it's time you knew a jib from a mainsail. Start with a hands-off course, if you can find one. The Coast Guard Auxiliary, a hard-working group of volunteer boatmen (and boatwomen), runs many evening courses around town, usually listed according to the number of the flotilla (local unit) that runs them. Five to ten flotillas make up a division (area unit), and there are 17 divisions in this district (four-state area).

The standard auxiliary course consists of 12 or 13 two-hour lessons, given once or twice a week; but many of these are almost over, because the teachers have sensed the season and boarded their boats, helping the Coast Guard patrol. The one class that you can join now is listed below; others may be held later this spring, and many more start again in the fall.

Why learn sailing in the classroom? According to Dave Carvey, one of the teachers, "Sailing is a physical process, but it also involves a process of thinking. You have to be able to combine the two to become a good sailor. We take people through the steps with diagrams, pictures and slide demonstrations until they become familiar with the process. Then, when they get on a boat, they don't have to learn everything at once. Some of the information is already internalized." Many people in Carvey's class have been sailing for years, but they want to go through a systematic class to learn the fine points. Others say they've only sailed in enclosed areas and need expertise in navigating open waters, or feel the need of other kinds of expertise, Coast Guard Auxiliary classes are well-organized and taught by pros. Best of all, they're free, except for the cost of the textbook, which comes complete with homework questions.

After you've taken a hands-off course (or even if you haven't) you can pipe yourself aboard one of the many hands-on courses offered at marinas along the Potomac.

Captain Kelly's classes are a typical example of how hands-on courses generally proceed. The first of his eight classes is a lecture at the Gangplank Marina, on rigging, safety, winds and many of the same topics auxiliary courses go through in detail. After that, each person in the class picks a partner and signs on for two years before the mast -- or what seems like two years but is really only two hours.

Partners take turns serving as skipper and crew, as Kelly shouts instructions through a megaphone. Nervous first-time sailors soon learn how to make figure-eights around buoys without capsizing or being blown up to Baltimore.

According to Kelly, "After they get over their initial fright, people's eyes shine, and they are really pleased when they realize they can do it." Most students start enjoying each new challenge, and by the end of the eighth class they can sail their boats on a triangular course and even do some racing.

Students who complete these kinds of hands-on courses are usually given certificates that say they're able to handle small boats (the Gangplank's are 15 foot sloops; Belle Haven Marina offers 19 foot Flying Scots and others teach on 20- or 21-foot boats), and after that they can rent sailboats almost anywhere for day or weekend jaunts.

By the end of a hands-on course, people either know for sure that sailboats are not for them or are permanently hooked. (Kelly says he can predict who will come out on each side). And permanently hooked often means the eventual purchase of a boat, so if you're planning to take a sailing course this year, start saving now.

But "permanently hooked" also means great fun, adventure and, yes, even romance. Keep this in mind: Once a month during the winter Kelly takes former students down to the Virgin Islands for ten-day cruises on larger boats, usually six to a boat. What could be more romantic than sailing with someone special under the tropical sun and stars? Meantime, here's an assortment of courses that will help you sail into spring. HANDS-OFF CLASS

For information about future classes, call Charles Payne, the public education officer for Division 14, at 703/451-3903. When you call make sure to mention that you're interested in S&S (Sailing & Seamanship), not BS&S (Boating Skills & Seamanship, mainly for power boaters).

BY FLOTILLA 2-2: Tuesdays and Thursdays 7:30 to 9, starting April 13, Kenmore Junior High School. 250-4933. HANDS-ON CLASSES

AMERICAN UNIVERSITY, Division of Continuing Education and Summer Session. Sailing and Seamanship, taught by Captain Kelly and associates. Course n017, four Wednesdays 5:30 to 7:30 and four Saturdays 9 to 11, starting April 7 or April 10, at the Gangplank Marina, 600 Water Street SW. $150. Certified graduates can join Gangplank Sailing Association, $300 for season, $150 for half-season. Call 686-2500.

OPEN UNIVERSITY. Basic Sailing, taught by Captain Kelly and associates. Four Thursdays 5:30 to 7:30, and four Saturdays, 11 to 1, starting April 8 or May 6, at the Gangplank Marina. $130. 966-9606. (Open U. also offers sailing courses taught in Annapolis and Galesville.

BELLE HAVEN MARINA, off the George Washington Parkway, 3/4 mile south of Alexandria. Basic adult learn-to-sail classes. Series of four lessons, 21/2 hours each, with textbook. Classes offered all summer (weekdays, evenings and weekends) starting April 3. Maximum of three students per class. $110, slightly less if two or three sign up together. 768-0018.

BUZZARDS POINT MARINA, Half and D Streets SW. Individual lessons onl 292- 227.

WASHINGTON MARINA, off the George Washington Parkway south of National Airport. Red Cross basic adult sailing program taught here 16 years by Woody Vashaw. Series of five classes, 6 to 8:30 Monday through Friday, May 24 through August 20. Test for Red Cross certification. $60. 548-0001. (Also childrens' sailing classes)

Captain Giles Kelly private lessons, $18 to 20 per hour. 244-5633.