JUST AS the starting horn sounded the mast snagged in a picturesque cherry tree - not a good beginning for a sailboat race. Before I could shake loose, the boat was dotted with wilting cherry blossoms and the 37 other El Toro sailors who had come to the Tidal Basin to race Saturday were long gone.

Taking off in cold, damp pursuit, I knew immediately I was out of my league. I have sailed in the Caribbean, to Bermuda and all over Chesapeake Bay and felt comfortable in sailboats up to 40 feet, but the borrowed eitht-foot El Toro was baffling.

All the rainy way to the first mark I kept the mainsail tight; every time the wind shifted a little I would come about. By the time I rounded the mark I was practically the fleet laggard.

At the jibe mark, where boats change course 90 degrees, I was still far back. Then I slowly began to creep up. Imp, which belongs to Ron Kiss of Rockville, was faltering and seconds before we arrived at the leeward mark I shouted, "inside overlap."

I was so exhilarated at actually passing somebody that for a moment I lost my concentration and before I could regain it Kiss had gone ahead again.

From then on it was nip and tuck; I would take a nip - it was awfully cold - and Kiss would tuck away a little more of the lead. Toward the finish line, the wind favored me slightly and I slipped over it a good six inches ahead of Imp. I had beaten Kiss and two other skippers and earned 35th place in the first of three races in the fifth annual Cherry Blossom Regatta.

At the start of the second race I was so confident that I went right down to the favored end of the line where the hot shots were congregating. I was about to squeeze between 10 boats and the marker when someone shouted, "Hey, you're barging." Barging isn't anything you want to try unless you're sure you can get away with it. Since I couldn't, I had to do a 360-degree turn and drop way, way back.

On this circuit I gradually learned that too much sail trimming of an El Toro cuts the speed and too much body movement ruins the sail shape. Since the boat weighs only 80 pounds, even something as innocuous as scratching a mosquito bite translates into a jarring motion that flaps the sail and slows the boat. Once it's going, the craft moves at a sprightly two or three knots, but you want to sit completely sitll and move the tiller and sheet with the greatest delicacy.

Unfortunately, all this knowledge was gained too late. Imp was far ahead at the finish and there were three other boats between the two of us. My finish was 31st, but seven of the 38 boats that had started the first race didn't compete in the second.

For the third race I was dead tired. Rain had fallen all morning and I was chilled and wet to the bone. But I wonder of wonders, I got a good start.

The first leg turned out to be a broad reach, which means the wind was slightly behind. At the first mark I had five boats put away.All morning the wind had been nearly nill; now it became steady at two with gusts to three and I decided not to tack until the next mark's layline - heading right for the mark. When I made it here, miracle of miracles, nine baots - including Imp - were far astern. I began to hunt the leader.

just ahead were "Jeremiah" and "Shafu" and I shouted, "Look out, I'm coming through."

But it didn't psych them out because racing sailors tend to be grim and determined. Nevertheless, I finished 19th in a fleet now reduced to 28.

Later, everybody went over to National Capital Parks headquarters to dry out, warm up and find out who won. Paul Numa Jr., 16, from Baltimore had come in third, seventh and fourth and was the overall winner. David Geisz of Springfield, president of the El Toro Atlantic Coast Region, took second with 2&4/12. Jacie Ayres of falls Church earned third with an 18/1/1.

Trophies were awarded in the 19-and-over senior division and the under-19 junior. In the seniors, Geisz was first; Jack Slater of SPringfield was second and Buzz Brown, who had driven all the way from Youngstown, Ohio, took third.

Numa and Ayres topped the junior division and Matt White of Springfield was third.

Even some of the losers were winners. Leigh Schaeffer of Reston who had trailed in the junior division, and Ed Fitzwater of Gaithersburg, who had been last in the seniors, were awarded new sails.