Diana Young was delighted when Irma hit the water. "We can finally park the car in the garage again," she said with a smile.

Wrong.

Irma is a 141/2-foot Marblehead skiff that Diana and Bill Young's 16-year-old son Jonathan had been building in the garage in back of their house on Upton Street NW for the last two years.

Last Saturday Irma was done. The Youngs and 40 neighbors gathered at the Washington Sailing Marina to drink mulled wine, spritz champagne and generally cheer young Jonathan, who spent the morning in a cold sweat, convinced the boat wouldn't float or that if it did it would capsize as soon as he stepped aboard and sink in front of mom and everybody.

When six stout gentlemen agreed to carry Irma the last 20 yards down the gangway to the water, Jonathan said, "I think I'll go somewhere until this is over."

But Irma sat as pretty as a slipper on the glassy Potomac, sparkling white where she wasn't varnished brightwork. While the giddy crowd pressed closer, almost sinking the dock at one point, the young boat- builder set to work rigging the mast and boom he'd fashioned by hand from squared spruce logs.

Anyone who has sailed knows the deepest terror of the sea is not drowning, capsizing in a storm or sinking in blue water. Instead, the worst thing that can happen is to be obviously unable to handle your own boat while people you have to live around are watching.

Jonathan checked his rigging, inspected the bilges, led his lines and fooled with the sail track so many times that folks began to wonder if he was going sailing at all.

Finally he and a colleague set off on the river to the shouts and cheers of the multitudes, to whom it evidently was not apparent that Irma wasn't sailing at all, but simply sliding backwards with the wind.

"Can't get the (center)board down," muttered Jonathan. So they dipped around a corner where folks couldn't see, hauled the boat onto shore and pulled the board down by hand.

Things improved. Jonathan got his sea legs and before long was hurrying back to the dock to take cousins, friends and passersby for a ride in his splendid little yacht. Everyone loved Irma. "She's really a beautiful little boat," said some folks in a fiberglass Albacore. And they were right.

Unfortunately, shakedown cruises are designed to locate the defects in a boat and they usually succeed. Before the day was over, Jonathan had revised his plans, which initially called for him to sail a mile down the river and leave Irma at a boatyard.

Halfway downriver a gust came along and blew so hard on the mainsail that it cracked the boom in two. He got a tow back to the sailing marina, the boat went back on a borrowed trailer and home to the Upton Street garage, where it will remain in repose until a new boom is fashioned. That should be all winter.

That small failure makes the launching of Irma no less a success. She does indeed float and she sails like the dream she is, albeit a tippy dream. Besides, if everything went right Jonathan would have nothing to do all winter long.

"He's the middle child, with bright, high-achievement sisters on both sides," said a neighbor. "I think everybody looked at this as the thing he could excel at, and everybody's delighted that he has."

For his part, young Jonathan Young, an 11th-grader at Washington's public School Without Walls, said he was pleased with the launching. "I had a lot of doubts. I thought it would just roll over and sink, but it didn't. I figure my greatest worries are over now."

He first became interested in boatbuilding when he was 12 years old and an uncle in Connecticut was considering building a small boat. The uncle took note of Joathan's interest and sent him someplans for a 91/2 -foot pram, which the youngster built in one year in his basement. He wanted a bigger challenge, and after some research settled on the Marblehead skiff, a classic design with a flat bottom and lapped, clinker-built topsides. He started work in August 1979. "I really didn't know what I was getting into," he said, "but I took it as it came and when I ran into trouble I sat down and thought it out."

In the end it cost way more than anyone guessed it would, about $1,200, half from his own money and half from his parents. He knew the oak frames would be costly, but the price of clear white pine planking shocked him.

"It was harder than I thought," he said. "When you look at all the little decisions, it's a lot more than slapping some planks on some frames."

Things get easier, of course, with practice. Now he's downright flippant about big jobs.

"Yup," said Jonathan Young, boatbuilder, "this winter I'll just spin out a new boom."