I heard 'crunch, crunch, crunch' and the boat slowed down," Dennis Cobb said. "The first thing I thought was that we had hit bottom, except it immediately did a 180-degree pivot to the right and started moving again. Then Pat looked below and realized it was a lot more serious; the cabin carpets were floating in eight inches of water."

Cobb was recalling the first seconds of an accident within sight of the Washington Sailing Marina that almost cost him his sailboat.

"I tried to start the inboard engine right away," he said. "It kicked over once and quit. We had been sailing with only the genoa, but I thought we might be able to make it to the big crane - it was less than 100 yards away - and get the slings under it. Then I realized the boat was filling up so fast the only chance we had was to head to the nearest beach."

Cobb, 34, who lived in Arlington, had decide on an afternoon sail to while away a spring Sunday. He nad his wife, pat, and sons Christ and Danny had set out from the marina in Alexandria in their 26-foot Westerly Center. A peppy west wind made it easy to reach up and down the Potomac. At 4 p.m. they were heading north past the marina, getting ready to drop sail and power in.

"I've sailed out of the marina since 1972 and I know how shallow the river is. When I'm close to the marina I'm very careful about staying in the channel," Cobb said.

"The Number 6 day beacon, which last year warned about the shallow water off to the east, was knocked down by last winter's ice. But I could see the temporary buoy which had replaced it and I was well to the south and west of it when we hit the thing that holed us. I didn't have any idea what it was I did know that if we didn't beach the boat in a hurry it was going to go down, glug, glug, glug."

The water was over the dinette seats by the time Cobb put his boat on shore south of the docks.

"After we hauled the boat we saw the hole. It was big enough to take four fingers, if not a entire fist. Everybody who looked at it said it could have been caused by only one thing: the steel day beacon that was toppled by last winter's ice."

The Coast Guard unit responsible for the coreect positioning of local buoys and beacons admitted the temporary buoy was obviously far north of the day beacon for which it was substituting. Chief Boatswain's Mate Dan Dunn, who is in charge of "Capstan," the Coast Guard tug berthed in Alexandria, said the temporary buoy had been placed this spring and should have been right over the toppled day beacon.

"We used a sextant and followed the proper procedures," said Dunn. "But the temporary buoys only have 100-pound sinkers and if they're struck by a boat or used as a mooring they can sometimes be dragged."

While some local sailors claimed they knew the buoy hadn't been accurately placed, Dunn said, none had complained to the Coast Guard.

"If we had known it was wrong we would have relocated it," he said. "By the Tuesday morning after Cobb's accident we had put the buoy right over the sunken day beacon and we would have done it Monday, except that the wind was too strong."

Those winds didn't do Cobb any good, either. Seventy-knot gusts swept through the marina and twisted the mast - which had been lowered after the beaching but was still hinged to the boat. There was extensive damage.

The mainsail, which had been left in the cockpit, was blown out and nearly beat itself to shreds against the crane.

"The luff was damaged," said Cobb, "the leach was ripped and there's paint and grime all over it. It's a mess."

That's not all. The Cobb spent most of Monday during the boat and removing its sodden contents. But they left a jug of diesel fuel in the forward cabin. Its seams opened some time Monday night and on Tuesday the Cobbs discovered that much of the fuel had fouled the area they had dried out the day before.

"Another mess," said Cobb.

He said only a marine surveyor could accurately estimate damages, but he thought it might run to about $1,500. He is insured.

"If the boat had sunk I'm sure my insurance company would sue the Coast Guard, because the temporary buoy was way off station."

Even after his boat is fixed, Cobb is afraid the memory might cast a pall.

"My wife can't swim and she was really upset until we made it to the beach. It's hard to know how she'll feel about sailing in the future. As for myself, I'll always wonder if the hold is properly patched. The whole thing makes us sick."