CAPTAIN CARL NELSON is 89 years old. Since his 28-foot sloop is 71 years younger, he was surprised when it began to leak badly a year ago.

It seemed to be coming from behind the icebox and when he took the box out, sure enough, there was water seeping between the hull planks. He stuck a knife in one of the planks to see if it had rotted and the next thing he knew the whole section of plank crumbled and water began pouring in.

The sloop was tied up at the end of a dock at the Washington Sailing Maring in Alexandria and the only thing Nelson could think to do was beach it. He succeeded in getting it far enough up so the hole was above the water line, but that was a low tide. At high tide the boat filled right up because he hadn't had anything to patch the hole with and he was tuckered out from the effort of bringing the boat to the shore.

All he could do was watch the water going in and feel sick.

The next day Nelson's younger friends wadded cotton caulking into the breach and got a pump going enough to float the sloop so Nelson could get it around to the big crane and have it hauled out of the water.

But when the weight of the vessel got onto the cradle, three pad plates poked right through several planks. They were rotted just like the planks behand the icebox and would have to be replaced.

But that didn't really bother Nelson. He had built the sloop from the keel up and felt there couldn't be mucch to making it whole again.

So he replaced a few planks and caulked them and put on a fresh coat of bottom paint and said "let's get her back in the river."

In she went - and so did the water.

"That's all right," the captain said. "Wait'til the planks swell, she'll be tight as a drum."

But it didn't happen. The pump had to be started, the crane slings had to be wrapped around the hull and the boat had to come out of the water again.

That's how it has gone most of the year. Captain Nelson replaces a few planks, tries a new caulking material, slaps on a little paint and drops the sloop in the river again. "Wait 'til the planks swell."

But the planks don't swell - at least not enough.

It is apparent to everyone but Nelson that the plank will never swell, the caulking will never be tight and the boat will never be dry again; the skill and the strength and the cunning with wood that the old man once had are gone forever.

"No, that's not so," the captain keeps saying. "This time I've got it. All I need is a few more planks and a few more days. When she's ready, I'm going to take her down the Potomac to Cheasapeake Bay - singlehanded all the way."

The captain has been singlehanding for a long time. His wife died years ago and he lives alone in an efficiency apartment in Alexandria. He says his health is pretty good, but his voice is not strong and his hearing is suspect and he must concentrate very hard when asked to dredge up old dates.

"I don't go to doctors," he says. "I don't need to. This year they renewed my driver's license. I'm not supposed to drive at night."

And every fine day he's at the marina with his sloop and when he wants to be in the boat's interior he nimbly climbs up the eight foot ladder he has put alongside.

"She will be fixed, she will be," he says firmly. "I'm confident. Why shouldn't I be? I built it, I can repair it. She's not done yet - and neither am I."