Duby Joslin is a sea dog who works in a foxhole and talks to a man who tails halyards in a sewer.

"The cockpits in the America's Cup boats are divided into sections," Joslin explained. "The one scooped out aft of the mast is where I'm stationed. Just below me is the guy who cranks the winches that control the sail halyards. We call him the sewer man."

The sewer man and the foxhole man and the 11 others who make up the team of the yacht Independence are competing this week in Newport, R.I., against two other American boats in the first of a series of trials that will select the defending boat in September's America's Cup races.

"Actually," said Joslin, 29, a lieutenant stationed at the Naval Academy, "this week won't be much different from what we've been doing every weekend since early spring. We've been racing Courageous right along; now Enterprise has come from the West Coast and it'll get in the act too."

Joslin has been something of an ongoing act on the America's Cup scene. He crewed on Intrepid in 1970 when it successfully defended.

Getting on Intrepid was the natural result of a life-time of being close to sailboats. When his father, also a naval officer, was assigned to the Naval Academy, Joslin earned enough money as a paper boy to buy a 10-foot sloop and to campaign it in races put on by the Seven Sailing Association.

He continued to race after his father was transferred to Newport and one summer won the junior championship at the Ida Lewis Yacht Club. When he got to the Naval Academy he was captain of the plebe dinghy team and later captain of the varsity. Summers he crewed on big boats in the Annapolis-to-Newport and the Newport-to-Bermuda races.

"Those big-boat races were what led me to Intrepid," he said. "I was stationed in San Francisco after I graduated from the Naval Academy and found out that Steve Van Dyke, whom I had sailed with on the ocean races, had been named Intrepid's tactician. I sent him a letter of congratulations and the next thing I knew he called and asked me, one, was I interested in crewing and, two, could I get the time off."

The Navy was agreeable and Joslin was given temporary duty at Newport. Intrepid, with Bill Ficker as skipper, beat the Australian boat Gretel II, four races to one.

An invitation to crew on Courageous came to Joslin in 1974, but this time the Navy said he couldn't be spared from his job aboard a nuclear-powered ship on the West Coast.

Last fall Joslin went to Marblehead, Mass., several times to sail on Independence and on Courageous as well. Courageous' skipper is Ted Tuner, with whom Joslin had sailed last year in the Annapolis Yacht Club's fall series, and Joslin believes he could have been selected for the Courageous crew if he had wanted - "but I was really impressed with the way Ted Hood was doing things on Independence and that was the boat I preferred."

As the foxhole man, Joslin's main job is to shackle and unshackle the halyards - lines used to raise the jib and spinnaker sails. When the spinnaker - the sail that looks like a parachute - is up, he controls the line that adjusts its shape.

"The most important thing is that I'm the link of communications between the skipper and the tactician and the forward end of the boat," Joslin said. "I'm boss of the bow man and the sewer man. The sewer man can't see much of what's going on and he in particular needs precise instructions on what to do next."

Joslin is 6 feet tall and 180 pounds, but admits that's smallish for a sailor.

"My opposite number on Courageous makes me look like a midget. He's 6-5 and 230. But the job requires quickness as well as strength and I can handle it."

There is much to handle and Joslin said he's always conscious of the enormous stresses and strains that are generated when the 64-foot boat is under way. "It doesn't seem to be going very fast, even though we've done almost 13 knots. There is a feeling of tremendous power. Less when we're going into the wind, much more when we leave the windward mark and set the sails for a reach. Then comes a great, sudden forward surge. It's like riding a locomotive."

Joslin is sure that whatever boat is selected it will beat the foreign challenger.

"It comes down to sails - and American sails are the best in the world."