The 190-year-old U.S. Coast Guard stands by its motto, Semper Paratus, Latin for "Always Prepared."

But yesterday the guard's headquarters in Washington was closed. It is closed every Friday because the 2,300 officers and employes who work there are on a four-day workweek -- and a three-day weekend -- as part of the federal government's experiment with flexible working hours for its bureaucrats.

While many federal agencies have flexible hours or shortened work-weeks, federal officials said yesterday that the Coast Guard may be the only major agency to shut down its headquarters on a weekday.

Under the shortened week concept, employes work four 10-hour days a week. Most agencies using the system schedule their employes so that offices are staffed every weekday, thus actually increasing the number of hours the workers are available for the public.

But Coast Guard officials are only in their offices four days, a system that seems to be popular around the agency.

"Having just gotten back from jogging and thoroughly enjoying my day off, I can tell you it's working very well," said the Coast Guard commandant, Adm. John B. Hayes, from his Chevy Chase home yesterday ". . . Of course it's pretty nice because when those holidays come along people have a four-day weekend."

Hayes said the four-day workweek "effects savings in energy at the [headquarters] building and in commuting. It cuts commuting by 20 percent. . . . Also you reduce by 20 percent the number of coffee breaks."

The admiral said it also appears that people work more effectively during a 10-hour day.

"For some interesting reason, people are more conscious of a better use of their time. . . . There is a tendency of people not to eat [their time] up with extra meetings. People coming in early have a very productive hour and a half they didn't have before without phone calls and without much interruption."

Coast Guard spokesmen emphasized yesterday that Coast Guard stations around the country are open 24 hours a day, seven days a week and that all but headquarters personnel work a five-day workweek.

People who have emergencies, or who need boating information, can always call these local stations, the spokesmen said.

Under the Coast Guard's Monday-through-Thursday week, headquarters personnel may come in as early as 5:30 a.m. or as late as 8:30 a.m. They leave 10 1/2 hours later, taking half an hour for lunch.

Most personnel at the headquarters building on Buzzard Point on the Potomac River in Southwest Washington like the system, but not everyone is brimming with enthusiasm.

"It's kind of six-of-one, half-a-dozen-of-the-other. It's kind of a long day but you get Friday off," said Lt. Cmdr. Bob Gronberg, who had the misfortune to be the headquarters duty officer yesterday while everyone else enjoyed the first day of their regular three-day weekend.

But there was outrage at the new schedule, too.

John Cotton, a Washington lawyer representing a client who had business to do with the Coast Guard yesterday, was surprised when he called all the Coast Guard numbers in the telephone book and could only reach duty officers who told him to call back Monday.

"I'm all for energy saving, but this is insane," said Cotton. "What if I had an emergency?"

Hayes said he decided not to have the headquarters' phones connected to message machines that would explain the Friday closing because "it was going to be pretty expensive to do that."

Businessmen, attorneys and others who have business with the Coast Guard were told informally through press releases and news accounts of the schedule change, according to the Coast Guard spokesman.

Cotton apparently didn't get the message.

In 1978, Congress called for a three-year experiment in "flexitime" and shortened weeks for federal agencies. Under the flexitime concept, employes work five days a week but can vary their hours at their own discretion.

The shortened workweek involves longer hours each day.The shortest workweek now being used in the federal government is three 13-hour days for State Department security personnel who guard visiting diplomats, according to an official with the U.S. Office of Personnel Management.

OPM will eventually study the experiment to determine what effect it has had on government efficiency, mass transit and traffic, energy consumption, service to the public, opportunities for full-time and part-time employment and on family and personal life in general.

Meanwhile, the Coast Guard headquarters sails on -- four days a week.

Jule M. Sugarman, deputy director of OPM, was mildly surprised yesterday when told of the Coast Guard schedule.

"Well," he commented, "no mail on Saturdays, no Coast Guard on Fridays."