"Mr. Yenckel," reads the screen of the super-sleek table-model computer, "you have just been named captian of His British Majesty's Ship Impetuous. If you have audacity and common sense, you can be victorious."

Here I am, commanding a 19th-century man-of-war, England's last hope against Napoleon's forces. I have been transported back in time to the pages of a Horatio Hornblower adventure.

The screen's story unfolds: On a blustery night, the Impetuous anchors off the coast of France. "Capt. Yenckel," asks the fictional boatswain, "should we rig the boarding nets?"

"Rig the nets," I order, typing the command onto the screen. There'll be no surprise midnight assaults on my ship. . . .

What's going on here is a new $20 computer game called "Interactive Fiction" -- one of hundreds of such games that are making the home computer a hot item for Christmas. In this game, you become a character in the story. The outcome depends on your answers at crucial points in the plot.

(I stepped out of my plot when the cabin orderly began serving wine, and I accepted, against my better judgement. I -- reading ahead -- saw myself destined to lose my ship in a stupor.)

Home computers -- small enough to be practical and cheap enough to be affordable -- first came on the market in a big way in 1977. Since then, an estimated 500,000 have been sold. That figure is growing by 40,000 to 50,000 a month.

By 1995, half the homes in America are expected to have one. And that's "the most conservative prediction," says computer enthusiast Robert L. Perry, author of "The Complete Illustrated Guide to Owing Your Home Computer" (Everest, 224 pages, $10.95 paper).

Many owners buy their computers, they say, "for the fun," or "to play with" -- at a price ranging from about $500 to $2,000 or more, for a viable machine and acessories. But to Perry, who also wrote the first "Mechanix Illustrated Guide to Personal Computers," their value goes well beyond that.

"For a family," he says, "the most important thing is what it can do in terms of education for the children -- and to acquaint adults with what computers are all about."

Since computers are fast becoming a part of our life, people have to approach the purchase of one, he says, "as an investment in their children's future.

"If they're not literate with the computer, they're not going to get jobs 10 or 20 years from now. There's going to be an explosive growth in computers in the next 20 years. They will be missing out on fantastic career opportunities."

Its educational purpose, though, reminds Perry, is not to help the child learn to think, but as a tutoring aide -- "a drill kind of thing for multiplication tables" (or other drills a parent may rapidly lose interest in).

The computer's uses seem to grow almost daily as more and more people experiment with them. Because they are much cheaper than standard office computers -- but still can do many of the same things -- a number of small businesses or in-home businesses are buying them.

Perry says he knows of department managers who on weekends carry home the office budget, which they can handle faster on their home computers. "It gives them a competitive edge over other department heads."

In the past, he says, "machines have freed our muscles from unpleasant labors; in the present, home computers free us of mental drudgery."

Perry lists "99 common things to do with a home computer," ranging from learning foreign languages and advanced mathematics to determining your biorhythms, calculating your income taxes, maintaining your personal financial accounts and operating your home security system.

A Washington printer uses his to keep tabs on his stock market investments.

A free-lance writer has banished her typewriter and composes her articles onthe computer's TV-like screen.

Bill Bowie of Olney, a self-proclaimed "computer freak," who bought his first machine a year ago, uses it in his carpentry firm "to keep the books." He is working on a program that will enable him to make quicker cost estimates in job contracts.

Bob Palus, a salesman at Computerland in Shady Grove, keeps his checking and savings account up to date electronically. He has also, he says, devised a program "for who to pick on a point spread in a football game -- about as good," he says, "as guessing."

The industry has spawned a number of users clubs here and throughout the country. Purchasers of the same brand frequently band together.

Bernard Urban, a Housing and Urban Development planner, heads Apple pi, a 400-member group most of whom own an Apple computer system. (Younger members -- the youngest is 7 -- are called Green Apples. The international association is Apple Core.)

Members, says Urban, include "physicists and pharmacists, a lady welder and a cab driver." They meet monthly at George Washington University to discuss common problems and new ideas.

"This thing has blossomed," says Urban who writes the group's newsletter on his apple. "People fall in love with their computers."

Owners of Radio Shack's TRS-80 home computer have formed the Association of Personal Computer Users, meeting regularly at Chevy Chase library.

The club's president, Phil Yeary, a Vitro Laboratories project leader, estimates he averages two to three hours a day on his machine. He bought it initially, he says, "to learn more about computers" and is now creating new uses for it -- some of which he hopes to publish.

With the use of a phone hook-up, computer owners -- including Apple Pie -- have set up communications networks or "community bulletin boards" in which they can exchange messages with one another locally or even across country.

For an hourly fee (in the $3 to $5 range), home computers can be plugged into such information services as the latest news, stock quotations or airline schedules.

While many potential buyers may hesitate because a computer looks so complex, Perry says you can learn to use the latest models "as easily as you operate your new stereo.

"Computer companies have made the home computer into just another home appliance. Everything you need to know is recorded on a cassette tape, a cartridge or a disk. If you want to run your monthly budget, for example, you plug in a cartridge and follow simple instructions."

In the future, he sees computers that will both talk and listen and be as small as pocket calculators.

Nevertheless, he says, "The situation has changed so rapidly during the past five years and promises to change even more rapidly during the next 10 that to attempt accurate predictions today seems foolhardy."