Eleven-year-old Tracie Harper let out a squeal as she jumped into the hydrotube at Prince William County's Ben Lomond Park. Then she squealed all the way down the water-spewing, spiraling, four-story aquamarine tube until she landed in the special pool at the bottom.

She was still squealing when she got out, dripping wet.

"I wish it would go on forever," said the Fauquier County fifth grader, who had begged her mother to drive her and two girlfriends to Lomond Park for the second time this week.

Indeed, the newly opened hydrotube has become the hottest--or coolest--thing in Prince William County among the grade school set, and in the evenings it is attracting an increasingly older crowd as well.

County park officials are equally enthusiastic about the water tube because they say it will generate much-needed revenue for park and recreation programs.

But what has the officials most excited is how the hydrotube came to be at all:

It was installed by a private entrepreneur, who is to pay the county park system for the privilege.

The success of the tube, which opened this June, has prompted the county Park Authority to sign an agreement with another businessman who is building a combination bumper boat and speedboat pond near the hydrotube in the 100-acre Lomond Park off Sudley Manor Road near Manassas.

"We think this is a turn that public park and recreation authorities need to take," Prince William Authority director Jim Spangler said last week of the public-and-private enterprise alliance. "It is the whole concept of parks and recreation being asked to pay for itself. A lot of communities don't want private business in the public parks, but we are finding it's working."

Spangler said Prince William is the only jurisdiction in Northern Virginia to have a private entrepreneur run a recreational facility in a public park.

The prefabricated hydrotube was built for an estimated $100,000 over the past year. The county will receive nothing this first year, but in subsequent years, it will receive 10 percent of the gross profits.

Spangler estimates the county will receive $15,000 a year from the spiraling tube.

The county has arranged a similar deal with the businessman who is building the cement speedboat pond on an acre of land in Lomond Park, one of the largest parks in the county. Richard Metz, park director, said the pond will have specially designed speedboats cruising its outer edge with bumper boats cordoned off in the center. It is expected to be completed by late July.

"It will be just like the one in Disney World," said Metz, who described the park's hydrotube as unique to Virginia. He said the nearest one to it is in Rehoboth Beach, Del.

Metz said the county negotiated with the businessmen over the prices to be charged for the rides and the number of lifeguards on duty. The hydrotube costs $3 for a block of tickets good for 10 rides. For 50 cents, the timid at heart may buy one introductory plunge. Most swimmers also buy the $2 daylong pool pass sold by the county park system that allows them to use the large county-operated pool nearby. A guard at the top of the tube spaces riders so they will not bump into each other in mid-route and another at the bottom plucks riders out of the landing pool.

Metz said some riders have suffered bruises and scrapes, mostly because riders were trying to stop themselves in the middle of the tube and were running into each other. Now the guards eject anyone who stops mid-tube.

Metz said the guards and all utilities are paid by the tube owners.

"If that's not enough, our pool attendance has increased by 20 percent," Metz said. "It's definitely increased park participation."

The tube has worked so well that Spangler said the county is now looking for a private entrepreneur to take over Lomond Park's miniature golf course.

"These arrangements have endless possibilities," he said.