WATER WORLD : Today’s tubs are good for more than just soaking. They’re built with conservation and time-saving in mind.

Like the human appendix, the plain old bathtub has outlived its usefulness. Most Americans shun soaping up and rinsing off in the same lukewarm broth. When cleanliness is the goal, we prefer to shower.

Still we cling to the notion that a bathroom is only half a bath without a tub, and that has spawned a new generation of tub-like products that we scurry to buy when time comes to update the bathroom.

We rip up our master bathrooms, less frequently our secondary baths, toss out the fiberglass or stamped steel-and-enamel tubs that the penny-pinching builders supplied, and visit people such as Roger and Sally Shore, proprietors of Sea Pointe Kitchen and Bath in Irvine, or Phillip K. Black, president of Regal Cultured Marble Inc. of La Habra.

And from them we purchase oversize tubs of cast resin or acrylic plastic, tubs with sinuous curves, some with high-power whirlpool devices and skin-tingling water jets.

Sally Shore says her company does about 60 top-to-bottom bathroom remodels a year. In 65% to 70% of those involving a master bathroom, the homeowner has the old tub ripped out and replaced with a whirlpool unit, usually in a solid-color cast acrylic.

The American Kitchen and Bath Assn. says acrylic is generally harder and more abrasion resistant than cast resin, the material for cultured marble and other stone look-alike tubs. Shore says the two are “about the same quality.”

The trend these days is to keep a regular-size combination tub and shower in the second bathroom and to install separates--a glassed-in shower and an oversized soaking tub or whirlpool--in the master bath, says Tim O’Conner, senior product manager of bathing products for Kohler Co. of Kohler, Wis.

“In the past, with a bath and shower combination, you had to have a rectangular shape in order to bring the wall material down to the top of the tub so water would drain when showering,” he says.

“Now, with separate tubs and showers, the tub doesn’t have to be a rectangle. Ovals are popular, and we do a lot of intricate rim designs because we don’t have to worry about the shower draining down into the tub. And as master bathrooms have gotten bigger and bigger, so have the tubs and showers. They seem to get a little bigger each year.”

Standard tubs--30 to 32 inches wide, 15 or 16 inches deep and 5 feet long--come in a broad range of materials: the thin, one-piece, fiberglass economy tub-and-wall surround units; the purist’s tub of hefty, expensive, enameled cast iron, and tubs made of new plastics that are easy to clean, come in finishes such as faux marble, granite and onyx and can be repaired if scratched.

The larger, lounging tubs usually come in stamped steel with a bonded enamel finish or in gel-coated plastic resin or formed acrylic. At 36 to 42 inches wide, 60 to 72 inches long and 19 or more inches deep, they would be too heavy and too expensive if made of cast iron. Besides, resin and acrylic tubs are much easier than iron or steel to drill and plumb for whirlpool jets, and whirlpools are where it is at.

Both Kohler and American Standard--a unit of New Jersey-based U.S. Plumbing Products Inc.--have large and growing lines of whirlpool tubs competing in a sort of Spa Wars.

“We have four new ones,” boasts Lou Kosta, national sales and marketing manager for American Standard’s acrylic products units.

The trick is to “design the whirlpool first, the tub second. What’s critical is the location of the jets and arm rests, the convenience of the bather,” he says. One of the company’s new models has a built-in bench designed “for ladies to (sit while they) shave their legs.”

American Standard, known for a traditional, masculine or rectangular look in its products, has “tried to soften things, to make them a bit more feminine,” Kosta says.

He boasts of American Standard’s new jet design, which he describes as “larger, with a 50% air-to-water mix for a softer whirlpool effect on the body. There are more bubbles and more water volume, and, with more air in the system, it gives a champagne feel.”

The company’s whirlpool tubs retail in the $1,100 to $3,000 range, Kosta says. With the higher prices come colors, larger sizes, more powerful whirlpool jet motors and in-line heaters that keep the water at a constant temperature.

Over at Kohler, O’Connor says that shapes, materials and sizes of whirlpool tubs “have been pretty constant for a number of years,” so it has been “little features that changed” until now.

The Wisconsin company has just introduced a luxury whirlpool tub line that features powerful two-horsepower pumps (the standard is one horsepower or less) with in-line heaters, variable flow control so the bather can adjust the vigor of the pummeling massage jets and a special neck massage system.

“To really work, a neck massage needs to be above the water line,” O’Connor says. Kohler’s unit accomplishes that without spraying water all over the bathroom by placing the massage jets inside a pillow that is covered with a Lycra material that contains the water.

The top-of-the line models start at $3,500 and run up to $6,500 for a tub with the neck massage and 10 jets that operate in sequence to massage the bather by working up the back and then across the shoulders. Kohler engineers designed the system after talking to a massage therapists, O’Connor says.

Then there’s the Rolls-Royce of home-massage bathing: Kohler’s new stand-up, high volume, water-conserving BodySpa that, like a lot of today’s plumbing fixtures, borrows and improves upon an idea that cropped up in the early years of the industry.

Kohler says it is so new that there are none available at Southern California retailers. The product was developed “because in our market research we noticed that most people are shower people and don’t have the time or inclination to relax in a bathtub for 20 to 40 minutes anymore,” O’Connor says.

The BodySpa has some features of the early “needle jet” showers that ringed the bather with a multitude of shower heads from ankle to head. William Randolph Hearst’s San Simeon mansion had one of the first in California.

But BodySpa also takes conservation into account.

“Like a whirlpool (tub), there is a basin that you fill up,” O’Connor says. But from the basin up, the unit is a fully enclosed shower stall with a series of six to 10 massage jets mounted on the walls.

Some units have seats, but the typical bather would stand. The unit uses just 37 gallons of water, which the basin catches as it is forced through the pumps at high volume--up to 80 gallons per minute. “After you fill the basin, you turn the pump on and it recirculates the water through the jets,” O’Connor says.

The typical water-saving shower head delivers water at 2.5 gallons per minute. An 80 gallon flow “is an unbelievable amount of water that gives a wonderful massage,” he says.

And the price? From $3,500 to $9,500--or 12% to 32% of the average California wage earner’s annual income.

A 1914 “modern plumbing” catalogue for J.L. Mott Iron Works of New York listed a porcelain needle jet shower with a plate glass door, a dozen body jets and a 10.5-inch overhead shower head, all nickel plated, for $418--or about six months’ pay for a laborer.

That same catalogue shows plumbing fixtures with faux-marble finishes, pedestal sinks and even a fully built-in, double shell bathtub. That’s a tub with a flush enamel exterior, just like they make today.

A 1937 American Standard catalogue reprinted several years ago by the National Trust for Historic Preservation shows that almost 60 years ago plumbing fixtures came in rich hues such as a Persian brown, Clair de Lune blue, a pale lavender called Orchid of Vincennes and a deep purple called Royal Copenhagen blue.

Kohler and American Standard tend to set trends in the bathroom because they are the only two U.S. companies that make all of the plumbing fixtures: tubs, showers, whirlpools, sinks, toilets and bidets.

Producing full lines enables them to match colors exactly, and that, in turn, often makes them the preferred suppliers for designers and bathroom remodelers.

Their palette has about 19 colors for bathrooms, including three or four shades of white, and they can be and usually are purchased by acrylic and resin tub makers, who use them to tint their products. The closeness of the match between a plastic-based tub and a main-line manufacturer’s cast iron sink and ceramic toilet depends largely on the skill of the tub maker.

The hot new color from Kohler is Skylight, a tone that O’Connor describes as “a very clear, clean color--the color of blue light . . . different than blues of the past because it is natural looking.”

Over at American Standard, there are no new hues this year, Kosta says. The company uses 19--including deep black and Chinese red, but, like Kohler, sees its whites as perennial bestsellers. Kosta says he favors beige-toned Warm White and gray-tinged Euro-White.

For places such as Phillip Black’s company, which turns out several hundred custom tubs a year, as well as cultured marble, granite and onyx countertops and sinks, color-matching isn’t a problem. The faux stonework products all are multicolored: Even white-on-white marble has several tints swirling through it.

Black says the tubs have been around since the 1960s and began appearing in high-end tract developments in the 1970s. The tubs are popular because they look like stone and can be cast in shapes and sizes that cast iron cannot match. They cost about the same as a cast iron product--from about $350 to more than $1,000 for oversized cultured onyx or granite models.