The two tallest structures on any cruise ship are the funnel and the communications mast. But you don't have to look too hard to see a challenger on the horizon.
Sitting aft of the funnel on Carnival Cruise Lines' latest creation, the Carnival Sunshine, is a water park with a stair tower that elevates brave climbers four stories above Deck 10.
From there, riders have to choose. They can take the long way down on Twister, a 334-foot-long slide that takes them over the edge of the ship with a transparent view of the sea 150 feet below. Or they can compete with their friends on the 235-foot-long Speedway Splash, a dual-lane, racing-themed slide that delivers elapsed times on scoreboard displays at the end of the ride.
That's in addition to a raft of other water features aimed at younger, older or less daring passengers.
Water slides, once a small part of a cruise ship's fun repertoire, have morphed into a whole day's worth of entertainment. On some ships, they have become big enough to be defining features.
Slides aren't everywhere. Contemporary lines are where the action is. Premium and luxury lines have neither the deck space nor the volume of family passengers to make slides suitable.
Carnival, Norwegian Cruise Line, Disney Cruise Line, MSC Cruises and Costa Cruises have all become players in the slide derby.
"We have purposefully and with a lot of thought taken the design of the parks as far as we possibly can push them with each new delivery," said Lania Rittenhouse, vice president of product development at Carnival.
Competition is driving the creation of taller, faster, more exciting slides. Four years ago, the Carnival Dream had the longest slide, a 303-foot version of the Twister. The following year, Norwegian added a plunge element and a bowl at the bottom of its slide on the Norwegian Epic.
Disney topped that in 2011 with the 765-foot AquaDuck water coaster, a two-person ride. And last year, MSC installed on its new MSC Preziosa a construct called Vertigo, which at 394 feet in length is now the longest single-rider slide at sea.
But there happens to be a big payoff in this derby. Slides are like catnip to the family cruise segment, a key line of business for contemporary lines. They're especially crucial to keeping the teen contingent in the family onboard.
"This is a perfect thing for them," Rittenhouse said of the line's younger set. "They're on their own without their parents. They can run around. They can be in the water park, then go to our outdoor play areas, where there's basketball, foosball, pool tables, mini golf. We definitely keep them in mind when we design these slides."
Rick Sasso, president of MSC Cruises USA, said slides are just part of a package at MSC to woo the family traveler.
"We have tremendous kids' programs," Sasso said, adding that its "Kids Sail Free" policy for ages 11 and under "is tremendously attractive. Our entertainment in general does attract families. The water slide is just a nice little addition." From playgrounds to water parks
The first slide on a ship looked nothing like today's models. In 1978, when Carnival made over the former Union-Castle mail boat, the Transvaal Castle, into the Festivale, Carnival architect Joe Farcus added a simple playground slide to the pool.
But the pool was small. For safety reasons, the slide could only be used when swimmers weren't in it. For later ships, a slide design with a runout lane was developed so the pool could be used independently.
Slide mania really took off about 10 years ago when ship size began to exceed 100,000 gross tons. Multiple big slides create a huge footprint on the deck, so until ships got big enough, slides had to remain fairly contained.
On some ships, such as the Norwegian Epic, the slides occupy areas around the funnel that hadn't been used before for passenger activities.
With space to spare, slides' heights have expanded from two-deck affairs in the 1980s and '90s to three and four decks today.
At the same time, theming has become more elaborate. That's especially true since the Walt Disney Co. entered the cruise business. As Disney tells it, the AquaDuck was created by Donald Duck's nephews Huey, Dewey and Louie, who control it from the AquaLab, a water play area for younger children, full of valves, gauges and control panels.
On the Disney Dream and Fantasy, the AquaDuck runs in an oval circuit around the entire top deck, a clear acrylic tube elevated on columns. Water propulsion jets keep the rafts moving through the cylinder.
Watching riders shoot past overhead is a secondary entertainment for spectators not participating in the ride. While not especially thrilling by land-park standards, the ride offers a unique vantage point from which to see the ocean, a view enhanced by the additional height of the cruise ship.
The AquaDuck and other highly visible slides serve as a type of mobile billboard as the ships move around the world. On the Norwegian Epic, "the new slide concept supported the new dynamic appearance of Norwegian Cruise Line," said Christer Karlsson, senior vice president of newbuild.
Two big contemporary lines have largely taken a pass on the slide competition. Royal Caribbean International has two-deck slides on seven of its older Voyager- and Radiance-class ships from the late 1990s and early 2000s. But since the debut of the Freedom of the Seas in 2006, Royal has showcased the FlowRider surfing machine as its headline water attraction.
"While water slides are a great onboard amenity, we have since continued to explore the wider world of innovative amenities to offer guests at sea," said spokesman Harrison Liu.
Princess Cruises ships don't have slides, but all have dedicated play areas for kids, spokeswoman Karen Candy said. The water feature on its latest ship, the Royal Princess, is a fountain that creates sound-and-light spectaculars on deck during the evening.
Designed by Toronto-based Crystal Fountains, the Las Vegas-type showpiece includes 85 blast, burst and stream jets set into a circular deck area between two pools.
During the day, the area can be used for sun loungers or as a dance floor at the sail-away party. In the evening it comes alive with one of four computer-choreographed dancing-waters shows.
Crystal builds fountains all over the world, including those at Grand Park in Los Angeles and Millennium Park in Chicago. Parallel growth on land and sea
The rise of water features and slides on cruise ships has tracked the growth of the water park industry on land. Experts cite the 1977 opening of Wet 'N Wild in Orlando as the genesis of the modern industry. Over the intervening decades, parks have gotten bigger and rides taller and more exciting.
The biggest park in the U.S., the Noah's Ark attraction in Wisconsin Dells, Wis., covers 90 acres and features 51 water slides.
Last year, more than 85 million people used one of 1,200 water parks in North America, up from 78 million in 2008, according to the World Waterpark Association.
An issue on both land and sea are the long lines that sometimes drain the fun from slide riding. Kevin Sheehan, CEO of Norwegian Cruise Line, said one of the goals for the mini-park on the new Norwegian Breakaway was throughput. At least 1,000 passengers an hour can use the Aqua Park's five slides, which include a pair of side-by-side models.
Rittenhouse said Carnival has beefed up the non-slide play structures on Carnival Dream and Magic to make them more attractive,
"It's important to us when we're carrying 700, 800, 900 children at a time that there's lots of space for them to run around, lots of toys for them to interact with, so they're not always waiting in a line," Rittenhouse said. "So they can have fun and choose not to get in a line if there happens to be one."
Carnival has some sort of slide on every ship, Rittenhouse said, as does Disney. Royal has slides on seven ships, as does Norwegian, and Costa features them on six currently sailing ships. Meeting tough design challenges
Building slides on a ship involves some considerations that are not important for land-based attractions.
"Because of the nature of its travel -- a ship is outside sometimes in very heavy winds, rolling seas -- the engineering takes an extra level of sophistication," said Andrew Mowatt, vice president for business development at Vancouver-based Whitewater West, a leading slide builder.
Unlike a platform on land, a ship flexes, pitches and rolls as it moves through the ocean.
"We work with the naval architects to make sure those dynamics are built into our designs," Mowatt said.
In addition, weight is a bigger concern at sea than it is for a slide on land. Cruise ships cannot get too top-heavy, because that intensifies the effect of rolling in rough conditions. So the weight of the slide structure and the volume of water both have to be diminished.
Mowatt said that for some Carnival ships, Whitewater has built slide framing out of lighter aluminum components rather than steel, even though aluminum fabrication is more expensive.
To promote stability, most of the water used on ship slides is stored in tanks in the depths of the ship and piped upward as needed, Mowatt said. "The big weight of the water volume that's in that circuit is down much lower than you would see on the top deck of a ship," he said.
Corrosion is another factor at sea, especially in pumps. Most ships use 100% desalinated seawater for slides because it is easier on the skin, but they can use blends of 50% saltwater or more, Mowatt said, so pumps have to be made with all stainless steel parts.
The need to wipe down the fiberglass periodically in salt air is one reason why Whitewater West uses a premium molding process that produces a smooth surface on both sides of the slide, to make maintenance easier.
Size is the other constraint at sea.
"On a cruise ship, it's ultra-compressed," Mowatt said. "It's like building on a postage stamp."
One example of how Whitewater accommodates a smaller footprint is the DrainPipe ride on Carnival ships, an adaptation of the bowl slide that has been in parks for a decade or more.
On land, riders shoot into the bowl, circle, and eventually drop through a drain at the center. On Carnival, however, riders exit on a staircase to the deck instead, so the whole bowl can be raised above the deck, preserving space for other uses.
Cruise ships also use smaller filtration systems than on land, where the typical filter is a heavy and bulky sand tank. Ships instead use pearlite systems that are more effective, save energy and water and take up to one-sixth the space of a comparable sand tank.
"Because of [U.S. Public Health Service] requirements, their water treatment systems are probably better than most outdoor water parks," Mowatt said.
Although Whitewater dominates the category, other makers have started to gain a foothold in cruise slides. MSC said its supplier is Piscine Castiglione, located near Verona, Italy.
Polin Waterparks and Pool Systems, a Turkish vendor, built the slides on the Carnival Sunshine as part of the $155 million revitalization of the former Carnival Destiny.
Indeed, slide upgrading has become a significant business, as cruise lines learn to fit more exciting and modern rides into older ships. In September, Disney plans to put a wilder slide on its 15-year-old Disney Magic. The AquaDunk starts high on the funnel and drops riders vertically before looping them out over the side of the ship.
A similar slide called Green Thunder was retrofitted to the Carnival Spirit in 2012 before it was sent to Australia to sail year-round. The plunge slide is accessed from a white stair tower that is a prominent part of the ship's profile.
"It was huge," Rittenhouse said. "It's the one thing they talk about everywhere there."
Green Thunder has been a way for Carnival to stand out in the increasingly competitive Australia cruise scene.
"It's like a calling card," Rittenhouse said. "We can say, 'Look, families, come with us. You're going to have a great time not only inside our ship, but outside our ship.'" Follow Tom Stieghorst on Twitter @tstravelweekly.