ONBOARD THE HARMONY OF THE SEAS — The man who stood in front of the huge window of the Harmony's Dazzles lounge, the pair of Ultimate Abyss slides snaking their way down 10 decks in the background, might have been unassuming except that he was wearing bright red engineer's coveralls, thick black-rimmed glasses and a devilish smile.

He called himself "the world's only thrill engineer."

"I encourage you to scream," he said. "You might not feel like it when you start, but there is evidence that screaming will activate your autonomic nervous system, and your emotions will quickly catch up."

In other words, start screaming earlier, and you'll be thrilled longer.

The Professor of Thrill, aka Brendan Walker, who had been measuring the reactions of test subjects as they shot their way down the Abyss, was convinced that Royal Caribbean International had a hit on its hands.

And judging by the level of buzz on the Harmony, he wasn't wrong.

The line to enter the Abyss (through the mouth of a psychedelic anglerfish) started forming as soon as Royal's guests on this preview cruise (primarily British travel agents) began boarding in Southampton, England.

Designed for maximum anticipation, the slides are enclosed tubes, so riders don't know what to expect. And they're still not sure when they enter the mouth of the anglerfish and hear the screams of previous riders rising from deep inside the tube, along with a kind of whooshing that sounds like the thing just swallowed someone whole. Even the floor of the fish is transparent, the better to view the 148-foot drop to sea level.

On the other hand, once the ride starts, it is euphoric. Walker suggested that a midride snap to the right was an element of surprise that really would increase a rider's thrill level. Or was it a snap to the left? It's hard to know when you're plunging into darkness at 20 mph while screaming and "wooohooo"-ing.

The Abyss was a relatively last-minute addition to the Harmony, the result of a challenge to the newbuild team from Royal CEO Michael Bayley. In the words of Martha Acevedo, the ship's design manager, Bayley wanted "something thrilling, something fun, something special."

Bayley, for his part, said that it was his son, via a Minecraft reference, who told him to "take it to the next level."

It gives the ship that final conversational touchstone that lends the all-important buzz to new hardware. From at least the time that Royal Caribbean brought out the rock-climbing wall and the ice-skating rink on the Voyager of the Seas in 1999 it seems like many new ships debut with at least one marquee feature that sets it apart from its oceangoing brethren. Think of the bikes on the Carnival Vista, the bowling alley on the Norwegian Pearl, the FlowRider on the Freedom of the Seas. Even the Queen Mary 2 boasted the first planetarium at sea.

The purple slides of the Abyss, which wend their way down from the Pool and Sports Zone on 16 to the Boardwalk neighborhood on Deck 6, are conceptually linked to the Perfect Storm, a trio of waterslides that tower over the Harmony's pool area.

None of Royal's other ships have waterslides. But Perfect Storm appears to make up for lost time. I say "appears" only because the weather on this chilly, drizzly, foggy English cruise was not conducive to bathing suits.

In fact, the Abyss was such a talked-about feature on the Harmony that its other major claim to fame, being the largest cruise ship in the world, was almost a sidenote.

Harmony overall

The Harmony is technically a sister ship to the Oasis and Allure of the Seas. But the age difference between the Allure and the Harmony is more than six years, and in that time, Royal has introduced three of an entirely different class of vessel, the so-called "smart-ship" Quantum class.

For this version of the Oasis class, the tonnage was upped to 226,963 gross tons and the passenger carry to 5,479. Company CEO Richard Fain said the extra "few inches on the beam" give the ship more stability, which in turn enabled the line to add a few things here and there: the waterslides, for example, and the Abyss.

The Harmony is something of a mashup of Oasis- and Quantum- class styles. The Bionic bartenders, the tablet stations for booking shore excursions and buying photos, Wonderland and Jamie's Italian restaurants were a few of the Quantum successes imported to the Harmony.

In turn, the Oasis neighborhoods, the Starbucks, the Rising Tide bar and 150 Central Park are taken from the Oasis and Allure of the Seas.

The Oasis introduced "neighborhoods," and Royal continues to refine and enhance the concept. With close to 6,000 paying customers onboard, it enables passengers to circulate in clearly defined spaces without feeling overwhelmed.

On a preview cruise, where the bar is open and the arc (two days, in this case) is short, it's easy to see the possibilities but harder to gauge what a true experience would be like.

Will passengers crowd the "Grease" musical and give it a standing ovation? Probably. Will hundreds gather for an 11 p.m. dance party in the Royal Promenade? I hope so. Will they gravitate to the adults-only Solarium for a quiet, shaded deck chair? Almost certainly.

Will they race up to the mouth of the anglerfish for a trip down the Abyss? I think I know the answer.

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