PORTLAND, Maine -- Moments before Captain Dan revved up the Superduck's diesel engine and steered the bright yellow behemoth onto the road, First Mate Eriq advised passengers, ''Don't be afraid to be obnoxious. Quack away!''
The Duck, 39-foot-long and weighing 13 tons, then set off on an amphibious tour of attractions in Maine's largest city.
For the first half hour, the vehicle was akin to an oversized open-air trolley that navigated major thoroughfares while Eriq and Dan traded bad jokes and chatted about local history and traditions.
It then morphed into an excursion boat that cruised for 40 minutes along the waterfront, providing views of three lighthouses and a fishing boat unloading its catch. Common sights included giant tankers at a terminal in South Portland and lobster boats and pleasure craft sailing in and out of Casco Bay.
On its land journey, the Duck rolled down the waterfront along Commercial Street before moving a few blocks inland past such landmarks as the Victoria Mansion, the Portland Museum of Art, City Hall and the Portland Observatory.
Every so often, a pedestrian within shouting distance would let loose with a quack or two, prompting some riders to respond in kind.
During the two years that the Duck has been in Portland, locals have grown accustomed to the strange-looking sea-and-land yacht. It's a particular favorite among kids at a local day care center who shriek with delight when it passes by.
Portland is one of a growing number of cities where duck boats have taken hold, offering an alternative to sightseeing buses and trolleys.
Some of the vehicles are amphibious military personnel carriers dating back to World War II that have been restored and reconditioned for peacetime use. Known by their original military acronym as DUKWs, they were first introduced in the tourism market in 1946 in the Wisconsin Dells, where about 120 of the vessels now operate.
As of 2000, there were more than 250 refurbished amphibious vehicles in service nationwide, according to the National Transportation Safety Board.
Bob McDowell, a duck boat operator in Branson, Mo., has been reconditioning DUKWs and supplying them to tourist operations around the country. Other reconditioned vessels include LARCs, which were built for the military during the 1960s, and the Alvis Stewart, used in the British military.
The two Ducks in Portland are modified trucks built for the sightseeing trade. The 49-seaters, called Hydra-Terras, are built by Cool Amphibious Manufacturers Inter-national, or CAMI, in Bluffton, S.C., which has units at such far-flung locations as San Diego, Ketchikan, Alaska, and Osaka, Japan.
CAMI's president, John Giljam, says any place that has waterways and tourism is a potential home for a Duck. Rich local history is also a plus, giving tour guides a springboard for their narrations.
One of the most successful operations is Boston Duck Tours, which began in 1994 with four reconditioned Ducks and now has 17 of them. The business draws 450,000 riders during the eight-month season from April to November and could handle more if it could add more vehicles, says Cindy Brown, general manager.
''We've been under-Ducked for years,'' Brown says, but local officials have refused to permit additional craft for fear of adding to traffic congestion caused by Boston''s ''Big Dig'' construction.
The industry suffered a setback when one of the restored models sank in 1999 after entering a lake in Hot Springs, Ark. Thirteen of the 21 people on board drowned. More recently, four people were killed last June when an amphibious tour boat, the Lady Duck, sank in the Ottawa River near Canada's Parliament.
Federal safety officials who investigated the Arkansas accident blamed inadequate maintenance and recommended that the vessels be modified with backup flotation devices.
McDowell says some operators went out of business after the tragedy but that the industry has since recovered.
''A lot of people have come and gone over the years in this business,'' he says. ''Since the success of the Boston Ducks, everybody thought they could get into the Duck business and make a go of it, but that's not always the case.''
Giljam says his aluminum boats have sufficient foam to achieve positive buoyancy and are completely unsinkable. Still, before the Superduck plunged into Casco Bay, Eriq gave a demonstration worthy of a veteran flight attendant as he showed how to don the life preservers stored under the canopy.
Then came the moment of truth: As Dan and Eriq jested about a hole in the hull that might need plugging, the Duck rolled down the launch at East End Beach and hit the water with a dramatic splash.
''Hey, we floated,'' said Dan, feigning surprise as he guided the Duck past a flotilla of sailboats at their moorings.
Chugging along to the inner harbor, the boat passed the old artillery battery site at historic Fort Gorges, a pair of 300-foot-tall oil drilling rigs being outfitted for deployment in Brazil and a cruise ship that had just made Portland its port of call.
But the biggest attraction of all seemed to be the frisky harbor seals that bobbed up from the water to greet the Duck. Riders grabbed their cameras and leaned over the side to snap pictures.
If you go ...
SEASON: Super Duck Tours begins its season in early May, but business doesn't heat up until summer. It peaks at five to eight trips a day in July and August, then gradually slows to weekend-only operations during ''leaf peeper'' season in October before shutting down for the year.
www.superducktours.com or (207) 773-DUCK.
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