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It’s a blast riding in the biggest heap on the street.

The 31-foot-long, seven-and-a-half-ton Sitting Duck turns heads as it lurches down Pennsylvania Avenue NW. We have just embarked on an outing in what may qualify as the world’s strangest tourmobile: a red, white, and blue boat with wheels, the latest incarnation of an amphibious vehicle that began life as a World War II military marvel. America’s Main Street hasn’t hosted such a display of war machinery in years. All we need is Lee Marvin and some machine guns, and we’d be ready for battle.

Other sightseeing trolleys seem pathetically puny next to the bulk of the mighty Duck, which strikes fear in the hearts of bike couriers and jaywalkers. Its shadow darkens sidewalks and sends joggers scurrying for cover. Even Washington’s notoriously intrepid vermin—squirrels and pigeons alike—flee the ground-shaking rumble.

But on the deck of our open-air craft, martial daydreams must face the ceaseless cavorting of our mascot, a Big Brothers volunteer in a duck suit. A microphone announcement has informed us that he is there to promote the charity’s duck derby. The oversize, hyperactive hatchling performs his interpretation of the San Diego Chicken dance, and quacks incessantly on a duck call. The publicity machine meets the war machine. Disney meets Douglas MacArthur.

Fellow tourists, though, approve of our gaudy vehicle and its honking mascot. From every street corner, the sightseers wave; many cheer. They react with the instinctive glee that tourists reserve for any dumbing-down of the landscape. Some point their cameras as though they’re aiming guns at a rare big-game specimen. Meanwhile, the Duck’s passengers snap photographs of the people taking pictures of them.

“You’re going to see a lot of people with big smiles on their faces looking at you very enviously,” brags our guide. “You’re going to be in a lot of home movies around the world.”

We have come to see the sights, but we have turned into one of the attractions.

After a tour of the Mall’s federal buildings, the Sitting Duck lumbers across the Memorial Bridge into Virginia. The waters of the Potomac beckon from below, the sun-glinted waves winking a challenge. It is time for the aquatic phase of our mission.

We amateur Argonauts begin to harbor doubts. Can this bulky war relic—now making a clumsy turn onto the George Washington Memorial Parkway—actually float? If so, why are life preservers tucked ever so discreetly beneath our seats? Our craft’s very name seems to foreshadow a horrible fate.

A pair of passengers from the local Big Brothers program sit stone-faced and silent in their seats, their intergenerational friendship apparently sabotaged by dread of the river. An inquisitive tourist—the kind who always sits near the front to pepper the guide with endless questions—asks repeatedly about the size and power of the engine, as if to assure himself that the Duck possesses the strength to stay afloat. Even the duck suit falls quiet.

We drive through a parking lot at Gravelly Point, where spectators in lawn chairs watch the planes ascend from a nearby runway at National Airport. We approach a boat ramp, but must wait an agonizing few minutes as a tentative motorist drags his cigarette-shaped speedboat out of the way. Capt. Skip, our skipper, grumbles impatiently, “Never trust a guy pulling a boat with a Mercedes.”

At last our moment comes. Jets roar overhead as startled fishermen turn from their lines to see what the hell’s going on. Capt. Skip revs the engine, and the unwieldy craft tilts down the ramp like some gargantuan Tonka sliding into a swimming pool. We hit the water with a walrus splash. Our wheels churn into the bank as far as solid ground lasts; then, when the ground gives way to water, the barge bobs up quite lightly. Capt. Skip pulls the appropriate levers, and the Sitting Duck is transformed into a swimming duck.

To our relief, we find that our craft is seaworthy, all right. This wedge-shaped hunk of steel could have survived The Poseidon Adventure. If Ted Kennedy had been behind the wheel of the Sitting Duck at Chappaquiddick, history might have turned out very different indeed.

But though we float extraordinarily well, we advance at an extremely sluggish pace. On land, the Duck reaches a top speed of 40 mph; in water, it can achieve a mere 7 knots. At one point, a family of real ducks passes us, their webbed feet far more efficient than the Duck‘s spinning tires.

The stretch of river unfolds, as bucolic as a puddle in a Jiffy Lube bay. Jet trash and beer bottles float like buoys in the brackish water, now a long way from its mountain source. Concrete boat ramps and asphalt parking lots litter the nearby banks.

Capt. Skip takes over guide duties on water. He points out the National War College, a gloomy assemblage of bricks. The passengers struggle to hear his narration, his microphone sometimes losing to the engine’s roar. To fill the time as we crawl upriver, he relates a brief history of the vessel:

During WWII, General Motors manufactured the Duck for the U.S. Army; a mostly female work force built an armada of more than 21,000 DUKWs, as the amphibious vehicles were known. Ingeniously fitted onto a truck chassis, the steel hull is safely watertight, Capt. Skip assures us; after all, the DUKWs proved their seaworthiness in the Normandy invasion, which wouldn’t have been possible without them.

In the late ’50s, a WWII vet tapped the DUKW’s recreational potential, offering tours of the Wisconsin Dells. Since then, other companies have sprung up in second-tier vacation spots such as Hot Springs, Ark.; Branson, Mo.; and Memphis, Tenn.

The Sitting Duck belongs to a company called D.C. Ducks, Capt. Skip tells his captive audience. Since early October, a three-vehicle fleet has hauled tourists around this city, the first major urban area in the U.S. to be served by DUKWs. Like the fleet’s other skippers, our captain has a U.S. Coast Guard master’s license as well as a commercial driver’s license.

Finished with his Ducks history lesson/promo, Capt. Skip focuses on his driving, and we move upriver, listening only to the engine’s spitting. Except for an occasional quack, the mascot duck remains mercifully mum.

As the boat approaches the 14th Street Bridge, Capt. Skip points out the section of the span where an Air Florida plane crashed during a snow storm more than a decade ago, killing all but five passengers. Beyond the bridge’s stone supports lie D.C.’s crown riverfront jewels: the Jefferson Memorial, the Kennedy Center, Georgetown’s ritzy Washington Harbour.

But there will be no ports of call for the passengers and crew of the Sitting Duck; we will venture no farther up this river of dreams. Our boat ride is severely time-restricted. Coast Guard regulations limit passenger crafts without restrooms to 30-minute excursions.

The passengers do not protest. The Smithsonian and other sights yet await us on dry land. But best of all, the Sitting Duck will once again be one of those sights. Capt. Skip turns our vessel back toward Gravelly Point, where the water meets the road.