The duck boat that sank in a Missouri lake last week, killing 17 people, was built based on a design by a self-taught entrepreneur who had no engineering training, according to court records reviewed by the Los Angeles Times.

The designer, entrepreneur Robert McDowell, completed only two years of college and had no background, training or certification in mechanics when he came up with the design for “stretch” duck boats more than two decades ago, according to a lawsuit filed over a roadway disaster in Seattle involving a similar duck boat in 2015.

Officials have not given an official cause for why Stretch Duck 7, the amphibious boat owned by Ride the Ducks, sank during a storm on Table Rock Lake near Branson, Mo., while carrying 31 people on a sightseeing tour.

The Seattle lawsuit did not directly tie McDowell’s design credentials to duck boat tragedies that occurred on water, but in recent days, safety experts, lawyers and U.S. Sen. Claire McCaskill, D-Mo., have raised concerns about the design and oversight of duck boats, a popular watercraft for sightseers that also has wheels and can drive on land.

“There are inherent dangers in these amphibious vehicles,” McCaskill said on the floor of the U.S. Senate on Tuesday, alluding to a 1999 disaster in Arkansas that left 13 people dead when a duck boat sank rapidly. “When they’re in the water, it’s almost like an enclosed bus.”

McDowell and Ripley Entertainment — which bought Ride the Ducks last year — did not respond to messages Tuesday.

In the 2015 Seattle incident, a duck boat’s axle broke while traveling on a highway, sending the boat crashing into a tour bus filled with international students, leaving five people dead and dozens injured. Ride the Ducks’ parent company was slapped with a $1-million fine by the National Highway Traffic Safety Administration afterward for failing to follow federal safety rules for auto manufacturers.

In 1996, Ride the Ducks began creating dozens of modified duck boats — including the one that sank in Missouri — by disassembling old World War II-era duck boats, lengthening the hull, replacing other parts, and then reassembling the craft, according to the NHTSA.

The new duck boats, nicknamed “stretch ducks,” were designed and developed by McDowell, who first took over a duck boat company in 1970 and later ran Ride the Ducks International, according to court records.

But the lawsuit filed in King County in Washington over the Seattle tragedy, alleging negligence, raised concerns about McDowell’s qualifications to re-engineer the duck boats.

“Mainly he learned what to do through speaking with … a high school football coach who previously co-owned the business,” the lawsuit said in one filing, citing depositions with McDowell.

McDowell “self-educated by going to auto parts stores and talking to different people,” including “a transmission person, as well as the maintenance people at the local Penske Truck group and the U-Haul down the street,” the filing said.

However, McDowell “did not consult with any engineers,” and the company did not consult with other manufacturers before building the stretch ducks, according to the filing.

U.S. Coast Guard spokesman Chad Saylor confirmed that the Missouri duck boat was a “stretch” duck boat. He said the boat was last inspected on Nov. 29, 2017, and was found “fit for route and service.”

matt.pearce@latimes.com