His wet suit undone to the waist, 21-year-old Josh Burton inspected the three-blade propeller of his 9.5-foot, one-person, man-powered submarine. Nearby, his sister sawed chunks of purple-tinged foam to insulate the hull.

It was the third day of the 11th International Submarine Races in Bethesda, and Burton had heard that the United States Naval Academy’s team already had broken the competition’s record. Twice.

His family’s backyard creation — Scuba Sub — had yet to test the dark green waters of the model basin at the Naval Surface Warfare Center’s Carderock Division. But Burton, a Virginia Tech engineering student, was convinced the latest design changes would send Scuba Sub gliding underwater.

“We’re still waiting to get across the finish line,” said Burton, of Frederick, who was a third grader when his family built its first submarine. “It’s been a long time coming.”

This year’s International Submarine Races drew 29 teams of high schoolers, university students and submarine enthusiasts from as far away as Oman, Venezuela and France.

The races, organized every two years by the Foundation for Undersea Research and Education, started Monday and run through Friday. Teams are allowed to race their submarines through a 100-meter stretch of water as many times as the schedule permits.

There are no guidelines, other than safety regulations. Any design for a human-powered submarine is fair game.

One submarine was painted like a blue whale, complete with cartoon eyes and a blowhole. Another looked like a tuna. The number of yellow submarines submerged in the basin would have made the Beatles proud.

“This is a systems engineering problem with a few wrinkles: It’s underwater, it’s human-powered,” said Daniel Dozier, the naval center’s submarine race liaison.

A panel of nine judges will award teams based on innovation, use of composite materials, design, team spirit and absolute speed, among other factors. Winners get bragging rights and, for some categories, cash prizes and trophies.

Many former participants have gone on to work for the U.S. Navy or government agencies, Dozier said, adding that he has handed out dozens of business cards to aspiring engineers.

Several team members likened the experience of pedaling a submarine underwater to full-on sprinting, but with a scuba mask in 67-degree water. Faculty advisers and coaches reminded teams to fuel up with Ga­tor­ade and energy bars between runs.

Commander David Robillard, the faculty adviser for the Naval Academy’s team, wore a bright orange life jacket and watched his divers lower themselves into the basin.

The $10,000 two-person, non-propeller yellow submarine took more than a year to design and build and is powered by two sets of blades that flap back and forth in the water.

“We put on the final paint Sunday morning, trailered down here, and were the third sub to get in the water,” said Robillard, who graduated from the Naval Academy in 1988. The team broke the competition record for nonpropeller submarines on Monday and again on Wednesday, clocking in at 5.885 knots, or 6.8 miles per hour.

Robillard explained the team adjusted the tension in the blades for greater torque, or turning force, and tweaked the location of the pedals.

After completing their first run for the day, the team from the Université de Sherbrooke in Québec headed to their tent in the parking lot. Boxes of tools, pieces of wood, fiberglass sheets and Goldfish crackers lined the trailer that they traveled in for 12 hours. A barbecue grill was set up in the back.

So far, the team’s one-person propeller submarine, Smash, has posted a speed of 3.6 knots, or just over 4 miles per hour. They hope to beat 6.1 knots, or 7 miles per hour, by Friday.

“We called our sub Smash because we wanted to ‘smash’ the competition record,” said Ludovic Trenblay, 24. “But we smashed it into a wall in our first run instead.”

After Wednesday’s first run, the team decided to build a set of new fins out of wood — “on-the-spot engineering,” Trenblay quipped — to minimize the bobbing motion of the submarine.

For show, each sported a mustache for the races. Except Mathieu Philippe Gauthier Lemieux, 22, the submarine pilot. He said he shaved his team spirit Tuesday.

“It got in the way of the scuba mask,” Lemieux explained, rubbing his upper lip ruefully.