On the sweltering rooftop of Tokyo's Big Sight convention center, a handful of enthusiastic but inept amateur engineers are braving the heat to compete in Hebocon - the tongue-in-cheek Crappy Robot Competition - where extremely low-tech robots, shoddily assembled from throwaway parts, race each other to the finish line.

Or, more often, simply flail around helplessly without ever getting near the finish line. To make things more interesting, this edition of Hebocon is the first to be held on water, so competing robots have the additional challenge of not simply capsizing before they get started - a challenge that not all competitors will meet.

The contest is being held in a large water tank that's booby-trapped with a fountain and two water pumps (although in true Hebocon fashion, one of the pumps breaks down midway through). Racing from one end of the tank to the other in sudden-death matches are ridiculous-looking robots that have been jerry-rigged from bathtub toys, bento boxes and polystyrene blocks, and are powered by torpedoes, fans, propellers and even dry-ice fog.

In the first heat, a bamboo boat inspired by a somen noodle chute and powered by a rubber-band propeller is an easy winner, despite losing its load of noodles. Its rival, a mechanical dancing chorus-line toy sealed in a plastic box, gets tied up in the hoses under the fountain.

Frequent competitor Anipole cobbled together her floating robot diorama in about two hours the evening before the contest. A doll in a camouflage outfit spins around a pole on top of a graveyard of toy jeeps. Unfortunately, during the race the dancer's legs snag on a jeep, causing the gearbox to burn out. Under the water, two tiny engines take the float nowhere.

Another crowd favorite is a boat with a paper fan for a sail. According to the maker, water poured onto the dry ice in the boat should generate enough fog to fill the sail and drive the boat to victory. Unfortunately, the boat capsizes a few seconds into the race, spilling dry ice into the tank and flooding one end of the pool with billowing white smoke, much to the delight of the children in the front row.

While the crappy robot contest celebrates amateurishness, the two contest MCs keep up a very professional vaudeville-style patter. Unlike in other contests, extra points are awarded for ineptitude and silliness. After one race, the MCs praise a watermelon-themed boat for its completely superfluous outrigger melons, before awarding a prize to a plastic peach with a figure on top that bobbed along the start line for two full minutes.

At the end of the afternoon's round, it was a studious-looking boy with a slick catamaran that wiped the floor with the competition, but there was more applause for the total losers, like the guy who failed miserably at blowing air from a foot-pump into his Perspex boat. Or the engineers who created a boat made from chopsticks and water wings, which unfortunately disintegrated when it was pulled by a string powered by a fan that was plugged into a notebook computer.

As the crowd of spectators drifted away, the winners of the round clutched their prizes and posed for pictures, while the creator of the somen noodle-chute robot could be seen fishing runaway noodles from the bottom of the tank.