It wasn't until the second and final heat for the alcohol-burning outboards that the bubble finally burst for Bob Thornton.

Thornton represented the old guard at last weekend's hydroplane regatta on the Potomac. At 52, he had the years to be father to most contestants and grandfather to some.

Tacked on the back of his over-the-hill hydroplane was a motor of classic vintage. Thornton isn't even sure how old the superannuated Johnson is. "They made them between 1933 and '37," was the best he could do.

With that motor he had swept his class the day before, beating out two youngsters with hot new Konig and Yamato engines that turn about 4,000 rpm more than he can.

How did he do it? "They couldn't get 'em started."

Then on Sunday he had blasted along to a second-place finish in the first heat when rough water forced the newer, lighter boats to run half-throttle for all three laps between the 14th Street and Memorial bridges.

It was in the final heat that Thornton got his comeuppance. A plug fouled in the old motor and he had to circle the course at a tepid 30 miles an hour. As he burbled by the judges' stand he beat the gunwale of the little boat with his ham fist.

No great matter. Thornton finished third for the day out of four boats, and the thirdplace money of $5 was only $7.50 below the winner's share. Which has to make outboard racing the lowest-return professional sport around.

Actually it doesn't seem professional at all. Almost all the competitors at the Potomac race were racing hobbyists. They travel from state to state all summer long on the weekends, beating the gentle freshwater swells at speeds up to 90 mph for token prize money.

They don't act like they're out for blood, either. Racers in this tight little fraternity spend half their time sharing propellers,, engines and even boats. They may end up racing against their own motor on someone else's boat.

Robert Smith, an Air Force fighter pilot, organized the Potomac race and is hoping it will become an annual event. He said that even at national races first prizes are often in the $50 to $100 range, though some go higher.

So if it's not money that induces these boatmen to make long pilgrimages to foreign waters, what is it?

For Thornton it's a sweet smell he can't get out of his mind. He saw his first hydros racing on the Schuylkill River in Philadelphia in 1937. "They had alcohol-burners back then, too," he said. "The fuel they used was called DuPont Dynax and they mixed it with castor oil, just like we do today. It had an aroma I'll never forget."

Thornton, who had fooled with motorboats and sailboats as a child, was hooked on hydros.

It was a lot the same for Smith, who has been meddling with motors and boats for 32 years and recently won the national marathon championships in a 13-foot runabout with a 20-year-old Mercury outboard.

"You've got to try it," said Smith. "All you've got to do is squeeze the throttle and turn left."