An eight-woman crew cut through the waters of Petaluma River on Sunday and quickly gained on another eight-woman boat.

But just as they charged ahead in near-perfect unison, the other team showed a surge of energy in a noontime heat of the Wine Country Rowing Classic off Shollenberger Park.

For the next 1,500 meters, as soon as one stern lurched forward, the other heaved and pushed ahead, with dueling coxswains urging them on.

"This is called emptying the tank," said chief referee Dave Hayes, 59, of Santa Rosa, who rode behind the teams in a skiff.

"There's something about moving next to somebody," said course marshal Ned Orrett, 63, manning the outboard motor.

"Everyone starts pulling a little bit harder," Hayes said.

The Petaluma-based North Bay Rowing Club hosted 558 athletes from 27 rowing clubs for the seventh annual Wine Country Rowing Classic.

Teams raced for 5,000 meters on the tidal slough, a winding course that ended just before the Petaluma Marina.

Athletes came from 20 cities in four states. The youngest rower was 14; the oldest in his 70s, Hayes said.

Most boats finished with times of 17 to 24 minutes.

Joggers, cyclists and dog walkers rubber-necked their way along Shollenberger Park's gravel path.

Groups of spectators and teammates cheered from benches and lookouts. Eileen Wheatman of Petaluma was ready with a camera to catch her coworker on the water.

"It is kind of exciting. I have never seen him row before," Wheatman said.

Sunday's race was the first of the season for Jackie Leary, who rows with Sonoma State University's varsity team.

Rowing is an early-morning sport when the water is calm, the wind low and the sun rising, Leary said.

"You feel really close to the water, and once you get the correct rhythm, it feels so smooth," said Leary, a sophomore environmental studies major.

One scull flipped during Sunday's event, but the rower was able to get back into his boat. Another rower missed a turn and briefly landed in the weeds.

The fastest time of 17 minutes and five seconds was recorded by the eight-member Marin Rowing Association men's high school team.

But with times adjusted for age, they came in just behind their teammates on the men's masters boat. That eight-man team passed the finish line in 17 minutes and 41 seconds but, with the age handicap, received a net time of 16 minutes and 48 seconds, according to posted results.

The dueling women's boats also were from the Marin Rowing Association, among the largest rowing clubs in the Bay Area.

Susan Mackay of Larkspur was on the boat that finally pulled ahead.

"It was exciting. It's easier to stay strong when there's a boat to race against," Mackay said.

You can reach Staff Writer Julie Johnson at 521-5220, or on Twitter @jjpressdem.