Tiny sailboats rocked from side to side Sunday morning along the Petaluma River, at the mercy of the momentary wind gusts. “Any wind is good,” George Pedrick, 65, who lives in Point Richmond, said.

Each pointed a boxy radio control at his own DragonFlite 95, a meter-long vessel with a pointy sail like a shark fin. Sometimes the boats rounded markers made from painted Clorox bottles. Other times, ...