The Valiant was a 162-foot luxury motor yacht built in 1926 at an estimated cost of $750,000.  Owned by West Coast Buick distributor Charles S. Howard of San Francisco, she featured a 14- knot cruising speed and 5500-mile range.  Although the Valiant was the first American ship to feature a double hull for storage, stability, and safety, this wasn't enough to save her. <P>The yacht was anchored off Catalina Island, and dinner preparations were in progress when all lights went out.  The gas generator's fuel tank was dry, so a crew member opened a valve to refill the tank, restarted the engine, and left.  The tank overflowed and gasoline flooded the engine room.  A tremendous explosion set the entire midships ablaze. <P>The Valiant burned for three days, fueled by 8000 gallons of diesel fuel.  She went down with a reported $75,000 in cash and gems.