RedactedNarrative: Operator/owner had just launched boat and was approximately 500 yards from shore, at about 20 mph.  Operator said he heard an explosion, turned and saw the rear cowling about 20 feet in the air behind his boat.  The boat engine compartment was on fire.  Operator grabbed the fire extinguisher and tried to put out the fire, but used up the extinguisher and the fire was not out yet.  The fire then started to spread in the boat, so he waived down a passing boat for help.  Operator and 3 passengers (names not included in Officer’s Report) were able to get off the boat with PFDs on, and get to the other boat for safety.  Operator said he left the boat because it was on fire and he had a full tank of gas.  Vessel then became completely engulfed in fire and burnt to the hull.  Fire department responded on the water to extinguish the fire.  Vessel was then towed to shore.