While patrolling waters of Lake Koshkonong and Rock River I was a vessel launching from Harbor Recreation near Neville.  Shortly after heard a loud backfire of boat and saw 2 subject in boat and fire coming out of engine compartment.  In approximately 2-3 minutes the entire vessel was engulfed in flames.  2 people on board were taken off by another vessel.  2 extinguishers were used but ineffective  Vessel continued on river and hit Hwy 50 bridge before continuing.  Vessel continued to burn for 15-20 minutes before sinking between Hwy 59 and I-90 bridge.  Vessel was removed from river by Rock River Marina.  Operator stated he ran ventilation for 2-3 minutes before starting that he filled fuel tank with 15-16 gallons today total capacity 30 gallons.  Vessel had trouble starting and then backfired.
Newville