RedactedNarrative: Vessel 1 was traveling at a cruising speed of 15 MPH when the boat felt like it was losing speed & power.  The Operator turned back & there was heavy smoke coming from the engine bay & also some hissing, crackling noises. The operator opened up the hatch by using the hatch switch  located next to the steering wheel & saw flames and more smoke coming from the engine bay.  He went to get the fire extinguisher but by the time he came back to extinguish the flames , the halon system had tampered down the flames.  The operator stated that he had just changed the battery & thought the fire might have been electrical.