A week before our sixth race to Mexico, we took Midnight Sun out for an overnight shakedown cruise. About 2 am everything went dark. At first I thought we had lost an alternator belt (we were charging batteries) but immediatley discounted that because we had five batteries on line. 10 seconds later, black acrid smoke started billowing out of the engine compartment. Lookinh into the engine room I could tell it was electrical ( the pile of white hot wires and burning insulation was my first hint). It took just seconds for a number 2 battery cable to melt completely through. That ended the fire but started the nightmare. What had happened was the main battery feed from my forward house battery bank (four group 130 batteries) fell across the exhaust manifold (broken zip tie) melted and caused a direct short involving probably 1000 amps of current. We are in the middle of replacing all the battery switches and cables. Still don't know about collateral damage (alternator, starter, refrig, etc)