Pervasive problems aboard boats are ground faults and short circuits. Both can slowly but surely drain the batteries, even with every switch turned off. Perhaps more important, these snafus can cause stray current corrosion, which can sink your boat by eroding away through-hull fittings, fasteners, rudders, shafts and drives. Here’s how to smoke them out and enjoy better service from the batteries. You’ll need a multimeter for both techniques.