Final answer: corrosion positive bus bar (I hope I have the terminology right) adjacent to the house battery. This took a while to figure out, during which process a preliminary conclusion was that the windlass motor was bad. Manually, we could operate the windlass with a winch handle, but flicking the switch caused the house battery volt meter to go all the way down and all of the cabin electronics to shut off (motors were not running during any of this investigation, nor was the shore power activated). The only problem was that the volt (amp) meter was showing the proper amount of resistance for a good motor. To my mechanic friend -- indispensable to have if you know as little as I do and own a boat -- tried various things and found out that turning on the cabin lights would also cause the house battery voltage meter to go as far down as would register and knock out everything for awhile. So he went back to the batteries and was dismayed at the condition of the positive bus bar, and, after removing the battery and cleaning all the corrosion, we flicked the windlass switch and it worked like a charm. Lots of loose connections were found in the process, so there is more work to do, but the mystery is solved. Thanks for all of your help and encouragement.