We were starting an offshore passage
from Norfolk, Virginia to Rockland, Maine
and were four or five hours out when the alternator
stopped working. We did some diagnostics while underway and found that when we put 12 volts to the field circuit of the alternator we got 100 amps. Our diagnosis was a dead voltage regulator
and then we had to decide whether to turn around and go back to Norfolk and waste at least a couple of days, alter course to enter another port and try to find a new regulator or to continue on the voyage and rig a temporary regulator.
I vaguely remembered that Nigel Calder had written about making a voltage regulator in his book "Boatowner's Mechanical and Electrical
Manual" 2nd edition page 71. As Nigel says "A test wire connected to a 12- or 15-watt DC lamp will feed approximately 1 amp to the field winding (12 volt system)... The higher the lamp wattage, the more amperage it will pass. Divide the wattage by the system voltage to find out how many amps."
I, of course did not have the book on board so my hazy memory was all we had to go on as we wired a 12 light socket into a circuit to feed the field wire on the alternator. We tried it and it worked so we tried different bulbs until we found one that we liked the output and we then wired the bulb in series with a switch so we could turn on and off our single
output level alternator. Since this was a five to six day offshore passage
we could not have this light turned on during the night to kill our night vision so we mounted the light socket inside of a cupboard in the galley
. We would monitor
the voltage level of the batteries and turn on our alternator when we needed to charge. A nice side effect was that we had light inside of the pots and pans cupboard!
I highly recommend Nigel's books
. I suggest that you get them and read them from cover to cover, even the stuff that you think that you will never use. You never know when a slight memory may solve a big problem for someone.