Live-aboard cruising means carrying spare parts and/or being able to effect repairs and improvise. This story is certainly of the improvise category.
Of course when we are under way we are always looking at gauges, engine temperature, oil pressure (not fuel though because that one is broken) and once in a while the state of the batteries. I say once in a while instead of my former every 30 minutes or so since we have stopped living with the shadow of refrigeration over our electrical heads! So, perhaps I hadn't really looked at the voltmeter in a day or two. With the engine running though it should have been around 14 volts and it was at about 12! That ain't right!
My first thought was that we had a broken belt causing the alternator to not turn at all. Nope, engine and belt are spinning just fine. Now what? Well, if you're me, you immediately turn to Nigel Calder's Boatowner's Mechanical and Electrical Manual and re-read the section about alternators and regulators. In so doing I re-learned how basically simple this electrical arrangement is. The alternator produces a current via spinning a magnet in the middle of several coils of wire. This is 19th century physics ala' Michael Faraday. Basically simple. To be able to control the output of the alternator the spinning magnet is actually an electro-magnet - another coil of wire around an iron core (remember making an electromagnet with a nail, wire and a battery in grade school? You should!) Now you can control the output of the alternator by controlling the current to the magnet. This is the 'field' as referred to in books and such and there is a 'field' terminal on the alternator somewhere for an alternator with external regulation.
I need to know if the problem is the alternator or the regulator. I'm thinking it is probably the alternator because that is the more expensive of the two parts and one of the rules of boat maintenance is that for interdependent devices it will always be the more expensive one that has to be repaired/replaced. A simple test is to disconnect the regulator and put a jumper wire from the +12 volts directly to the field. I did this and started the engine. The voltage climbed very fast telling me that there was output from the alternator and that the problem was with the regulator. I checked for the fuse that was supposed to be installed in line with the regulator +12 v but there was none. OK, I need to order a new regulator.
At this point we are in Ponce, Puerto Rico. It is a half mile walk to the only working phone we could find and miles to anything like civilization. I do manage to put in an order with 'our guy' in Florida and expect to have a new regulator shipped to Salinas in about 10 days. Great except I don't plan to sit still nor read by candle light for 10 days. I just need a way to control that current back to the field. I need a variable resistor. . . . probably about 0-25 ohms at 10 watts would do it I'm thinking. A nice thought but I'm guessing I'd walk a long way looking for "Resistor Mart"! But, this is not rocket science. I should be able to find something on board to provide that resistance.
Every electrical device has some resistance. It is a measure of how much that device resists the flow of electricity. A device that allows 1 amp of current when presented with 1 volt of potential is said to have a resistance of 1 ohm. 1 volt and 1/10 of an amp would be 10 ohms of resistance. It's just voltage/current. First I tried to just put a 12 volt light bulb between the +12 v and the field. That was too much resistance as the voltage barely crawled up to 12.2 volts.* Now what? Well, wire has resistance but not much. However the longer the wire the more resistance. I connected two 50 foot reels of #22 wire together in series and tried that. That has a total resistance of about 4 ohms. Hmmm... not bad, The voltage came up higher but I have little control over it. Then I thought of those 15 feet of seizing wire I recently bought at the Sunday flea market in Luperon, Dominican Republic. I stretched out the remaining 12 feet and measured the resistance with my multi meter (you HAVE one, right?). Good news, it's about 10 ohms! And it is bare wire. That means I can make a variable resistor out of it between 0 and 10 ohms. One end of the wire will go on the +12 and then I'll connect the field wire somewhere within the 12 feet and pick off whatever resistance and hence field current I want. I don't really want 12 feet of wire laying around so I wrapped the wire around and around a piece of starboard with plenty of space between the coils so they couldn't touch each other. An alligator clip on the field wire allows me to attach it where I like. I installed a switch as well to turn the thing off. The switch allows me to turn it off all together and skip the little sparks that are inevitable when disconnecting the alligator clip.
After assembling this 'dumb regulator' we give it a try. It works! I can manually control the output of the alternator with my alligator clip. A better test will be as we make the short 8 mile run to Cayo Isla De Muertos south east of Ponce. No problem. I let the voltage settle in around 14.2 volts with the engine at 1500 RPMs for 30-60 minutes. Then I increased the resistance and lowered the voltage to about 13 volts for the rest of the time. The seizing wire gets a little toasty but doesn't melt the starboard nor does it smoke which is always a good sign with electrical things!
When I get my new 'smart' regulator I'll certainly keep my 'dumb' regulator as backup. Cheaper than buying two!
*Note that you can't measure the resistance of a bulb with an ohm-meter. With an ohm-meter the bulb won't light and you'll be measuring the resistance of the cold filament. This is WAY less than the resistance of a hot filament. For that you need to measure the voltage AND the current and then divide!
Scott Welty is one part maritime MacGyver, and two parts Mister Wizard ( or should it be Captain Wizard? ) His book, The Why Book of Sailing: The Curious Sailor's Guide to the Science of Sailing and Seamanshipand his columns in a number of sailing publications blend fun with understanding of "why" and "how" things work on a sail boat. Scott has taught high school and college physics for over a 30 years, including a 4 year stint at Chicago's Museum of Science and Industry. When Scott and his wife aren't filling the cruising kitty, they sail the "Enee Maire", an Endeavor 42.