The Electrical System
by Scott Dorrer
Adirondack Chapter, ACBS
I didn't have any idea what this article was going to be about after the last one was done, but a near tragedy prompted me to write about the electrical systems in our boats. The event that gave me the idea for this article goes as this. After two years in storage we finally got the water pump repaired for the A-70 engine in the 1929 26' Chris-Craft that most of you know as Shady Lady and it was time for a water test. We soaked up the hull, changed the oil and checked over the whole boat giving it an okay for the water test. The boat was launched and she started with no hesitation whatsoever. The day was pretty nice so we decided to go for a ride down the Hudson to Castleton about five miles down the river. As we were returning, about one mile from the launch ramp I noticed a smell that wasn't there before and looked down at the gauges only to see smoke bellowing from under the dash. My excitement kicked in along with my adrenaline and as soon as I had the engine shut down I pulled the wires under the dash that I could see were smoking and thought that this would be the end to the shorting. Wrong! By this time the main harness going to the engine compartment had also started to melt off the insulation so affected wires were still shorting. The next line of defense was to turn off the main battery switch and this would surely end the short. Wrong again! When that did not stop the short the only thing left was to disconnect the batteries and finally the short was then stopped.
Once back at the shop the questions started to arise. What caused the short? Why didn't the fuses interrupt the short? Why didn't the battery switch interrupt the short? All these questions were answered that afternoon. The cause originated at the spotlight. The positive and negative wires run through a metal conduit to the light, once they get to the light the negative attaches to the arm of the light with a small metal clamp and the positive is routed under the clamp on its way to the switch. The positive had worn through over time and shorted on the ground clamp causing a direct short. Why didn't the fuse blow? Simple, the light was not fused.
The next question about the battery switch took only a few more minutes to figure out. The switch was a master disconnect that was installed on the negative side of the batteries. This would have stopped the flow of electricity if the accessory grounds had not been attached directly to the battery. This in effect only cut the high amperage ground needed for the starter letting the accessories to still be grounded keeping the short circuit alive.
I used to to think that circuit protection was not a necessity, after all most of these antique boats have been around for years without it and they somehow avoided disaster. To protect against shorts I now use two devices; fuses and a main circuit breaker.
Fuse blocks and holders now come in a variety of sizes. The fuse blocks that I am now using are of the modern spade type. They come in anywhere from a single that can be crimped into the positive side of a circuit, up to a ten gang block. Most of the accessories will give the maximum amperage draw and can be fused appropriately. For accessories that do not give the amperage that you can calculate it by dividing the watts by the voltage (W/V). If you like the old test method of finding the amperage draw you can make a simple tester with an old ammeter. Hook up the accessory with the meter in series and that will give you the amperage. A final way of testing would be to install the fuse block and hook up all of the circuits, then use the ammeter already in the boat to determine what the fuses should be by turning each accessory on individually and reading the amperage off the meter.
Once I have all the appropriate fuses figured out, I then decide what the total amperage draw will be with all the circuits powered up and install a circuit breaker. This may seem like overkill but if the generator cutout of voltage regulator sticks, the individual fuses will not stop the generator from burning up the charging circuit. The circuit breakers that I use can be bought at any automotive supply and are cheap insurance against overload. I installed the circuit breaker in the line that goes from the battery terminal on the starter to the to the ammeter. I also do not hook up anything directly to the battery anymore - all power must go through the breaker.