Transformational Technologies- An Incomparable Tool
We cranked the motor and set course to the northwest from Warderick Wells towards Norman's Cay. We were in the beautiful Exuma island chain in the Bahamas, headed for the secure anchorage at Norman's before a 'norther' came through.
A line of dark clouds built to the west and closed in. As we approached the relatively narrow reef-bound entrance to Norman's, I tried to crank the engine. Nothing. Not even enough power to set off the alarm buzzer. And this from an engine that had fired effortlessly three hours earlier, on a boat that was only a year old.
The squall line was almost upon us. When it came through, the winds would switch from the southwest to the northwest, so there'd be no problem sailing eastwards into the shelter of the island, but anchoring without power in the midst of the cruising fleet would be stressful. Instead of dropping the sails, as had been our intention, we reefed down and got the anchor ready to go.
The squall line hit as we closed the reef entrance, with northwest winds gusting to 45 knots and driving rain. In no time at all we were weaving through the anchored fleet. The holding is none too good in much of Norman's so I wanted plenty of dragging room in hopes this would give me time enough to solve the cranking problem. At the widest part of the relatively narrow anchorage I rounded up. We tacked to the windward side, letting fly the sheets and dropping the hook just before we ran out of water.
We got lucky. The anchor took hold and we were spared the task of sailing it out and resetting it in the midst of the squall. It was time to do some troubleshooting. I pulled out my trusty digital multimeter and began checking for voltage in the engine wiring, working back from the key switch. Nothing. And nothing at the solenoid, and nothing anywhere else on the engine. I checked the isolation switch for about the tenth time to make sure I was not doing something really stupid. It was on.
A voltmeter measures the voltage difference between its two probes. It can be used in two ways. One is to measure from the 'hot'(positive) side of a circuit to the negative side, in which case you read the system voltage (e.g. 12.0volts on a 12.0 volt system). The other is to place both meter probes on the same side of a circuit (i.e. the positive side, or the negative side), in which case you should get zero volts because there is no voltage difference to be measured. This is called a volt drop test. I learned long ago that it is the most useful troubleshooting test on a modern, electrically-loaded boat.
I turned off the battery switch and measured across its two terminals: 12.0 volts. Just what you'd expect, because now one side of the switch was at battery voltage, and the other, through the rest of the system's wiring, 'bled down' to battery negative, so I was, in effect, measuring from battery positive to negative. I turned on the switch and got zero volts. Again, just what you'd expect, because I was now essentially measuring at two points on the same side of the circuit. How come I had no power anywhere else in the system? I was baffled.
On the back of the switch was an aluminum plate, connecting one side of the switch to an emergency paralleling switch. As an afterthought, I tested from the aluminum plate to the terminal on the cable feeding the rest of the engine circuit and got 12.0 volts. How could this be? I was testing from one piece of metal securely clamped to another piece of metal. There was absolutely no way there should have been any voltage differential to measure, but I was getting full system voltage, suggesting a break in the circuit.
I undid the nuts to the two switches and removed the aluminum plate. It was then that I noticed a layer of oxidation on the surface of the aluminum. A little more investigation showed that this was creating a total insulating effect - from allowing full cranking current to pass at Warderick Wells, we had gone to not even allowing milliamps to get through three hours later. If I had not seen it myself, I never would have believed it.
A little bit of sandpaper fixed the problem temporarily. The permanent fix was to replace this, and several other aluminum 'buss' bars, with copper. I also discovered a number of high current fuses with aluminum connecting plates. These too were replaced.
The immediate lesson here is that aluminum has absolutely no place in an electrical system on a boat. The abiding lesson is that the voltmeter is an incomparable and irreplaceable tool. All boat owners with electrical systems should know how to take basic voltage readings and how to do a volt drop test.
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