Mystery math

I was hanging dinghy davits on the rear of a good-sized Hunter when a big old power boat stumbled into the next slip over. It was quite an arrival with the boat thumping and bumping off of nearly every piling in sight. The Captain blamed the ungainly landing on his not being able to find neutral on either one of his engines. Fair enough.

A day or so later another tech and yours truly were tasked with finding out why the Captain in question couldn't find his neutrals. It turned out that we could not find them either. The shift levers felt like they were anchored in thick, cold, oatmeal; both stiff and mushy at the same time. We popped the control covers at both the upper and lower helm stations and discovered every single screw in each set loose, allowing the mechanisms to flex and bind at will. Even worse, the clamps at the cable ends at the transmissions were within a thread or two of just falling off, allowing the cables to slip and flex. It was kind of amazing that the Captain managed to hit the slip at all. I haven't a clue as to how such a state of affairs came about. That was what we found when the boat landed in our laps. How it got that way doesn't really matter.

There was an additional puzzle. Though I am not all that familiar with power boat shifting levers, it appeared that some parts were missing; parts that determine exactly where the detentes are felt as the levers move. But the threaded holes where I thought such parts should go appeared to have never had any parts screwed into them, ever. A bit of Internet searching came up with a parts diagram (yeah, I was amazed as well) that listed the missing parts as a “kit”. Apparently not every installation includes them, though it is hard to imagine why. In any case, it looks like the neutrals might really be missing, as in not on the boat at all. Maybe they have never been on the boat. Maybe the design engineer decided that they don't need to be on the boat. There is simply no way of knowing.

Kits were ordered and I would have liked to put them in just to know, but they ended up on back-order. The Captain needed to be on his way so we offered to forward them to wherever when they arrived. And with that he fired up his big old power boat and headed out...all the way to the middle of the yard's basin, roughly two boat lengths.

There he discovered that his starboard engine had refused to start (not sure why he left the slip without it) and that his entire DC electrical panel had died. A bit of a scramble ensued, a small power boat was launched to help corral the wayward yacht while fenders, lines, and poles were brought into play. I watched from the far side of the basin, having been earlier dispatched to check something else on a different boat. When that task was done, I found myself assigned (along with my original partner in the neutral search) to figure out what had gone wrong with the big boat.

While the other tech worked on replacing a badly corroded starboard-side engine ground he had found, I went looking for the DC panel's missing electricity. Turned out it was tucked away in the forward port corner of the engine room, a tight little spot forward and outboard of the port engine. (That was the one that had started. Leaning against it was a mistake I made only once.) Inside a plain looking box were two 12 volt, 100 amp fuses that fed electricity from the batteries to the DC panel. Both had failed under some kind of massive load. As usual, we were working without any kind of schematic, so the exact wiring details were anyone's guess.

Though a couple of people suggested that the corroded engine ground could explain the blown fuses, both the other tech and I were skeptical. Corroded connections normally reduce current flow but, hey, this is the boating industry. Maybe it has its own physics.

New fuses showed up while the two of us were off trying to figure out a reluctant system on a third, unrelated, boat. I bailed on him to put the new fuses in the big boat, which brought up the DC panel. But the starboard engine starter still refused to turn the engine over fast enough to get it running. General consensus was that the start battery was toast.

At this point, I must digress a bit. I openly admit to being a completely anal aviation tech, one trying to be useful enough in the marine industry to fill our cruising kitty. Fixing things is fun enough, but knowing just what was done to fix a thing, and what is was that caused the thing to fail in the first place, is better. When it comes to mechanical things I don't like unsolved puzzles.

I was told to just jump the start battery to get the engine running, but two blown 100 amp fuses not directly involved in the starter circuit was a puzzle that chafed. Things will tack weld themselves at 200 amps; smoke and sparks can get downright exciting at 200 amps. Excitement I wanted nothing to do with tucked in an engine compartment with 2/0 wires running in every direction.

My partner for the day had hit a stopping point on our other project and so joined in. He is good, knows more in general about boats than I do, and is used to working in the blind. Still, he indulged my reluctance to just start jumping things; though I suspect he would have gone straight to jumping the start battery (what we would have called a “smoke check” back in my aviation days). He is also about half my age, but I don't hold that against him.

We put a voltmeter on every battery we could find, both with the battery charger on and with it off. It is a preliminary check that really doesn't do much more than rule out a battery that has suffered an internal self destruct, but that is a good thing to rule out. We checked the voltage drop across the start relay, bypassed the ground half of starter circuit, then bypassed the power run from battery to relay. Nothing appeared amiss, except...

This boat is equipped with an enormous gyro bolted athwart ships to act as a stabilizer. Spinning that thing up has to chew through a noticeable amount of wattage, and it was starting to spin as soon as the ignition for the starboard engine was turned on. Though I don't know much about such gyros, that just seemed wrong, and my partner in crime agreed. A breaker was found to kill the thing. With nothing else obviously amiss we jumped the starboard start battery and hit the switch.

No smoke, no sparks, the engine fired right up. The starboard start battery was, indeed, toast.

Still, the math bothers me. It doesn't seem completely impossible that a dying battery facing the combined loads of an engine start and stabilizing gyro's start-up surge (which does flow through the DC panel), was enough to do in the fuses. Amps can do unexpected things as voltage falls away under big loads. But it doesn't really add up, and I am not convinced that we know what went wonky, where, when, or why. There is no parallel switch on this boat, no battery isolation switches, the battery that has gone toes up is labeled as both a “start” and “house” battery, and there are two other batteries in the engine compartment have no markings at all. Mysteries abound.

But if the boat starts with a new battery and the fuses don't blow, the owner will be happy, pay his bill, and be on his way. A tired battery, two equally tired fuses, a funky ground, some sneaky circuit some where that has a gyro spinning up when it shouldn't? Maybe, in this case, 1+2+1+1+1 = 5.998, and that is all the answer I am going to get.