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Magnetic anomaly almost causes disaster

Jan 1, 2003

To the editor: Recently I experienced a near disaster under circumstances that I have never read or heard about. I therefore thought it worthwhile to record what happened.

My 40-foot sailboat Moonraker was proceeding up the St. Clair River enroute from Detroit to Port Huron, Mich., where we would begin the Millennium 600 race from Port Huron to Chicago. Besides me, there were three old friends aboard, all sailors. Two had raced with me for over 20 years and were to be the watch captains on our successful (second place finish) race to Chicago.

The St. Clair River current is swift enough to make it very worthwhile for up-bound boats to hug the banks. Despite this, we found ourselves in over 2 knots adverse current even when we hugged the left bank as closely as we dared. Therefore, we crossed to the right bank, the Canadian shore, in hopes of finding less current. Along this shore was dockage for ships, mostly tankers, to moor near the chemical and refining plants. Actually, the dockage was really a long row of clusters of pilings against which the ships could moor.

We knew that, unlike the shoaling left bank, here there was no fear of grounding, since the water was deep enough for ships. We felt perfectly comfortable just clearing the dockage by about 20 feet, so we went that close to find less current. In racing, we often pass within two feet of steel buoys. Therefore, 20 feet was a big, safe clearance for us. So safe, in fact, that we were steering by autopilot, just adjusting a degree or two now and then. We had used this particular autopilot for many years, and it had earned our confidence. The day was clear and beautiful. We were enjoying the peaceful leisure before the big race.

We traveled in this mode for a while, with the helmsman monitoring the position and course and making occasional, small changes to the autopilot. Suddenly, he saw that he needed to subtract a few extra degrees because the boat for some reason turned slightly right, toward the next cluster of pilings.

In the briefest period of time, we were all suddenly aware that the boat was headed dangerously to the right. The helmsman had, by then, twice punched in "left 10 degrees" in rapid succession. We were still turning right very rapidly.

At this instant, the helmsman pulled the tiller free of the autopilot and gave a hard left rudder that caused us to avoid collision, but just barely. The nearest piling was off of vertical, sloping outward at the bottom, so that our chine, rather than our sheer, would have struck first, possibly causing underwater hull failure.

Had the autopilot malfunctioned? Simultaneously, two of us realized the reason the boat had turned right. It was not a malfunction of the autopilot. The problem was elsewhere. The water was approximately 30 feet deep. The steel pilings towered above the water perhaps 20 feet and surely were driven into the river bottom at least 20 and probably 30 feet. Thus the pilings were approximately 70 to 80 feet in length. They were in clusters of six, each piling angled slightly off vertical for stability, lashed together with wire rope.

So what occurred was clear. Such a massive steel structure has the potential for locally changing the earth's magnetic field. If the steel pilings had been horizontal, the effect would have been obvious and even greater but, because the "dip" in the earth's field is large, vertical iron can have a large, although local, effect.

In slightly technical terms, the earth's magnetic field much prefers to pass through iron, which is highly permeable, rather than through air and water, which have low permeability, or even through earth. Consequently, the direction of the magnetic field is distorted so that it points toward any steel object. The distortion is very localized, significantly affecting a region surrounding the steel object that is only a few times larger than the object itself. Once you are in this region, however, the effects become increasingly dramatic as you get nearer to the object, as we had demonstrated. All of this is well understood by physicists and also by mariners skilled in the compensation of compasses aboard steel ships, but these are vanishing breeds.

Our 20 feet, while on a northerly course, was close enough for the direction of the earth's field that was guiding our autopilot to be slightly changed in the precise direction to draw us closer. And as we drew closer, the effect was dramatic and nearly disastrous.

The near miss in the river might suggest that an autopilot should never be used in proximity to hard objects. This is a tough issue for me. If you have great confidence in your autopilot, then surely it will avoid any momentary lapses of attention that characterize many humans. Also, the autopilot is surely more accurate than most humans. So it's a judgment call. I will still use my autopilot a great deal, but surely I will stay far away from using it very close to big masses of iron, including ships, of course.

Nils Muench is a racing navigator and boat owner who lives in Bloomfield Hills, Mich.