Scott wrote: "I postulate that at some point local magnetic disturbances caused a deviation limit in the AP settings to be exceeded. The AP sounds an alarm and stops providing rudder input, but I'm not certain it returns the rudder to center. Therefore if the rudder was commanded hard over in an attempt to deal with the disturbance . . ." ****************** Scott, do you think the magnetic disturbances might be military signals such as a powerful radar or other focused transmission? Submarine? Sunken ship below? While we don't have submarines on Lake Superior, there are significant iron deposits, particularly near the north shore which have played havoc with compasses forever in this region leading to shipwrecks before the days of electronic navigation. However, that probably is not much of a problem elsewhere. David Sorenson Duluth, MN ____________________________________________________________ Easy-to-use, advanced features, flexible phone systems. Click here for more info. http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw34S62Ha80aKyMD4uHXDAFnKJsX4sjDuPHIYbObP569TWzwA/

> Scott wrote: > > "I postulate that at some point local magnetic disturbances caused a > deviation limit in the AP settings to be exceeded. The AP sounds an > alarm and stops providing rudder input, but I'm not certain it returns > the rudder to center. Therefore if the rudder was commanded hard over in > an attempt to deal with the disturbance . . ." > > ****************** > David Sorenson replied > While we don't have submarines on Lake Superior, there are > significant iron deposits, particularly near the north shore which have > played havoc with compasses forever in this region leading to shipwrecks > before the days of electronic navigation. However, that probably is not > much of a problem elsewhere. REPLY Magnetic anomalies can be found anyplace where you find tectonic plate activity. When KVH first came out with their Azimuth 1000 flux gate model, they sent me a pair to test and report on. During testing and comparison against a Sunnto floating card compass I discovered a small area local anomaly in my own back yard. Very peculiar. The floating card compass did not react to it but the fluxgate did show a 20 degree deviation over a small area. I finally got talking to the Geological Institute of Mining in Sudbury where I learned that folding of subterranean rock strata can produce intense but small and localized magnetic deviations of as much as 90 degrees. As David mentions, the northern half of the Great Lakes are known to have many magnetic anomalies. Since the West coast is a series of subduction zones with fault lines and other extreme earth movement, it is not surprising to hear of local magnetic disturbances in that area. Until a few decades ago, the only instrument we had for measuring magnetic fields was a floating card compass. The emergence of flux gate compasses driving auto pilots and having a GPS instrument for comparison has only been available to recreational boaters for two decades. This means even the official hydrographic chart surveys may not include new data about local magnetic anomalies. Arild