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New Bay Bridge Magnetic Anomolies

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  • New Bay Bridge Magnetic Anomolies

    Brian Boschma, a man of many talents, a short handed sailor and all around good guy thinks outside the box. And sometimes outside the boat. For instance this little gem on something most of us would never think about. However if you are navigating in dense fog, and are counting on your compass to get you safely through the narrow passage under the eastern span of the new Bay Bridge, you might be surprised to learn what Brian has discovered...

    The new Bay Bridge stanchions, on the NE side of the bridge exhibit a substantial magnetic anomaly. In two recent passes through the bridge I have measured shifts of over 30 degrees when passing under the bridge and within a few boat lengths, approximately 80 feet, of the stanchions. The anomaly is most pronounced near the center of the road bed above you. If you are steering by autopilot this anomaly can lead to a collision with the piers.

    I happened to be testing an AP on Apr 11 and opted to engage to see the effect. As you approached the piers from the north the AP would attempt to alter the course significantly towards the east. Traveling from south to north and with the piers within a few boat lengths would have certainly resulted in a collision. My measurements were to the east of the E pier, this is outside the new construction zone (the area with scaffolding, between E and Yerba Buena, is restricted).

    I then made another pass paying close attention to a flux gate compass with heavy damping, while moving at about 3 kts. The shift was apparent in the readings. The old piers, to the south, showed no such disturbance. I pass this along as a safety tip. I already know of one boat that mysteriously hit the new construction when an AP made an unplanned abrupt turn.
    ~Brian Boschma~
    " I just found out my nest egg has salmonella" Photo Gallery

  • #2
    Obviously the Bay Bridge has an inferiority complex as regards her much more famous sister, the GG Bridge. The Bay Bridge would like to have its own demon to scare sailors with, and a little more respect!


    • #3


      • #4
        Must be all the cameras they have installed.


        • #5
          Homeland Security anti-terror system. Keeps would-be bridge attackers confused by distorting the magnetic reading.

          I remember seeing somewhere reports of a "mystery spot" somewhere between Alcatraz and Gashouse Cove, though I've never noticed any compass anomalies myself.


          • #6
            Perhaps an accident waiting to happen?

            Racer X (J105) was on AP under power at 7 knots on the way to the Big Daddy pursuit race. DH crew was moving around the boat to finish rigging for the race. We were passing under the bridge in the temporary main channel when the boat turned violently to port. Dove into the cockpit, switched off the AP, no time for evasive action or reverse, turned all the way to port and went between two of the new bridge abutments.

            It will be ugly when someone is not as lucky as we were collides head-on with the new abutment.


            • #7
              Might the problem be corrected somehow? Huge magnet(s)?... no, they'd have to move with the boats somehow, wouldn't they?

              There was a story recently that a number of the tie-bolts encased in the tower's concrete were failed, so that some sort of external brace is going to have to be designed and added to secure the roadbed. Perhaps that will make the problem less severe... or worse.


              • #8
                There are methods to "degauss" ships after construction. I suspect the same techniques could be applied.