How the Aerial Transfer Bridge Worked


Diagram of Aerial Transfer Bridge by Zenith City Press

Duluth cityty Engineer William Patton’s annual report for 1903, published in early 1904, describes the Duluth Aerial Transfer Bridge’s physical details: “The plans, as approved, are for a stiff riveted girder of 393 ft. 9 in. span, supported on steel towers resting on pile and concrete foundations, with the bottom chord of the bridge 135 ft. above high water. The ferry car is suspended by stiff, riveted hangers from trucks running on tracks placed within the bottom chords of the trusses. The car is proportioned to carry a loaded streetcar of 21 tons, and the remainder of the floor loaded with 110 pounds per square foot.”

The ferry car itself was fifty feet long and over thirty feet wide, with a seventeen-foot-wide center roadway and seven- foot-wide walkways on either side. The middle thirty feet of each walk was enclosed to form a cabin. Cables powered by one of two fifty-horse-power electric streetcar motors (the other was a back-up) installed beneath the gondola would drive the ferry car at four miles an hour, crossing in about one minute and ten seconds. The motors turned a drum on the side of the car; cables wound around the drum and up to a traveling pulley—a truck holding many sets of wheels—that rode along the truss. The revolving drum set the car in motion. Like a streetcar, operators used a removable handle at the “front” of the gondola to set the car in motion; for the return trip, the handle was moved to the other end of the car, now moving in the opposite direction. If the electric motors failed for any reason, operators could use a hand gear to move the car safely to shore and out of the way of oncoming vessels. When at rest, the ferry car hung over dry land, allowing ships to pass freely.

Aerial Bridge Regulations

The following regulations were put in place by Major Thomas Potter of the Corps of Engineers in 1905:

Lights: The car shall carry at night two red lights, in a vertical line, one over the other, not less than six feet apart, and located over the center of the car, at a height such that the lower light will be twenty-six feet above the water, and of such character as to be as visible all around the horizon for a distance of at least one mile.

Gong or Bell: There shall be maintained on the car a loud sounding gong or bell, under control of the operator, such as can be heard distinctly at the distance of half a mile. A stroke of one gong shall be given one-half minute before starting from either side of the canal, and two strokes just before starting. When the car is in motion during thick weather, three strokes will be given as frequently as every fifteen seconds. At any time the car should come to a stop over the channel from failure of the motor or other cause, the gong shall be sounded continuously until the car shall be clear of the waterway. In case the signal lights for any reason be extinguished, the operator will use the gong signals prescribed for thick weather, until they are again relighted.

Right of Way: The car shall be run so as to give clear channel to vessels and shall always keep out of their way. No special signals shall be required of vessels on account of this aerial ferry, but it is advised that when approaching the ferry in thick weather the whistle be sounded more frequently than one minute apart…so that the car operator can more correctly locate the vessel. Vessels are not expected to turn out for the car except in case the car shall be detained over the channel by accident. The car shall not start on a trip across the channel when an approaching vessel, which is moving at a moderate speed is within 500 feet of the car’s course. In case the vessel is believed by the operator to have a speed of eight miles an hour, or greater, this distance should be increased to 600 feet or more, so as to make ample allowance for safety. On the other hand in case of a tug towing a raft at the usual very slow speed, the interval may be less than 500 feet. The car shall in no case attempt to pass between two vessels of a tow.