EBDG-Designed Project Honored by ACEC

The replacement of three historic river ferries along the Missouri River in Montana has been recognized by the Montana chapter of the American Council of Engineering Companies (ACEC). The Missouri River Ferry Rehabilitation Project won an Honor Award at the Montana chapter’s annual engineering competition. The vessels were designed by Seattle’s Elliott Bay Design Group (EBDG) and Helena, Montana-based Morrison-Maierle, Inc.

The Montana Department of Transportation-owned project involved the design and construction of three replacement ferry boats and their cable drive systems, as well as new terminal structures and site improvements. EBDG developed the vessel hull design, propulsion system, electrical/mechanical systems and safety features on the ferries. Morrison-Maierle, Inc. was the owner’s primary consultant responsible for the design and delivery of construction documents.

The three boats are the last of two dozen cable ferries that once operated along the Missouri River in Montana, linking local population centers with farms and ranches on both sides of the river. The ferries operate at Carter, Virgelle and McClelland/Stafford in the vicinity of Great Falls to carry cars, farming equipment and other vehicles across the waterway. Over the years, most of the ferries have been replaced by bridges. These three locations serve a sparse population, and given the low traffic volumes on the local county road systems, it has not proven financially feasible to replace the ferries with spans.

The original ferry boats were constructed of thin wall sheet metal hull, rough sawn lumber deck planking, rudimentary cable-supported ramps and old farm tractor engines mounted on their original chassis as the power plant. As the 1940s-era ferries aged, it became increasingly difficult to repair and maintain the vessels. Much of the equipment used had become obsolete, and the aging cable system was becoming a safety concern.

The new boats feature an upgraded drive system: a 17 HP diesel engine running through a right-angle gear box to a 24-in.-dia. drum that pulls the vessel on the cable. They run at a speed of 4 MPH and feature a watertight hull. The ferries are each 50 ft. long by 22 ft. wide and are designed to carry 50,000 lbs., or the equivalent of about two cars. The project team faced a significant challenge during the design of the replacement ferries and their systems. Very little information was available to help EBDG predict the effect of the river’s sideways force on the vessels, needed to determine the size of the cables holding the ferry in place. When initial load estimates dictated the use of seemingly excessively large cables, the project team decided to seek more definitive figures by conducting a model test at the University of British Columbia in Vancouver. Scale models of the ferry hull design were tested in a 1,200-ft. long test tank under worst-case scenario conditions. Recomputed load values following the study were half of what the team had originally predicted, resulting in a savings of approximately $250,000 in cable, structural steel and concrete construction of the support towers. As an ACEC-Montana Chapter Honor Award winner, the Missouri River Ferry Rehabilitation Project is eligible for entry in ACEC’s Engineering Excellence Awards – a national competition that recognizes the winners from local chapters. The gala will be held May 8 in Washington, D.C.