Since World War II, the Navy’s steel pontoons or “magic boxes” have been extensively utilized as components of amphibious landings as causeways, lighters, tugs and other items. These pontoons were designed to transfer materiel and equipment from ship to shore in deep water. During various Pacific campaigns the ocean waters were deep enough to support these pontoons and items made from them.
That was great as long as naval amphibious warfare remains in deep waters, but what happens when supplies are needed up rivers or on shallow banks? As the U.S. entered the Vietnam Conflict this problem became more and more prevalent until the Navy reached out to our good friend Dr. Arsham Amirkian
Dr. Amirkian, chief engineer at the Naval Facilities Engineering Command (NAVFAC), conceived of the AMMI Pontoon as a possible solution to the cargo off-loading problems in Vietnam caused by the lack of adequate deep water port facilities. This pontoon gave the Seabees a unique component for rapid port construction and operation from the amphibious assault.
The AMMI Pontoon had several advantages over the magic box, as pointed out in the table below, but the thing that gave the AMMI Pontoon a major advantage over the magic box is the framing system called “biserrated orthotropic.”
General Differences Between the “Magic Box” and the Ammi Pontoon
|Metrics||Magic Box||Ammi Pontoon|
|Dimensions||90x22x5 feet||90x28x5 feet|
|Weight||67.5 tons||50 tons|
|Drawn water||20 inches||8 inches|
|Weight supported||100 tons||290 tons|
|Freeboard||10 inches||10 inches|
Unlike the conventional framing of watercraft, where use is made of rolled-section stiffeners of L- or T-shape, all AMMI Pontoons feature a novel rib system that not only increases the structural strength, but also reduces framing weight. It is the “biserrated orthotropic” that makes the AMMI Pontoon so unique. Dr. Amirikian named the system to convey the two characteristics of the bent-plate stiffeners used in the system: 1. The trapezoidal trough shape of the member and 2. The serrated openings that occur along the upper edges of the two sides.
The strips of plating for the stiffeners are cut from a large plate along two paralleling serrations, rather than along straight lines that are used in conventional pontoon building. By this means, the width of the strip is increased by a distance equal to the depth of the serration, without an increase in its weight. Subsequently, when the strips are bent to form trough shapes an increase of almost the same extent is also obtained in the depth or height of the member, with a corresponding increase in their strength.
Another bonus for the “biserrated orthotropic” design is that when these members are welded to the shell plating, the serrated openings, which occur in an alteration sequence in the two faces, provide access for depositing the back or interior welds of the connecting edges for full fusion. The same openings also making it possible to inspect and maintain the otherwise-concealed interior surfaces of the ribs.
Another distinguishing feature of the AMMI is the end-connectors and the tube pile spud wells. These items enable the pontoon to be used to build bridges both floating and elevated. How does it do this? Stay tuned for next and final instalment to see how!
Meet the Archivist: Ingi House
Ingi House is originally from Kansas where she got her B.A. in history from K.U. and M.L.S. from E.S.U. After working for the Dole Institute of Politics she moved to the East Coast. In D.C. she worked at the National Archives and Records Admiration and then at the Defense Acquisition University where she became a Certified Archivist. Her continued enjoyment of military history lead her to switching coasts and coming to work for the Seabee Museum where she is collection manager for the archives and records manager liaison