Both types were twenty-two feet in length and took considerable time to set up, requiring several men to lift into position and pin the individual sections together.
[1] Early in 1864, the commander of the Army of the Cumberland, Major General George H. Thomas, was seeking a light-weight, easy-to-haul and erect pontoon bridge to move his troops across unfordable rivers and streams.
Captain William E. Merrill, Thomas's chief engineer, improved on Rosecrans's prototype, making it lighter and stronger.
The new design yielded a portable boat that was lightweight, small enough to carry on a standard supply wagon, and easier to construct in the field.
Merrill had the first ones constructed in the army's engineer workshops in Nashville, Tennessee, under the supervision of Lieutenant James R. Willet.