During trials of the first submarines, the propulsion system was found to be prone to failure for a variety of reasons. Most failures were attributed to the fifteen-tank diesel fuel system: the tanks were designed to fill with salt water as they were emptied to maintain neutral buoyancy, but water would regularly enter the engines due to a combination of poor design, gravity separation of the fuel and water being insufficient, and operator error resulting from poor training. Problems were also caused by bacterial contamination of the diesel fuel, which, along with the salt water, would cause the fuel pumps to rust and other components to seize. The fuel-related issues were solved by installing coalescers, improving training and operational procedures, and adding biocides to the fuel.