When performing a whirling calculation on a propulsion system, the hull structural supporting stiffness at the propeller is always a difficult parameter for the analysis team. Most of time, either a fixed boundary condition or a generic stiffness from analysis experience will be used as a rough estimation. This situation becomes more critical for a P-bracket support because the stiffness can be much lower than other types. IDC Engineering recently conducted a whirling calculation study by creating a detailed model of the P-bracket and hull structure to provide a more accurate supporting stiffness for the propulsion shafting. The study also considered the gyroscopic and submersion effects.