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en-USJuly 2012 en-US en-USwww.sname.org/sname/mt (isolation mounted) levels measured on the equipment of interest can be used in con -junction with the stiffness of the isolation mounts used to calculate the forces applied to the equipment?s foundation. For a forced analysis, the forces must be applied at the operating frequency. Given the variability in equipment and the poten -tial for variability in the resonances, it is advisable to apply the forces for a number of frequencies in a number of small steps around the exact operating frequency, typically ± 2 Hz, to ensure there are no res -onances in close proximity. Propulsion systems with wide operating speed will require the full range to be analyzed. Each source should be analyzed separately as they are unlikely to be operated in phase onboard the ship. Fluid loading ere are two common types of uid loading that must be considered for an FEA model of a ship: tank loads and water loads due to immersion. Tank loads are applied using the additional mass feature of plate ele -ments to smear the mass of the uid over the walls of the tank. Depending on the num -ber of tanks and the percentage of the ship?s displacement that they contain, dierent tank load conditions may need to be modi -ed. Ships on which tank loads are a small percentage of the total displacement may not see signicant eects between dierent loads. Water loading due to immersion may be handled in a number of ways depend -ing upon the FEA code used. A number of systems include methods for calculating Figure 3: Hull Mode Example (sh ape exaggerated). Forced analyses use forces applied at the various sources of vibration, primarily including forces generated by the propeller and propulsion machinery.