Machinery vibrations in the Skjold class of MTB's
MetadataShow full item record
- Institutt for marin teknikk 
During test and verification of the Norwegian Navy’s fast patrol boat KNM Storm some problems with the lift fan machinery have been experienced. The problem observed is that occasionally, triggered by the firing of the 76 mm cannon, the port fan diesel engine is set into vibrations. In order to investigate the possible causes and the transient dynamic behavior of the system, several models have been explained, built and simulated throughout the master thesis. The bond graph notation and the computer software program 20Sim have been used. A non linear rigid body model of the diesel engine is built. The rigid body model is connected to a model of the complete drive train, including the fan load. Excitation models for the inertial force and moments from crank kinematics, counteracted torque loading of the fan, displacement of foundation and air pressure pulse propagation into the fan compartment are built and simulated. The eigenfrequency calculation of the rigid body (diesel engine and gear) has been reviewed.The inherit physical properties of the rigid body show that we can expect amplification and phase shifts in the frequency range of approximately 4.5-25 Hz. Installing snubbers at the engine brackets may lead to increased transverse stiffness and eigenfrequencies within the diesel engine operating speed range. A velocity pulse, in transverse direction based on SKF measurements, to the diesel engine and gear foundation, reveals a 0.7 mm transverse displacement contribution of the diesel engine and gear at the center of gravity. The pseudo bond graph model of the air blast pressure pulse propagation from deck through the fan compartment reveals that the pressure pulse will flatten out, with a decrease in pressure amplitude and an increase in pulse length, at the fan volute intake. The effects on pressure pulse at fan inlet have been investigated. Disturbance in the fan inlet flow field by a pressure pulse causes a negative torque loading transient as well as a positive engine speed transient. This affects the inertial forces from crank kinematics excitation vector and the counteracting torque fan loading excitation vector, which results in a diesel engine displacement. The potential of diesel engine displacement from the counteracting torque loading is greater than the velocity pulse to the engine foundation. The governor engine speed set point has been given a 2 Hz oscillation at 2100rpm. This results in a 2Hz oscillation of the fan load, counteracted torque loading, displacement in y-direction and angular displacement around the x-axis. The 2 Hz transverse displacement of the diesel engine is in accordance with the observations by crewmembers and yard. We can also observe the phase delay between the engine speed regulator and the actual engine speed and fan load. In order to create a continuation of the oscillation, a phase difference of 180 degree is required. A further model implementation of the physical effects of the gas dynamic processes together with the actual speed governor properties and exact physical data of the diesel engine are needed in A non linear rigid order to possible recreate this phenomena, and increase the accuracy of the simulation results.