Marine propulsion shafts
Designing marine propulsion shafts according to ice class requirements can be split into two parts:
- Calculating the ice class impact and determining the response torque spectrum
- Code check
Nauticus Torsional Vibration tool is used to calculate the maximum response torque amplitude(QAmax) on the shaft due to ice impact. It uses the steady state approach to calculate the ice impact. The Fourier transform method is used to convert the excitation from time domain to the frequency domain. Then such frequency domain excitations (harmonics) are considered in steady state torsional vibration calculation.
The response torque spectrum is assumed as Weibull distribution and can be determined by the maximum response torque amplitude (QAmax) calculated from torsional vibration and total number of load cycles.
The fatigue strength calculation is perhaps the most complicated part of the code check. Different from the “open water” loads, there are no dominating load cases in ice-class load. So Palmgren-Miner’s approach is needed to calculate cumulative damage ratio. Nauticus Shaft Fatigue integrates the Weibull distribution spectrum into S-N curve to calculate the cumulative damage ratio.