Underwater free-vibration analysis of full-scale marine propeller using a Fibre Bragg Grating-based sensor system

A detailed experimentally-based study has been carried out on instrumented, full-scale marine propeller blades in order to investigate their vibration behaviour, both in air and underwater. To obtain data with minimum perturbation to the characteristics of the blades, a Fibre Bragg Grating-based sensor network system was designed and implemented, for the first time. The individual vibration frequencies at each measurement point and thus the broader vibration patterns seen for each of the blades were obtained, with excitation both in air and in water and the results are compared favourably with those obtained from Finite Element (FE) analysis. The vibration patterns obtained show that the same modes of vibration occur in air and in water, although in some natural frequencies the mode order is seen to change from one blade to another on the same propeller. The extensive performance survey carried out and experimental data obtained have also shown that while the effect of the added mass of water on the natural frequencies of the blades in the fundamental modes is considerable, this effect diminishes as the natural frequencies of the blades increase. The results obtained from the optical fibre sensor network were compared to those from previous work in this area using different and less satisfactory techniques and it was confirmed that the ratio of the natural frequencies in water to those in air increases in a linear manner as the frequencies were increasing. Additionally, the natural frequencies of a blade were measured under different depths of propeller immersion