Improving hydrodynamic efficiency of composite marine propellers in off-design conditions using shape memory alloy composite actuators

The bend-twist coupling of fibre-reinforced composite laminates has been studied extensively to introduce self-adjustment in pitch angle of marine propellers for improving the hydrodynamic efficiency in off-design conditions. A recent study has shown that for full-scale propellers designed against material failure, the twist generated in the deformed blade due to this effect is not sufficient to cause any noticeable change in the hydrodynamic efficiency. In this paper, we study the use of shape memory alloy (SMA) for inducing variable twist in composite propellers for achieving the same. Prestrained SMA fibres are embedded in the composite in the martensite phase. When SMA fibres are heated upto the austenite phase, a large recovery stress is developed due to the shape memory effect. This stress is used to create twist in the propeller blade through appropriate positioning of the SMA actuators. A full-scale propeller of 4.2 m diameter, made of graphite epoxy with Nitinol fibres as SMA, is considered. The study establishes for the first time that using SMA composite actuator elements, it is possible to generate sufficient twist that can cause the desired improvement in the hydrodynamic performance of full-scale marine propellers at off-design conditions over a wide range of advance ratio.