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Abstract
Nickel-Aluminium-Bronze alloy is the most common material for ship propellers but, more recently, composite materials have been used in propeller construction. One of the advantages of the use composite materials for ship propellers is that they allow the possibility of shape-adaptability. A shape-adaptive propeller is one that is designed so that the blades deform with load changes in such a way that the propeller performance is enhanced in comparison to that of a conventional “rigid” propeller, e.g. the goal of shape-adaptability may be higher efficiency over a greater range of operating conditions, or reduced cavitation and noise. This shape-adaptability can be achieved through the choice of the appropriate blade geometry, and the optimum arrangement of composite materials.
A design method for composite shape-adaptive propellers is developed and tested in this work. The method has a baseline rigid propeller as the starting point. A novel method is used to identify a design condition at which the rigid and flexible propellers have identical shapes, and therefore performance. Optimisation procedures, to find the unloaded shape and composite material properties, are a necessary part of the shape-adaptive propeller design method. A shape-adaptive flexible propeller has generally improved performance compared to that of the baseline rigid propeller. As well, a coupled hydrodynamic/structural analysis is required so that the performance of flexible and rigid propellers can be compared.
The results of a study of the twist characteristics of straight and curved cantilever beams with various amounts of skew are reported. The cantilever beams are structural analogues of hydrofoils and propeller blades, and the aim of the study is to identify the desired material and geometrical characteristics for shape-adaptive propellers and hydrofoils.
Two case studies, that demonstrate the developed design procedures, are described: Design of a flexible hydrofoil, and Design of a flexible propeller.
In both cases, the desired shape-adaptive behaviour is achieved but the performance gains are small. It is felt that the example structures do not have sufficient flexibility to take full advantage of shape-adaptability. Possible means of achieving greater flexibility are optimisation of the baseline shape (planform and thickness distribution) for flexibility, and selection of composite materials that maximise flexibility, within the strength requirements.
An expression is developed for estimating the efficiency gain of a shape-adaptive propeller operating at the design condition in a ship’s wake. Calculations show that, even for what is considered to be a relatively flexible propeller, the expected efficiency gains are small (< 1%). However, small gains in efficiency across the range, can be significant for the operation of a ship. Furthermore, efficiency gains are higher at off-design conditions as a shape-adaptive design broadens the efficiency curve.