Composites final frontier

Dr Tim Searle, the commercial manager of The Advanced Composites Manufacturing Centre at the University of Plymouth tells Design Engineering about building composite propellers

The concept of a composite propeller for commercial marine applications is highly challenging. However, the benefits from the successful implementation of continuous fibre reinforced polymers are significant. To that end, the Advanced Composites Manufacturing Centre at the University of Plymouth has been carrying out some pioneering work developing a range of all composite marine propellers.

The traditional materials for manufacturing propellers include: manganese bronze (high tensile brass HTB1) and nickel aluminium bronze AB2. Whilst the material cost is important, usually this is only a small proportion of the overall component cost. Processing the material is the largest cost. As the geometry of a component increases in complexity, then it becomes progressively more cost effective to manufacture the part from composite materials.

Common to metallic and composite manufacturing routes is the need for a master pattern of the propeller design. For metallic propellers this starts life as a single blade wood or plaster pattern. This remains the production pattern for producing the sand mould if only small numbers of the particular propeller design are required. However, if larger numbers are required then a full bladed aluminium pattern is made. The sand mould can be produced quicker from this.

Once the pattern is available, work can begin on forming the sand that will define the rough casting shape of the metallic propeller. Producing the sand mould is a skilled and time consuming job.

Having produced this tooling, the propeller is cast and left to cool. After cooling, the mould is taken apart and the sand broken away. The sand is discarded, the casting is fettled and taken for machining. Here the blades are ground and polished to section and the keyway and shaft taper are cut. Finally, the propeller is balanced, measured and shipped to the customer.

In composite, the key difference is the requirement for a reusable set of moulds that accurately replicate the final propeller geometry. The advantage is that only minimal finishing will be required after moulding. The disadvantage is that this type of tooling is expensive compared to making a sand mould for metal casting. However, there is considerable industrial development underway to produce a format of cost effective tooling.

Resin Transfer Moulding (RTM) with accurate composite mould tool faces has been used successfully to produce all the propellers under development. The entire fibre reinforcement pack, together with a discrete metallic shaft interface, (accommodating the keyway) is assembled in the mould, which is closed and low viscosity epoxy resin is injected at between 2bar and 4bar pressure. If required, a gel coat can be applied to the tool faces prior to fibre assembly. RTM allows for accurate component geometry, high fibre fractions, typically up to 55% by volume, for relatively low tooling costs.

A 21in. propeller with a toughened epoxy gel coat finish produced by RTM has been produced at the ACMC. Common to both composite and metallic propellers is the need for a machined shaft interface. Thus some machining time must be costed to the composite propeller. However, this metallic boss can be produced much more cost effectively on its own. It can be produced as a stock item, defined only by its length, shaft taper, and key dimensions.

A key to success in RTM is the need for a well made fibre preform. Preforming automation is expensive, and particularly for complex shapes is not well advanced. However, cutting the fibre manually using templates and assembling the preform by hand can be accurate and effective.

Equipment to automate RTM is available for only modest investment. The handling of tool closure, resin injection, flushing, cleaning of mixing heads, associated pipe work and finally the opening of the mould on resin cure, can now be handled automatically. The entire cycle for modest components can be less than 4min with total equipment investments of less than £40 000.

This also allow for a ‘clean process’ where the only time resin is seen and handled is in the cured finished part and styrene emissions are negligible.

Figure 1 shows a 26in. diameter all composite propeller manufactured for sea trials. The design is a monolithic glass epoxy composite propeller manufactured in a single stage moulding. The propeller retrofits to the propeller shaft directly replacing the metal propeller which was used as the pattern for producing the tooling for the new composite design. The propeller deign was fitted to a 38ft Aquastar, a semi-planning boat powered by a 260hp engine.

The trials with the composite propeller indicated that there was no performance penalty when using a composite propeller that had more compliant blades than the conventional nickel aluminium bronze propeller. Added to which the propeller is 25% of the weight of the metal equivalent and can be made with repeated geometrical accuracy.

Five different designs of composite propeller have been made to date. The total running time with these designs is now approaching 300h. More details can be found at: www.tech.plym. ac.uk/sme/acmc.htm.

Advanced Composites Manufacturing Centre Tel: 01752 232651