Compensating with Composites

When it comes to selecting a propeller shaft material, composites offer a number of benefits, particularly in faster ships where weight is critical.

Geislinger manufacture a range of advanced composite shafts called the Gesilco shaftline which, it claims, offer outstanding weight advantages compared to steel shafts. Although the company is not the only manufacturer of composite shafts, it is one of a select few. Thomas Bozecski, Gieslinger?s Sales Manager, told The Motor Ship that the company currently manufacturers about three to five packages a year, mainly for fast ferries ? both monohulls and catamarans ? supplying the drive shafts between gearbox and waterjet.

Gieslinger has supplied shafts to Austal, North West Bay Ships, Leroux Navale, Lurssen, Oceanfast and Rodriquez, among others and the company?s latest delivery was for Aremiti 5, being built by Austal for Aremiti Cruise Tahiti.

Bozecski says that the company deals with “mainly the builder, but also designers are asking for quotes”.

One of the advantages of composite shafts is their low mass density, which results in higher critical speeds than conventional steel shaft solutions and allows much longer bearing distances. The installation of composite shafts in the high speed part of the drive line, (between engine and gearbox) takes advantage of this and allows the designer to increase the distance between engine and gearbox without additional bearings.

Depending on the type of fibre reinforcement, the weight of the composite Gesilco shaft is approximately 15% of that of a solid steel shaft and about 30% that of a hollow steel shaft. However, for the overall weight of a shaftline, steel flanges and intermediate shafts for bearings have to be taken into account.

Due to long bearing distances the radial stiffness of the shaft system is significantly reduced allowing hull deformations or even engine transverse and vertical deflections to be compensated by the shaftlines. However, all this comes at a cost and Bozecski says that carbon shafts are 30 ? 40% more expensive than that of steel shafts. But this might be reduced to about 25 to 30% in total when the reduced number of bearings required are taken into account.

Design and materials

The Gesilco shaftline com-

prises Gesilco shafts, steel intermediate shafts, steel adapters, bearings, bulkhead seals and additional, misalignment compensating Gesilco couplings.

The shaft is manufactured by filament winding technology from glass and/or carbon fibers and epoxy resin. Bozecski told The Motor Ship that the company does not use an aramid, such as Kevlar because “Kevlar is higher in price and therefore not competitive – we just use Carbon and glass fibre,” and explains that “glass has some better fire resistance ability but is higher in weight”.

He says that it takes about three days to build a composite shaft ? “one day for winding, one day for autoclave run and one day for final machining (fitting of flange hubs etc.)”.

The epoxy resin is cured at high temperature, which results in a service temperature up to 100° C. It is possible to manufacture a lot of possible fibre orientation angles within the shaft laminate by using the highly automated filament winding technology. The strength and stiffness properties of the shaft with regard to torque and bending stiffness are mainly driven by the fibre orientation.

While the company usually only offers two types of reinforcing fibres with two different fibre orientations, it can also offer specialised shafts with individually optimised fibre orientations.

A standard range of diameters is available for the Gesilco shafts, and covers the need of nearly all applications in ship drive lines. The diameters vary between 200mm and 600mm for a nominal torque range between 10kNm and 700kNm. However, shafts with higher nominal torque are also available. Bozecski explains that “there is in principle no limit – in case bigger diameter has to be manufactured a new mandrel has to be produced.” He says that the biggest shafts the company has produced are for the Corsaire vessels, which had a nominal torque of 673kNm.

Diameters of the shafts and steel intermediate shafts fit commercially available bulkhead seals and bearings and shafts are available with lengths up to 14 metres depending on the shaft diameter. He told The Motor Ship that this is not a manufacturing limit but “more a matter of span between bearing which has to be determined with whirling calculations performed by us together with torsional vibration analysis”.

The Gesilco shafts can be supplied with a special fire resistant protective outer layer. In combination with a sprinkler system mounted just over the shaftline, SOLAS fire resistance requirements for the installation of the lightweight shaftline can be met without additional housing.

Installation

Gesilco shaftlines can compensate for radial misalignments and the company offers a torsional vibration calculation (TVC) service. Additional calculations like whirling analysis of the shaftline and finite element stress analysis in case of required compensation for radial misalignments are also performed.

The shafts have been

developed in line with

DIN/ISO 9001 and are

delivered with certificates

from the main Classification Societies.