Propeller design pushes on

Malcolm Latarche
Malcolm Latarche

20 February 2017


In the search for more efficiency, one of the biggest steps of recent years is the development of IT tools such as CFD that can help predict the performance of the whole propulsion system rather than looking at individual parts as has traditionally been the case. Furthermore, the same tools can be used to determine the effect of hull modifications that will add further efficiencies. Following on from the use of such tools, propellers have been identified as an area where big improvements in efficiency can be had for relatively little outlay. Getting the best efficiency is not so much a matter of designing new propeller shapes but more of matching existing designs to the vessels hull shape and operating strategy. At the end of 2016, Rolls-Royce reported an order for two of its Promas rudder/propeller systemss by Danish ferry operator Mols Linien. The Promas system integrates the controllable pitch propeller, a propeller hub cap, a rudder with bulb and a twisted leading edge into one hydrodynamically optimised unit. Gearboxes, steering gears, two tunnel thrusters in the bow and a control system are also part of the delivery. But this is more than a contract for equipment as the deal includes cavitation tests at the Rolls-Royce Hydrodynamic Research Centre (HRC) in Kristinehamn, Sweden. At the facility, the performance of the combined propeller and rudder system will be controlled and tested prior to manufacturing. The HRC includes a large cavitation tunnel where a model of the ship’s hull, with the ordered propulsion set up, will perform in different operating conditions. Model testing can lead to important and cost-saving adjustments in a product or ship design. There are other projects in place globally exploring the use of new materials especially composites which reduce weight. Benefits are also being gained from modifying propellers with boss cap fins being especially popular. One such device is the recently unveiled Wärtsilä EnergoProFin. It is a propeller cap with hydrofoil section fins on the downstream side of the propeller. According to the company, it can reduce a vessel’s fuel consumption by up to 5%. Propeller caps with fins have existed for three decades, but their application has so far been primarily on FPPs. The view of the industry has been that applying an energy saving device such as Wärtsilä EnergoProFin to the technologically more challenging CPP would not be possible. Wärtsilä took on the development task and succeeded in creating a product that offers vessels equipped with a CPP an opportunity to reduce their energy consumption. The payback time for the investment is, even with the current low fuel prices, generally less than two years and can be as short as less than a year. In another development, propulsion specialist Steerprop has worked with The Switch to develop permanent magnet technology for its range of large contra-rotating propulsors. According to Steerprop managing director Jussi Seppälä, market demand for more compact designs and greater efficiency has prompted the development of company’s new CRP ECO LM fully azimuthing propulsor with electric motor and a flexibly coupled shaftline. By using permanent magnet technology instead of conventional methods, unit size can be between 30% and 50% smaller with a weight reduction of up to 40%, partly conferred by the hollow rotor design. This lowers inertia, an important point if shaft line vibration is a consideration. The new PM-model will initially be available in Steerprop’s large CRP ECO units (5MW to more than 10MW), and for the smaller CRP units (0.8–1.2MW and 3MW). The price will be at a slight premium to Z-drive, induction motor units