The ideal propeller form is far from settled as manufacturers explore new fabrication techniques and modern materials

If you told a naval architect 10 years ago that a ship would be allowed to sail with a hollow propeller, they would have been horrified. Jump forward a decade and such a development is in sight, thanks to a European-funded research project.

French marine and naval industrial company Naval Group and engineering school Centrale Nantes have made the first hollow propeller blades by metal additive manufacturing, a form of 3D printing. The program, which is funded by the European Commission under its Horizon 2020 framework, has the ambition of producing innovative propeller demonstrators to improve the operational capabilities of ships, while reducing environmental impact

In order to improve vessel propulsion, Centrale Nantes and Naval Group are using additive manufacturing to design large components – including propellers of up to 6 m in diameter – which are beyond the capabilities of traditional manufacturing technologies. This is achieved by a technique known as wire arc for additive manufacturing (WAAM), one of the very few 3D printing methods that can be used to make such big components.

The development paves the way to produce propellers with more complex geometry. The partners’ first demonstrator is a one-third scale hollow blade for a container ship propeller. It was printed in stainless steel in less than a hundred hours and weighs about 300 kg. There are huge benefits in being able to safely manufacture more complex structures: in the full-scale product, weight reductions of over 40% will be possible.

The project is just the latest step in a long-running (and long-named) initiative titled Realisation and Demonstration of Advanced Material Solutions for Sustainable and Efficient Ships (RAMSSES). Naval Group’s Patrice Vinot, propeller package manager for the project, says: “Although additive manufacturing is increasingly present in industry, the programming and design of complex parts, such as propeller blades for ships, represents a considerable challenge.”

Full-scale ambition

As the name suggests, demonstration is key to the RAMSSES project. These demonstrators will not be limited to scale models, says Mr Vinot. A full-scale, hollow-bladed propeller for container ships is under development.

“The potential of the process revealed by this new case study means that we now anticipate unparalleled performance for the propellers of tomorrow,” says Mr Vinot. “Taking part in projects such as RAMSSES and coordinating our network of academic and industrial partners will allow us to bring 3D printing into shipyards for the long term.”

Sirehna, a Centrale Nantes spin-off and a subsidiary of Naval Group, is piloting the blade design. The company’s work has so far led to improvements in efficiency and endurance, as well as a significant reduction in radiated noise and vibrations.

Centrale Nantes’ expertise in trajectory generation and additive manufacturing has been critical to producing the blade. The school and the engineering group has a long history of collaboration which was formalised in 2016 with the opening of a joint laboratory.

Centrale Nantes Rapid Manufacturing Platform head Jean-Yves Hascoët says: “Addictive manufacturing technology has been developed over the last 35 years on the Rapid Manufacturing Platform. All these years of research come to fruition through projects like this, which represents a transfer of our technologies into a real industrial environment.”

Beyond materials and manufacturing techniques, there is room for analysis-driven innovation too. Late last year Philadelphia design firm Sharrow Engineering reported that it had been awarded patents for a new propeller design in the US, Japan, Canada, and the EU, with further patents pending.

The company has completed the final phase of testing on what will be its core propeller product. Over the past six years, Sharrow Engineering has analysed testing data collected at the University of Michigan’s marine hydrodynamics laboratory and implemented a supplemental test programme using manned vessels in lakes, rivers, and bays. Results demonstrate that the propeller is 9%-15% more efficient than the conventional Wageningen B-series design.

Clean sheet

The design was verified by Applied Universal Engineering. Founding partner John Dannecker says that Sharrow Engineering took a “clean-sheet approach” to propeller design.

“Using the latest generation of commercially available engineering software and state-of-the-art computing architecture, Sharrow Engineering has developed a proprietary design capability and process for their propellers,” he says. “This process pointed to a new propeller geometry that reduces fuel consumption and demonstrates superior vibration and noise characteristics.”

Sharrow’s propeller provides a wider peak efficiency curve for greater utility over a wide scope of operational ranges and registers a 17% reduction in torque while accelerating.

Sharrow Engineering CEO Greg Sharrow believes that, now more than ever, squeezing fuel efficiency from propeller design is important. “Stricter regulations on sulphur levels will add many millions more to the expense column for individual operators and billions to the industry’s overall fuel spend. We believe our product can be the difference between ships that burn more fuel and more cash, and those that find calm seas heading into 2020 and beyond.”

If reducing fuel spend is not enough incentive for shipowners to revisit propeller designs, there are environmental schemes that offer discounts on energy saving technology. One such scheme is the Green Award Foundation. The programme aims to protect the marine environment by certifying vessels with a low environmental impact.

Companies that share those goals support them by providing incentives, and Mitsui OSK Lines (MOL) is among them. The Japanese shipowner has 11 vessels certified under the scheme, while its technology sales division MOL Techno-Trade, offers the owners of certified ships a discount on its propeller boss cap fins. Owners are granted a 3% discount on the product.

MOL’s boss cap fin is no newcomer – it has been on the market since 1987, when it was jointly developed by MOL, West Japan Fluid Engineering Laboratory Co., Ltd., and Mikado Propeller Co (the predecessor to Nakashima Propeller). To date, over 3,300 vessels have installed the device, which offers fuel savings (of 3-5%) by eliminating the hub vortex generated behind the rotating propeller and improving propeller efficiency.

Important design improvements are being driven by cutting-edge techniques, materials and cutting- analytics. Combined with added incentives to invest in proven technology, cost-conscious shipowners have many reasons to look astern, anew.

United Shipbuilding Corp designs controllable pitch propeller

Russian shipyard group United Shipbuilding Corp (USC) has introduced a low-noise, controllable pitch propeller (CPP).

The VRS-M propeller was originally designed to work with a 20 MW engine of the type found on Russian Navy corvettes and frigates. It was developed at USC’s Zvezdochka Ship Repair Center, with the prototype built at the Vega Experimental Plant in Borovsk. During the project the company also developed the capabilities and systems required to begin mass production of the propeller, which it is now offering to commercial vessels.

USC said that, compared to similar propellers, its product boasts an extended service life. It is designed to offer high stability in emergency situations and the automatic locking function means that the propeller can still be operated, even if the pitch adjustment system has been shut down.