By using two propellers that spin in opposite directions, ships can move more efficiently through the water – reducing the power needed to move forward.

‘We see that propulsion efficiency increases when we utilise some of the energy lost from the front propeller at the same time as the rear propeller gets better water flow. This can provide more than 10% better efficiency compared to conventional propellers,’ comments Øyvind Rabliås, Researcher, SINTEF.

The system has already been used to test a propeller designed by Norwegian company Brunvoll and has delivered ‘valuable data on performance and efficiency’, according to SINTEF.

Although contra-rotating propellers have existed for decades, they remain uncommon on commercial ships due to their complexity and cost.  

‘Together with our research partners, we’ve spent a lot of time on understanding and developing solutions for this. Now we believe that the technology is mature and ready for wider use,’ says Jahn Terje Johannessen, Senior Hydrodynamicist, Brunvoll, designer and manufacturer of propulsion and manoeuvring systems.

SINTEF’s testing equipment was developed as part of Norwegian cruise line Hurtigruten’s Sea Zero project, which aims to operate the first zero emissions small cruise ship by 2030. Large battery packs, retractable sails, air lubrication systems and an energy-optimised hull are among the other innovations being evaluated.  

‘This design simply means that we need less energy to achieve the same speed compared to conventional propellers. Brunvoll’s design is also more efficient than the contra-rotating propellers that exist today,’ comments Gerry Larsson-Fedde, Chief Operating Officer, Hurtigruten.

SINTEF built two types of dynamometers: one integrated into ship models for towing tank tests, and another for open water and cavitation experiments, the latter of which refers to erosion to surfaces caused by a hydrodynamic effect of propelled water.

‘One version is suitable for integrating into ship models that are tested in the towing tank, meaning that it is built into the model, and the other version is used in open water tests and in cavitation tests,’ says Rabliås.

‘Contra-rotating propellers are new to us, and they are not common on ships either. That’s why it is so important for us to be able to thoroughly test the ship design in the cavitation tunnel. Then we’ll know that it will work in practice,’ adds Larsson-Fedde.

While the potential for improved efficiency is clear, challenges remain. Contra-rotating systems require a complex shaft-in-shaft arrangement and careful fine-tuning of multiple parameters to optimise flow and performance.

‘The design process is also more complicated than for conventional propellers, both because of complex flow phenomena and the larger number of parameters that need to be adjusted, compared to one propeller,’ notes Rabliås.