This is how two contra-rotating propellers can make ships more energy efficient

By using two propellers that rotate in opposite directions, a ship can use less energy to move forward. New knowledge means that more ships can use the technology, including Hurtigruten’s Sea Zero project for its coastal cruise ships.

SINTEF has now designed and manufactured new measuring equipment for model testing of contra-rotating propellers. This could be good news for many large ships.

“We see that propulsion efficiency increases when we utilize 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,” says Øyvind Rabliås, a researcher at SINTEF.

Although contra-rotating propellers are not new, they are still rarely used on commercial ships. The reason has been their cost and more complicated design than traditional layouts. That may be changing now.

“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. He is a senior hydrodynamicist at Brunvoll, a leading provider of propulsion and maneuvering systems.

Good test results

SINTEF’s new measurement system was developed when Hurtigruten decided to use contra-rotating propellers for its zero-emission cruise ship in the Sea Zero project. The goal is an emission-free Hurtigruten ship with a sustainable and circular solution by 2030.

Contra-rotating propellers have significantly better efficiency than today’s propeller systems.

“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,” says Gerry Larsson-Fedde, chief operating officer at Hurtigruten.

From drawing board to reality

A broad interdisciplinary team at SINTEF Ocean, from technicians and instrumentation engineers to design experts and researchers, has developed the new measuring equipment for model testing contra-rotating propellers. Together, they arrived at a solution that is well suited for self-propulsion tests.

Two dynamometers, which are instruments used to measure force in rotating systems, have also been developed.

“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.

The system has already tested Brunvoll’s propeller system for Hurtigruten. The results provided valuable insight into the system’s performance and helped identify the most efficient propulsion solution for further development.

“It was an added bonus for Brunvoll that the project is so versatile. We have a lot of different industrially relevant issues in our work to create a demonstrator,” says Johannessen.

Hurtigruten is also very pleased to have its ground-breaking ship tested.

“It’s fantastic for us that we can test all the parts of the Sea Zero design at such a professional and advanced laboratory as SINTEF has. 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,” says Larsson-Fedde.

Some challenges too

Propulsion efficiency is increased by recovering part of the energy loss from the front propeller and by better inflow to the rear propeller. That is why it is possible to achieve more than a 10% increase compared to conventional propellers. However, contra-rotating propellers require a complex shaft-in-shaft system.

“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. This applies to both the diameter ratio and the propeller’s revolution ratio between the two propellers, for example,” says Rabliås.

It will be exciting to follow the developments in the future for Hurtigruten as well.

“The goal of Sea Zero is to design the world’s most energy-efficient ship, and in that quest we have to leave no stone unturned. The propulsion itself uses a lot of energy, and so contra-rotating propellers are very exciting for us to look at,” says Larsson-Fedde.