Tests in calm water are not enough

The right combination of hull and propeller saves fuel. But in the ship industry, they are only optimized to function optimally in calm water. The results of a research project within the Swedish Transport Administration's industry program Sustainable shipping run by Lighthouse should change that.

The hull is designed separately, the same with the propeller, which is pressed into it a little later. In the best case, they are tested to function optimally in calm water. A little simplified, this how it still works when it comes to designing ships.

“The idea of ​​the project has been to optimize the interaction effects between propeller and hull, even in waves. We’ve been studying different kind of wavelengths; from short ones to wavelengths that are significantly longer than the ship. This has not been done before, says Arash Eslamdoost, a researcher at Chalmers who leads the research project Propeller-Hull Interaction Effects in Waves.

It´s easy to understand why. Figuring out how the propeller and hull work optimally is not easy – not even in still water. And in waves, it is next to impossible. It is only recently that computers got enough computing capacity for simulations of flows around waves to be made. Just the calculation of a single specific wavelength takes one to two weeks.

“This is of course very expensive and probably nothing that will be done in the industry.”

But the nice thing is that the results obtained in the research project will be able to be used by industry.

“Our goal is to be able to contribute with clear recommendations. Perhaps it is only for the best that the industry tests in still water and then adjusts with our results regarding the effect and influence from waves. It makes it a little easier”, says Arash Eslamdoost.

During the research, a couple of wavelengths have been discovered which are generally problematic and create great resistance for the hull.

“Wavelengths that are roughly the same length as the length of the hull have a major impact on the behavior of the hull. The movement is then greatest and the resistance maximal. Such waves must be avoided. You just have to swerve or slow down or those fuel costs will be very expensive.”

There is also a smaller wavelength, which corresponds to approximately 50-60 percent of the length of the hull, which should also be avoided, explains Arash Eslamdoost and says that he is very proud of the uncertainty analysis that the research team used.

“Many actually use the same calculation methods as we do but we also do a verification and validation analyses. While others only examines one wavelength, while we do multiple simulations with varying data. In this way, we can predict the entire picture and the flows at different wavelengths.”

So how much fuel can be saved by optimizing the hull and propeller?

“It is very difficult to say. Hopefully we can answer this when we have done the second part of the project. So far we have analyzed the hull and now work begins with the propeller”, says Arash Eslamdoost.

The second part of the project will focus on interaction effects between the propeller, hull and waves.

“We have already done some tests with only propellers where the aim has been to understand how waves and propellers interact with each other. After that, we will also take in the hull to get the whole picture of how the hull, propellers and waves interact with each other.”

The three-year research project Propeller-Hull Interaction Effects in Waves (Part 1) has been carried out by Arash Eslamdoost, Mohsen Irannezhad and Rickard E. Bensow from Chalmers and Martin Kjellberg, SSPA. Part two of the project is expected to be completed by the end of next year.