The propeller can have optimal efficiency in open water. The hull can be developed to cause minimum resistance. Yet they do not work together.
"In fact, a higher resistance hull can provide better operating conditions for the propeller, so overall energy consumption will be lower,” Jennie Andersson, a PhD student at Mechanics and Maritime Sciences at Chalmers, says.
The problem is that the propeller usually sits at the back of a ship. This means that the water flow to it is affected by the hull while the propeller in turn affects the flow around the hull because it sucks water. To measure the efficiency of the system, a 60-70 year old method is used, based on model tests, where the interaction effects are divided into fixed mathematical factors.
- Nobody understands those numbers. They say nothing about the physical conditions, whether flow or hydrodynamics. They can not be used to analyze or improve a system.
However, the method that Jennie Andersson has developed and presents in her licentiate thesis, does the trick. It is based on flow calculations, known as CFD (Computational Fluid Dynamics). The first attempts with this type of method were made in the 60’s, but the problem is that it requires a lot of computer power.
"It is only now that it is possible, and there is a trend in the industry from model tests to flow calculations," Jennie Andersson says.
Her method measures the delivered power to the propeller in terms of different energy fluxes in the flow. For example, a flux is axial kinetic energy (water accelerated in the right direction) another transverse kinetic energy (movement in the wrong direction). However, the new thing is that the method also measures the friction losses of the system, so-called viscous losses.
“The method gives an overall view of the system. It's really a fairly simple method based on Reynold's transport theorem. It’s just that nobody has applied it on propellers and hulls in this manner previously.”
Unlike earlier, the method now makes it possible to figure out how propulsion systems work, not just in model scale, but also in full scale – something that the industry is of course interested in.
"A big problem today is that the shipowners are installing ESDs, energy saving devices, without having a proper understanding of how they work or if it actually does work. This method allows you to save a lot of money.”
How much you can save on fuel consumption exactly is hard to say, Jennie Andersson says.
”There's very little to save on a propeller in open water. It’s just about 0.1 percent. The same applies to the hull, but on the system as a whole, the savings are significantly greater. Then we talk at least about five percent.”
However, there is another problem incorporated into the industry. Propulsion systems does usually not come in one. The hull is manufactured by one supplier, the propeller by another and third may make the rudder.
“This means that there is no person in the chain that sits on all geometries. This is making it difficult to optimize the system. Any party in the chain must have access to all components if it is to work to full extent,” Jennie Andersson says.