Wärtsilä has recently developed an important energy-saving device, that is, the industry

New EnergoProFin changes the game in propulsive efficiency

In the fiercely competitive world of shipping, every drop of fuel matters. Naturally, any way to sail farther on less can translate to substantial cost savings for operators, not to mention benefits to the environment. Working toward that goal, Wärtsilä has recently developed an important energy-saving device that is likely to spark a high degree of interest in the business: the industry’s first hubcap and fin setup for Controllable Pitch Propellers.

Text: Anton Voermans Photo: Wärtsilä

The underlying issue in fuel savings is the efficiency of a ship’s propulsion system. In an ideal world, every bit of energy put into the shaft would be used for driving the ship forward. In reality, only about 50-70 percent can be utilised due to various types of kinetic losses. One area of concern is rotational loss, a consequence of the rotating propeller’s pushing against the water and thus putting the water into a spin. Rotational loss consumes roughly 5 percent of the energy that goes into the system.

Since its introduction three decades ago, a favored solution to reduce rotational loss has been to place a cap with fins aft of the propeller hub. The cap reduces the swirl, while the fins effectively catch and absorb the force of the rotating water, eliminating the vortex and feeding the energy back into the propulsion drive train. See figure 1.

Wärtsilä’s line of such devices, the Wärtsilä EnergoProFin, has proven to be a highly attractive add-on as it boosts propulsive efficiency by a fairly consistent 2 percent at a reasonably low cost. Over 200 units have been sold thus far.

Fig. 1 - Computational Fluid Dynamics images of downstream vortex with a typical hub cap (1a) and with an EPF (1b).
Fig. 1 - Computational Fluid Dynamics images of downstream vortex with a typical hub cap (1a) and with an EPF (1b).

However, this type of solution has historically been limited to use on Fixed Pitch Propellers (FPPs), where the pitch angle of the blades never changes.

The pitch for FPPs is optimized for energy efficiency in only one operating condition – a specific sailing speed in the case of bulk carriers and container ships, or maximum bollard pull in the case of tugs. Other types of ships, notably ferries and fishing vessels, use Controllable Pitch Propellers (CPPs), where an actuating mechanism inside the hub can change the pitch depending on the operating needs, be they sailing, low-speed maneuverability or raw pulling power.

CPPs, with all the mechanics fitted within them, have relatively larger hubs compared to FPPs. The resulting higher hub ratio means that they can produce a more intense swirl. Additionally, when a cap and fin device is used, the angle of the fins is designed to operate with a specific pitch angle. When the pitch is changed on a CPP, the angles of the fins and the blades no longer match. For these reasons, the prevailing view in the industry has been that recapturing the rotational loss in a CPP by using a cap and fin device simply would not work.

That view has now changed – Wärtsilä’s Services Hydrodynamic and Mechanical Design Engineering team has developed a new type of Wärtsilä EnergoProFin specifically for CPPs that overcomes the above challenges. It is already being used on two vessels.

Fig. 2 - Side-by-side images of the EnergoProFin for FPP (left) and for CPP (right).
Fig. 2 - Side-by-side images of the EnergoProFin for FPP (left) and for CPP (right).

 

Eye-opening research

The catalyst for this breakthrough was an EU-funded research project entitled GRIP (Green Retrofitting through Improved Propulsion), which was carried out from 2011 to 2015. Under its auspices, Wärtsilä and nine other European companies joined forces to overcome some of the fundamental problems in propulsion efficiency. Specifically, they were studying various energy-saving devices in the market to determine which in fact worked and why – questions that were not fully understood in the industry. The approach would lead the participants to take a hard look at the basic principles of the energy-savings equation.

Armed with the deeper knowledge of hydrodynamic principles gleaned from GRIP, along with state-of-the-art CFD, the Wärtsilä team was able to use its experience in producing the Wärtsilä EnergoProFin to develop a solution for CPPs. Figure 4.

Testing the design presented challenges that are all too familiar in the shipbuilding industry. Using a modeling tank comes with a fundamental scaling problem – while it is certainly possible to shrink the ship and its hardware to model size, it is never possible scale down the properties of the water such as density and viscosity. Full-scale tests would therefore be preferable in theory, but ever-changing conditions at sea make comparative tests impossible, even if the high cost of such tests were not a factor. The development team took the best middle-ground approach, using a combination of full-scale and model-scale CFD computations instead. The CFD computations at model scale can be compared to the results that were coming from the relatively accurate modeling tank. Because the CFD calculation methods were correct at the model scale, the engineers could be confident that they would be accurate at full scale as well.

Computer modeling was also employed during the design phase to optimize castability, thus minimising the chance of flaws forming in the structure of the finished product.

The hard work resulted in a new type of Wärtsilä EnergoProFin that is compatible with a CPP. It produces roughly equivalent efficiency gains as the FPP version, though there is more variation depending on the specifics of the individual propeller. The upper end of the range exceeds even 4 percent.

A distinctive feature compared to the design for the FPP is that the hubcap is open at the aft end. The hub aft is already seawater resistant and the open end does not influence performance. The angle of the fins, meanwhile, is optimized for the pitch setting that the ship will be using most. Efficiency gains will be highest at that pitch, though it is important to note that there are gains at other pitch settings as well. In other words, in off-design conditions (up to 12 degrees difference in pitch), there is still a positive effect. See figures 2 and 3.

The device can be mounted on both new build and retrofit propellers, though integrating it requires specialized expertise from Wärtsilä engineers. It should be noted that, while the Wärtsilä EnergoProFin for FPPs can be used with competitors’ propellers, the current EPF-CPP model can be fitted only onto Wärtsilä propellers with a 4C-3A(O), 4D or 4E hub. Installation on other (including third party) hubs is under investigation.

Fig. 3 - EnergoProFin CPP brings also fuel savings in off-design conditions (up to 12 degrees pitch deflection in this case.)
Fig. 3 - EnergoProFin CPP brings also fuel savings in off-design conditions (up to 12 degrees pitch deflection in this case.)
Fig. 4 - EnergoProFin with Controllable Pitch Propeller.
Fig. 4 - EnergoProFin with Controllable Pitch Propeller. 

Assessing the benefits

Development of the Wärtsilä EnergoProFin certainly marks a real achievement for the teams involved. It also serves a testament to the efficacy of joint industry research projects like GRIP and to the power of recent advances in Computational Fluid Dynamics.

The more fundamental significance of this device, however, will be in the sizable energy savings that it can bring to the shipping industry as a whole, and in the benefits to individual operators looking for practical ways to minimise costs and emissions. As information about the new energy-savings option spreads among ship operators, more of them are likely to be taking a close look at their fleets’ energy profiles to assess ways to incorporate this new technology into their propulsion systems.

In the meantime, Wärtsilä’s engineers will continue to work to make these important energy-saving devices compatible with a wider array of ship and propulsion designs, bringing the benefits of lower fuel costs and lower emissions to more customers.

Author: Anton Voermans, Head of Hydrodynamics Department, Propulsion System Services, mail: anton.voermans@wartsila.com

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