Energy-saving devices (ESD) are special-purpose appendages to the hull, propeller and rudder of a ship, enhancing the vessel’s propulsive efficiency. They modify the flow field around the ship in a favourable manner so as to improve the hydrodynamic interaction between the hull, propeller and rudder. Depending on the application, ESDs may create or reduce the swirl in the flow field (e.g. pre-swirl stator, rudder bulb) or locally accelerate the flow (e.g. duct, nozzle). The improved flow conditions enable the propeller to operate more efficiently such that the power required for a certain ship speed is reduced. ESDs are designed for new buildings or retrofitted to existing ships, enhancing their life cycle. HSVA has long-term experience in designing well-established ESDs such as pre-swirl stators (PSS) or rudder bulbs. Moreover, the model basin is active in R&D in order to devise new ESD concepts for future applications.
Pre-Swirl Stators (PSS)
The PSS is a set of number of fins installed right before the propeller with the intension of introducing certain pre-swirl for the propeller. The pre-swirl fins are normally equipped to a ship with high block coefficient either for new-building or retrofitting projects. Generally it is expected that a power gain of 5% in average with a 2% uncertainty range can be achieved by such an energy saving device. The full scale trails in the EU project GRIP has proven the power gain of the PSS designed by HSVA for a bulk carrier built by ULJANIK BRODOGRADILISTE DD has reached up to 7% in this specific case (see photo).
The largely accepted working principle of the pre-swirl fins is to modify the tangential propeller inflow to improve the asymmetric tangential wake due to ship, which helps the propeller operate more homogeneously and to shift the propeller load toward the center plane. That is why the most pre-swirl fins are installed on port side of the ship. By installing pre-swirl fins, the rotational energy losses of the propeller slipstream can be reduced. The side effect of it is that the required rotational rate of the propeller will be reduced by ca. 5%, this should be considered before installation of the pre-swirl fins.
The hydrodynamic design of the PSS is crucial for its performance and needs individual adaptation for each ship. HSVA has set up an efficient procedure to design and optimise the PSS using the BEM and sophisticated RANS methods for both model and full scales. The combined BEM and RANS methods together with the ready-to-use parametric models of PSS make it possible to find the most optimised design of PSS in a short time period, as normally required in the industry. The predicted power gains by the self-propulsion RANS computations due to the installed PSSs have been confirmed in several reference cases either by the model tests or full scale trials.
Streamlines passing through the propeller hub region indicating the reduction of the hub vertex due to PSS
Rudder bulb – axisymmetric or asymmetric
The “original” rudder transition bulb is the axissymetric “Costa” bulb. The “Costa” bulb is a bulb attached to the rudder directly behind the propeller boss. The bulb reduces the hub vortex losses. Often the bulb is integrated into a rudder and sometimes also connected to the propeller boss with a very small gap. The properly designed rudder bulb can reduce the power needs by about 3% with a 0.3% standard deviation. This indicates that this device is working rather stable. In addition this device can be designed and constructed with relative ease in the shipyard, rudder bulbs are therefore widely accepted in the industry.
The saving potential of a rudder bulb depends on the existing propeller and rudder: extent of the hub vortex and effect of the hub vortex on the rudder. During the EU project GRIP, HSVA has designed an asymmetric rudder bulb by employing a fully parametric rudder bulb model (see picture). Higher potential in recovering the hub vortex losses has been found due to the asymmetric nature of the single-screw propulsion system. This concept has been supported by detailed CFD analysis using the adjoint solver @FreSCo+ in the HSVA.
Streamlines passing through the propeller hub region indicating the reduction of the hub vortex due to an asymmetric rudder bulb
“BLAD” (Boundary-Layer Alignment Device) is a new propulsion-improving concept developed by HSVA within the EU-funded R&D project TARGETS. The BLAD concept consists of two flow deflectors mounted asymmetrically to the stern of full-form vessels such as bulk carriers and tankers (Fig. 1). These deflectors build on two different energy-saving effects:
- Reduction of the hull resistance by a modification of the pressure distribution in the aft-ship region
- Improvement of the propeller inflow by enhancing the flow uniformity and creating a swirl against the propeller rotation
The BLAD concept differs from established ESDs in that the appendages are mounted far upstream of the propeller. This ensures that the deflectors are not exposed to the unsteady flow created by the propeller. They are thus free from dynamic loads, resulting in lower structural demands and a smaller risk of failure as compared to traditional ESDs. Also, the flow deflectors are easily installed in a hull region with relatively flat surfaces.
BLAD deflectors applied to a Capesize bulker in model scale
HSVA has successfully demonstrated the functionality of the BLAD concept for a model of a Capesize bulker (Fig. 2), using CFD methods. In the future, the model basin will push the BLAD development further, considering full-scale computations of smaller bulkers and tankers (e.g. Supramax class).