3.1 Power Saving Devices
Enhancing Efficiency
The maritime industry is constantly striving to improve operational efficiency and reduce its environmental footprint. A significant contributor to a vessel's fuel consumption and emissions is the propulsion system.
Innovative power saving devices (PSDs) have been developed to optimize propeller performance and hydrodynamic efficiency. These technologies contribute to reduced fuel consumption, lower emissions, and improved economic performance.
This chapter looks at some of the most prominent PSDs employed on modern merchant vessels.
Wake Equalizing Ducts (WED):
A WED is a stationary, streamlined device fitted around the propeller. It functions by influencing the water flow leaving the propeller (wake). The wake is the turbulent flow generated by the propeller as it pushes water astern. A poorly shaped wake can lead to significant energy losses.
A WED channels and straightens the wake, reducing its swirl and energy dissipation. This allows the propeller to operate in a more uniform flow, resulting in improved propulsive efficiency and reduced fuel consumption. WEDS are particularly effective for slow-speed, high-propeller loading applications.
Pre-Swirl Stators (PSS):
A PSS is a fixed ring positioned upstream of the propeller. It features strategically designed blades that generate a swirling motion in the water flow before it reaches the propeller. This pre-swirl helps to align the water with the propeller's rotational direction, minimizing energy losses due to flow mismatch.
The optimized flow entering the propeller allows for increased thrust generation with less power input, leading to fuel savings. PSS units are widely used across various vessel types due to their effectiveness and ease of installation.
Propeller Boss Cap Fins (PBCF):
A PBCF is a relatively new PSD that attaches directly to the propeller hub. The PBCF disrupts the formation of a low-pressure zone (hub vortex) behind the propeller. This hub vortex contributes to energy losses as it draws unwanted water into the propeller.
PBCFs feature strategically designed fins that break up the hub vortex and improve the overall flow pattern around the propeller. This results in reduced resistance and improved propulsive efficiency, translating to fuel savings. PBCFs are particularly advantageous for high-powered vessels with large propellers.
Grim Vane Wheel (GWW):
The Grim Vane Wheel (GWW) is a rotating cylinder positioned within the propeller’s nozzle. The GWW features helical vanes that generate a secondary flow within the nozzle, counteracting the propeller's tip vortices. These tip vortices are swirling energy losses at the blade tips.
By mitigating tip vortices, the GWW allows for improved propeller performance and reduced fuel consumption. However, GWWs are a complex technology with limited commercial adoption due to potential cavitation issues and design challenges.
Contra-Rotating Propellers (CRP):
A CRP system utilizes two propellers operating on the same shaft but rotating in opposite directions. The aft propeller recovers the rotational energy lost in the wake of the forward propeller. This significantly reduces energy losses and improves overall propulsive efficiency.
CRPs offer substantial fuel savings, particularly for high-speed vessels. However, their complex design and higher maintenance requirements make them less common compared to other PSDs.