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Measures for improving the Propulsion Efficiency

These consist of hydro-dynamically based thrust augmentation devices also called as energy saving devices. These devices can be considered to be operating in three basic zones of the hull as follows

i.   Located before the propeller.
ii.  Located at the propeller.
iii. Located after the propeller.

Before the propeller

1. Pre-swirl Stator (PSS)

It consists of a set of blades right in front of the propeller. The aim is to reduce the rotational losses in the propeller slipstream by introducing a rotation in the inflow to the propeller.
PSS increases the hull efficiency and hence the total propulsive efficiency. DNV classification society has reported a 6% power saving in design draught but just about 1% in ballast in a Kamsarmax bulk carrier that gives indication of the potential of PSS for the full form vessels.

2. Becker Mewis Duct

It is a Pre-swirl Stator with Accelerating Duct. The role of the
duct is to homogenize the axial wake component. It increases the efficiency of the pre-swirl fins by providing a more important water inflow to the stator. Also the duct contributes to the total thrust by virtue of the lift created by the accelerating flow over its walls.

3. Schneekluth Wake Equalising Duct (WED)

It aims to improve the overall propulsive efficiency by reducing
the amount of separation over the afterbody of the vessel by helping to establish more uniform inflow into the propeller by accelerating the flow in the upper part of the propeller disc and minimizing the tangential velocity components in the wakefield.
It also helps in the selection of largest possible propeller diameter. 

At the propeller

1. Propeller Boss Cone Fins(PBCF)

The idea of fitting fins to the cone of the propeller, is to enhance the
efficiency of the screw propeller by reducing the energy loss associated with the propeller hub vertex. Principally fins of flat plate form and having span of the order of 10% of the propeller blade span are fitted at a given pitch angle to the cone of the propeller. The number of fins correspond to the number of

propeller blades.

2. Unconventional Propellers

Kappel propeller has been developed by Prof. J.J. Kappel. The pressure on the suction and discharge side of the propeller at the tip equalises due to the tip vortex and hence the efficiency of the tip region is low. Kappel propeller minimises the flow over the tip and the outer region of the propeller therefore retains high efficiency. The efficiency of Kappel propeller is appreciably more than
the efficiency of the conventional propellers.

Behind the propeller

1. Rudder Bulb

It comprises of a large bulb having a diameter of about 30-40% of the propeller diameter which is placed on the rudder close behind the propeller boss. This has been in application as Costa Bulb since 1950s. It is aimed to prevent flow separation and excessive vorticity behind the hub by effectively extending the propeller boss. Additionally in some cases there are fins fitted to the bulb and these fins then produce a lift force. A component of this lift force then acts in the forward direction to produce thrust augmentation.


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