The propeller of a ship can be optimised in order to reach a hydrodynamic design. By minimising the number of blades, the blade area is reduced, resulting in less resistance, but the reduction potential is mainly related to the diameter size and the amount of revolutions per minute (RPM). With a large propeller rotating at a low speed, high propeller efficiency can be reached. Propellers with a large diameter are mainly suited for deep-draught ships. An optimal design of a propeller can be limited by factors like the possibilities of enlarging the diameter and loading.
To reach the above described propeller optimisation a new propeller has to be installed. In case this is not possible there are several ways to upgrade a propeller to optimise conditions and reduce emissions. Examples of propeller upgrades, mostly in the form of devices to be installed, are summarised in Table 1. A full description per measure can be found here – A and B.
Losses of propeller energy can be avoided by recovering the rotational energy in the flow from the propeller or by making use of pre-rotation of in the inflow before it enters the propeller. The reduction potential on average is between 5 and 10% of total ship propulsion power.
Table 1 Overview of examples of propeller upgrades
|Measure||Applicability||Reduction potential||Payback time||Costs|
|Change of rudder profile and propeller||Tanker, container, RoRo||2-6%||Medium|
|Upgrading the tip of the propeller||Tankers||0.5-3%||Medium|
|Propeller boss cap with fins||All ship types||1-3%||Investment costs between USD 20,000 and USD 146,000|
|Optimisation of propeller blades||All ship types||< 2%||Very short||10-15% higher propeller costs|
|Contra-rotating propellers||Single-screw ships (very fast RoRo vessels or ice breakers)||3-6%||Investments costs, but unknown|
|Free rotating vane wheel/Grim wheel||Cargo ships||10%|
|Ducted propeller||Tankers, bulk carriers, tugs, offshore supply and service vessels||5-20% (average 10%)|
|Pre-swirl devices||All single-screw ships||1-9%|
|Post-swirl devices||All new ships||1-9%|
|Integrated propeller and rudder units||Cargo vessels, RoPax vessels and container vessels operating at relatively high speed|
|Wing thrusters||RoRo and ferries||< 10%||Medium||Investment costs, but unknown|
|Pulling thrusters||RoRo and ferries||< 10%||Medium||Investment costs only (unknown)|
Source: 2009 IMO Second IMO GHG Study 2009 (pages 171-173)