Last Updated: Apr 2012
What is it?
Hull and propeller optimisation and retrofitting is the use of new technologies to address the estimated 16% of propulsion energy lost by hull friction and the 13% lost through the propeller. The goal is greater efficiency in these areas by reducing drag through the water, leading to lower fuel use and reduced emissions. Newbuilds are able to deploy technologies in the design phase not always available for existing ships, but there are still options for vessels currently in service.
How it works
There are a range of options available from hull and propeller modifications and retrofits to best practices for cleaning and maintenance. Some options may be regarded as non-traditional; for example, the International Maritime Organization (IMO) Second Greenhouse Gas (GHG) study estimates that there can be a 1-2% efficiency gain from keeping ship topsides uncluttered and more streamlined on some ships, such as grinding weld beads flat, repositioning cranes and other deck structures, or adding 'spoilers' and airflow enhancers over the funnel and deck house.
For propellers, there are a number of design options (coaxial contra-rotating propellers, free rotating vane wheels, ducted propellers, pre and post-swirl devices, integrated propellers and rudders, guide vanes, and alternative propulsion systems) that are not available for ships currently operational. Upgrades available include more frequent polishing of the propeller, which can give an efficiency increase of up to 3%, according to IMO figures. Finding the optimal pitch for a controlled-pitch propeller can also lead to an efficiency increase.
Retrofitting a new propeller is an option if a smaller diameter propeller operating at a high speed can be replaced with a larger diameter unit operating at a lower speed. Depending on the vessel design, there can be a 5-10% improvement in efficiency and consequent reduction in fuel consumption and emissions.
To reduce the friction on hulls, anti-fouling coatings are used, which either self-clean by releasing a copper-based product to inhibit organic growth, some of which are silicon based to prevent organisms fixing themselves to the hull. Self-polishing coatings must be renewed every 3-5 years (in a drydock) but there is some benefit to hull cleaning in between dockings, especially through a hull resistance management programme.
Hull and propeller performance monitoring systems, such as the CASPER system produced by Propulsion Dynamics, monitor vessel performance over time so that a shipowner can manage hull resistance and efficiency gains from differing hull coatings, cleaning routines, and propeller cleanings and upgrades (as well as any other hull appended efficiency enhancing measures). Fouled hulls and propellers can add up to 40% resistance, according to figures from Propulsion Dynamics. Fouled hulls cause an average increase in fuel consumption for ocean commercial vessels of between 4.2 tonnes per day for an Aframax tanker to 14.1 tonnes per day for an average Panamax containership, according to Chris Geater, Marine Operations Manager for engineering firm Thomas & Coffey.
The IMO has reported that efficiency gains of nearly 12% can be made through regular hull cleaning (usually through dry docking) but that the current lack of accurate data and "variability in the effect of hull cleaning" is a problem that makes definite conclusions about efficiency gains difficult. A MARINTEK study concluded that a feasible increase in dry dock frequency could increase efficiency by 5%.
The IMO has also noted additional coatings or hull modifications, such as "introducing riblets that mimic shark scales" or using advanced polymers or nanotechnology coatings. The latter are currently only in development but could offer up to 15% efficiency gains. Also mentioned are air-bubble lubricating systems for hulls. DK Group offers a retrofitted Air Cavity System (ACS) technology that it claims can reduce fuel use in low-speed vessels by up to 15% as well.
In April, 2012 Gamma Light & Heavy Industries Ltd. also introduced a brand new propulsion system called The Gamma Propulsion System (TGPS) that can reduce energy consumption "by at least 75%" compared to current propeller-based systems. More information can be found under Costs and Trials.