Enhancing propeller performance

01 May 2004

When it comes to surface condition, the hull is generally considered more important than a vessel?s propeller. However, in terms of energy loss per unit area, propeller surface condition can be significantly more important. Also, in economic terms, ensuring a prop?s condition through effective maintenance can be relatively cheap and provide a high return.
Consequently, the hydrodynamic performance of foul release systems and the benefits of their use on propellers has been the subject of much study and increasingly more ship operators have been compelled to coat their propellers. The resulting benefits of such measure is the potential of fuel savings from increased propulsive efficiency in addition to lower maintenance costs and a cleaner environment.
Involved in this field of development is International Coatings which, in 2002 and together with a major oil company, requested that the University of Newcastle?s School of Marine Science and Environmental Technology provide scientific evidence on the observed fuel benefits with a tanker?s propellers coated with a foul release system. The results of the pilot study were reported at the International Conference on Environmental Sustainability. In the study the effect of foul release technology on the propeller open water performance, mainly on the torque, was taken through the corrected viscous drag coefficients and the propeller performance was simulated using a state-of-the-art unsteady propeller analysis code.
The test revealed that a foul release coated blade surface is equivalent to a new or well-polished blade surface. According to a major UK propeller manufacturer at the time, a roughness equivalent to Rubert B (on the scale of the Rubert Propeller Roughness comparator) represents a new or well-polished propeller while Rubert D to E would be equivalent to the blade roughness after one to two years in service.
Through relating the corrected drag coefficients to the Rubert scaling, the results determined the loss in propeller efficiency due to blade roughening with respect to Rubert B, which represented a foul release coated propeller. By considering the design operation condition of the subject vessel at advance coefficient, J=0.48, the propeller efficiency losses proved to be around 3-6%. On the other hand, the propeller gained a similar amount of efficiency when coated with a foul release system.
Further research and model tests, using the Emerson Cavitation Tunnel, were conducted to determine the efficiency and roughness characteristics of coated propellers with those of uncoated propellers of varying degrees of roughness. Model tests also considered the effect of coating thickness on the performance, cavitation and noise, as well as investigating a coating?s durability over time, in response to concern over detachment at the blade tips. International Coatings also collaborated in full-scale trials using the University research vessel Bernica and a number of larger commercial vessels with and without coatings to compare the power at the shaft.
One of the Hydrex Group of companies, Subsea Industries, has also gone to some length to develop a coating that not only protects a propeller from corrosion but also increase its efficiency. EcoSpeed, which has successfully passed through all research stages in propeller coating applications, is said to completely shield propellers from all corrosive elements in their operational environment. The propeller is electronically inert and so cannot leach ions of copper or brass into the ocean through electrolysis. With the de-alloying process being stopped, the service life of the propeller is increased.

Glass coatings
The coating is made up of glass (this being a natural barrier to water) platelets suspended in a reinforced vinyl-ester resin, which is said to enable EcoSpeed to form an impenetrable and complete protective barrier around the propeller material. Unlike earlier glass particle coating technology, however, this coating uses larger glass platelets specially designed to prevent the ingress of water while immersed in the marine environment. The hardness of the EcoSpeed coating is claimed to virtually eliminate cavitation damage, which obviously saves on replacement and maintenance.
This new coating technology is extremely resistant to the corrosive elements that would normally destroy almost all other protective materials over a short period of time. With the coating also keeping the propeller operating at its maximum efficiency and maintaining a self-cleaning effect, fuel consumption is also substantially reduced. Other benefits of the coating are the fact that it is non-toxic and requires no toxic chemicals (such as thinners) during application, it is relatively cheap and is claimed to last for at least 15 years with almost negligable wear. The bottom line is considerable savings for ship owners in terms of time and money.
Slipring system
From a corrosion point of view, coated propellers are often preferred because uncoated propellers act as cathodes (due to the noble alloy metal) and attract electric current from the hull through the water. This then has to find its way back to the hull, which can result in spark corrosion damage in the shaft bearings. Coatings actively reduce such corrosion but there is the question of speed and fuel economy that doesn?t enter the equation with uncoated propellers. So to avoid damage and gain the best out of uncoated propellers a slipring system needs to be installed.
Cathelco Jotun has, for many years, supplied slipring systems for propeller shafts, in combination with Impressed Current or Sacrificial Anodes Cathodic protection systems. Commenting on slipring arrangement for grounding a vessel?s shaft and propeller, Jotun?s product manager, Miles Buckhurst says "The best method of protecting a ship?s hull has long been a combination of coatings and cathodic protection. The cathodic protection is usually considered the back up to the coating system."
The Cathelco Jotun slipring arrangement allows for continuous earthing of the shaft (propeller) whether it is in operation or not. "While the slipring arrangement allows the hulls? cathodic protection system to protect the propeller," explains Buckhurst, "it is not possible to pull sufficient current to completely cathodically protect the propeller. Cavitation is reduced, but only as a secondary consequence of reduced galvanic corrosion in that area."
This is not, however, considered to be the most important aspect as a potential difference will occur between the hull and the propeller due to the use of different alloys and the rotation of the propeller. "If the shaft is not earthed intentionally, the galvanic current between the propeller and the hull will attempt to pass through other paths. This is usually via the engine bearings, where this is not a continuous link, and in order for the current to flow it must spark across the bearings. This creates spark corrosion in the bearings and eventually the bearings must be replaced at high cost whilst in dry-dock. This is considered by most ship operators as a problem that should be avoided and with the slipring ring arrangement it is," says Buckhurst.

Installing a slipring system

The slipring is made to fit the intermediate shaft diameter and is fitted in the engine room. It comes in two halves, which are easily bolted together. The reason for the ring being 50 mm deep is to keep the silver contactway clear off the shaft and free from the oil and dirt contamination associated with the shaft surface.
The silver graphite brushes for earthing the slipring are mounted on a single bracket for easy installation.
The slipring is simply bolted around a cleaned area of the intermediate shaft, perpendicular to its axis. The bracket holding the brushes is attached suitably so that the brushes make contact with the silver contactway in the ring and a cable runs from the double brush holder and is used to ground the ring to the ships hull. A mV-meter is recommended for monitoring the potential difference between the hull and shaft. This should read less than 75 mV in order to give reasonable grounding. The meter is fitted on a separate isolated brush.

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