Armada's Technology & Advantages of Propeller Cleaning
"Polished props -- the smoother way to greater economies"
Smooth propellers save money is the message ship operators are being told. But owners must be able to weigh the cost of propeller roughness against the expense and timing of maintenance schemes -- an equation requiring detailed understanding of blade roughness and its measurements.
EXTENSIVE STUDIES carried out by the British Ship Research Association (BSRA) have shown that a power loss of up to 6% can result from a roughened propeller. Although it is well known that hull roughness causes serious energy loss, it is perhaps surprising that, according to the BSRA research, the propeller with its small surface area, can generate energy losses amounting to half that of the hull. In BSRA's latest paper on propeller roughness, prepared by D. Byrne, P. A. Fitzsimmons and A. K. Brook, propeller maintenance and its role as a cost-effective energy-saving measure was examined. Data on propeller roughness was drawn from BSRA records compiled over the last 30 years involving over 130 propellers of varying ages.
||Ten-year roughness (µm)
||Equivalent annual value ($'000) with fuel inflation of:
The information was combined with roughness measurements taken recently, both with a purpose-built gauge and proprietary equipment on propellers in dry-dock and on replicas of propeller blades. The objective of the paper was to determine whether any extra investment in improving propeller conditions is warranted when balanced against the expected reduction in operating costs. The percentage of power lost due to a roughened propeller was computed for a 39 000dst single-screw containership from which model and full-scale data was calculated; at the speed of 23 knots the power loss varied from 0-6%. Data from the BSRA records also suggested that many other propellers in service could be resulting in a power loss of up to 4.5%.
The two main methods of roughness measurement are peak to valley roughness height (Rtm) and center line average (Roughness average or Ra). When working on hull roughness, a characteristic of new or smoother hulls suggested that measures of height alone might be adequate to estimate hull roughness effect. But analysis of many propeller roughness records has shown that roughness measurements based on height scales alone are insufficient -- a texture parameter, in addition to height scales, is necessary to classify a blade surface.