The importance of fuel economy in present day ship operation has been demonstrated many times in recent years. Oil costs have fluctuated for years. The requirement to maintain economical operation is therefore vital.
Notable advances have been made in the achievement of the highest possible propeller efficiency (average roughness of a polished propeller) to suit required service operating speeds.
The potential advantages of maintaining both hull and propeller smoothness has motivated considerable theoretical and practical research over the years and a number of important papers have been published on the subject.
The purpose of this paper is to endeavor to identify the types and causes of the propeller roughness, to relate degree of roughness to increase in power requirement to meet a given ship speed and thus to additional fuel consumption and cost, to advise the steps which may be taken to ensure that any increase in fuel costs is restricted to a minimum, and to illustrate the magnitude of the financial savings available.
Causes of Roughness
Some degree of roughening of propeller blade surfaces is inevitable in normal course of service. The causes of attack may be:
- Marine growth-primary and secondary
- Impingement attack
- Corrosion-chemical and/or electro chemical
- Cavitation erosion
- Inexpert maintenance
Marine growth of the animal or vegetable variety forms on the propeller blade surfaces while the ship is idle and the propeller is stationary. In its worst form, i.e. as barnacles, the resultant loss in propeller efficiency is very serious. There will also almost certainly be an increase in power absorption resulting in a fall in propeller RPM, with a tendency to overload the machinery.
Cavitation erosion is usually concentrated on localized, sometimes small, areas of the blade and may be due to cavitation arising from flow irregularities due to an unfavorable wake distribution or to the effects of physical damage to, or incorrect shape of, the blade leading edges. Cavitation erosion can be very deep, in some cases leading to complete wastage of the outer parts of the blades.
The development of surface roughness could be accelerated if the propeller has been ground by inexperienced operators, perhaps using too coarse grinding discs, and if insufficient attention is paid to the correct formation of the blade shape.
Definition of Roughness
It is important always to define roughness in the same units preferably using the same parameters, or at least to be able to relate the different kinds of measurement.
The International Standards Organization lay down requirements for surface finish of propellers in standard IOS 484 part I 1981 and has the surface requirement of three microns Ra for class S finish and six microns Ra for class I finish.
The rate of increase in roughness during service depends on the nature of the trading pattern of the ship, e.g. which ports are visited and the time idle in port, but measurements after service have shown that, physical damage apart, the general surface roughness can easily have increased by 15 microns Ra after 12 months, which, as will be seen later, will have a significant effect on the performance.
Methods of Measurement or Assessment of Surface Roughness
There are a number of surface roughness measurement instruments on the market, some which may only be used under permanent laboratory conditions.
A simple method of surface condition assessment is provided by use of a comparator gauge produced, with the assistance of S.M.M., by Rubert & Co. of Cheadle. This is comprised of six samples of surface finish ranging from Ra = 1 micron to Ra = 30 microns. The surfaces represented are very accurate replicas taken from actual propellers. The benefits of this comparator are that it can be carried in the pocket, and a special version can be used underwater if required. With practice a reasonably accurate impression of the surface condition can be assessed, upon which a decision whether or not to service the propeller can be made.
Blade Surface Maintenance
Overall blade surface wastage caused by impingement or corrosion leads to turbulence which increases the drag of the blade section resulting in loss of efficiency. The development of turbulence intensifies the attack.
If treated at an early stage the roughness can be removed by light and fine grinding with little loss of blade thickness. If maintenance is delayed the increase in depth of the roughening will be accelerated. This means that the loss in efficiency, and thus increase in fuel consumption, will be greater and because more grinding will be necessary the costs of rectification will be increased.
Therefore the rule applying to propeller surface maintenance should be “little and often”.
Translation to Fuel and Cost Savings
The effect of the improvement in propulsive efficiency on the annual fuel consumption depends on the following factors:
- Average service power
- Mean specific fuel consumption
- Days at sea per year
- Fuel price per tonne
Two typical examples, for a bulk carrier of 64000 DWT and a 1400 TEU container ship, are tabulated below.
|Ship type||64 K Bulker||1400 TEU|
|Average power kW||7870||13975|
|Days at sea||250||300|
|Fuel price /t||$280||$280|
|Annual costs||$2.8 m||$6.76 m|
|% savings after polishing||3%||3%|
|Daily fuel savings||$336||$676|
Whether or not it is considered necessary in the long term to change the propeller design to suit a revised operating condition, there is no doubt that very significant fuel cost savings are available for a very small outlay by grinding and polishing the propeller.
- L. Townsin, W.E.G.E.M.T. 1983 “Bottom Condition and Fuel Conservation”
- Fagerjord, Det Norske Veritas, Paper Series No 80 P022 October 1980 “Possibilities of Improving Propulsion Efficiency”
- Svensen, I.Mar.E 1982 “Techno-Economic Reasons for Selecting Fuel-Saving Priorities”