Summing up it can be concluded that CLT propellers offer higher efficiency. The efficiency gain can be used to achieve fuel savings from five over to ten percent at constant speed or alternatively for increasing the ship's speed up to 3.5 per cent whilst keeping the fuel consumption constant.
Depending on the type of ship and the characteristics of the propulsion plant, fuel savings substantially even higher than ten per cent can be achieved, especially in the case of full-bodied slow-speed large ships with heavily loaded propellers. Traelers can achieve fuel savings mentioned above and can also increase the pulling force by some twelve per cent.
Although SISTEMAR is no doubt the most leading company in the field of know-how in theory and design of propellers with placed end plates at the blade tips other companies are also trying to offer propellers with end plates. Known are proposals of such propeller design made by the Potsdam Ship Research Institute in cooperation with Schottel they tested propellers with end plates claiming 10% higher efficiencies than those of conventional propellers [4]. Another company J. J. Kappel Marine from Denmark is also involved in design of propellers with end plates, claiming an efficiency increase of 7% compared with a conventional Wageningen B-series design [5].
These two above named companies also undertaking the design of unconventional propellers have been mentioned just to demonstrate that there is a good enough reason to use this type of propellers in ship propulsion systems.
SISTEMAR can offer these days a mature design of an unconventional CLT propeller for powers ranging from 100 BHP to 58000 BHP covering a wide range of propeller revolution rates. So the question arises why these type of propellers are not yet becoming a standard conventional type of propeller fitted to a propeller shaft?
The answer to this question is not an easy one as this is a complex issue and final assessment of the CLT merits (fuel savings or propulsive power increase) is affected by many factors like:
- traditional conservatism of the shipowners superintendents, afraid of taking any decision which introduces novelty into a ship system. This attitude usually stems from lack of knowledge in a specific field of technology development.
- existing on the market makers of conventional propellers will try to undermine the merits of a CLT type propeller not having the vast amount of theoretical knowledge and practical know-how needed to be accumulated to design a propeller with end plates being superior from a conventional one.
In SISTEMAR the results obtained through direct calculation methods are generally in good agreement with the full-scale results of a CLT propeller, without the need of any model testing programme. In some cases however, the results of model tests with an equivalent conventional propeller are used as a basis for the design of a CLT propeller. And still in other cases it is needed to carry out a complete model testing programme, because of special requirements by the shipowner in such cases SISTEMAR has developed a new and very sophisticated method of extrapolation of model field results to predict full scale results.
- the achieved savings by installing a CLT propeller instead of a conventional one may range from 5% to let say 12% or even more. But assuming reasonable average savings 6% to 8% it is of paramount importance to conduct reliable and sufficiently accurate measurements of engine power and ship speed during sea trials. If this condition is not fulfilled committed errors may cancel the net gains in efficiency or even indicate that we suffer losses. As far as the measurement of ship's speed during sea trials is concerned the likelihood of making errors is very small with todays applied GPS systems, but measurements of developed power by the main engine during trials on the measured mile may turn to be quite controversial and erroneous.
The usual method of propulsive power measurement on board ship applied by the shipyards during sea trials is the usage of a torquemeter. Whatever type of torquemeter, will be used there is a necessity of its proper and skilful calibration. If the calibration is not carried out properly serious devations in plus or minus may occur leading to heated disputes between the shipowner, ship-yard and propeller maker.
As the author himself was involved in such a case a more detailed description and comments shall be given in order to understand how matters can be complex and how to verify objectively the engine power output.
The described further case concerns a series of 45000 dwt bulkcarriers built in one shipyard. Out of this seven ships the first two were fitted with a conventional propeller the next three were fitted with a CLT propeller.
3. ENGINE LOAD CONTROL SYSTEM, PROBLEM OF A CORRECT ASSESSMENT OF PROPELLER EFFICIENCY
Measurements by means of a torquemeter of power delivered by the engine to the conventional propeller fitted to ship "A" were noticeably lower when compared with the load indicator readings on the engine. On the contrary during sea trials of ship "B" equipped with a CLT propeller the engine power measured by the torquemeter have shown power readings higher when compared with the engine load indicator.
The engine manufacturer for his own sake during all sea trials was checking the engine load by using a graphic were the power developed by the engine is a function of the product of load index (Li) by propeller rpm (n) was defined. In figure 1 the correspondence between Li × n (rpm) and KW developed by the engine has been represented by a black straight line. In this Figure 1 the power measured by the torquemeter during the speed trials of ship "A" and ship "B" have been also represented. From this plotting it can be concluded that the scattering of torquemeter measurements has been very important and it should be noticed that torquemeter measurements during ship "A" trials were lower than the real power developed by the main engine, while the torquemeter measurements during the next trials conducted on board ship "B" have been noticeably higher then the powers developed by the engine.
a) For instance, the measurement taken during the official trials with the ship "B" corresponding to 11400 KW would request that in case that the engine should have run on its cubic curve law the load index should have been 7.4 but the real value measured on the engine was only 7.1. It is obvious that the power measured by the torquemeter is not in accordance with the main engine load index.
b) In the case of the power measured by the torquemeter corresponding to 9650 KW, the expected load index when the engine is operating on its cubic curve should be 6.71 but the value measured on the engine (load indicator) was only 6.30. So it is also evident that in this case the real load index of the engine indicates that the power measured by the torquemeter is deviating quite far away from the real one value. From Fig.1 one could deduce that the engine is severely overloaded during trials on ship "B" what was really not taking place, because not only the load indicator was showing lower load but also the turbocharger revolutions and exhaust gas temperatures were below nominal values. Therefore, the readings of the torquemeters must be disregarded.
