Propulsion Performance Research: The highlightsFebruary 8, 2019
Since 2016, VAF Instruments has been publishing white papers to create awareness on the importance of performance measurements on ships, specifically about the essential value of thrust measurement for the individual performance of hull and propeller. This article provides a brief summary of the research and conclusions done by our Research & Development team which is published in four separate white papers.
Separating propeller performance from hull resistance
Thrust measurements by VAF Instruments’ TT-Sense® allow for individual tracking of propeller performance and hull resistance. The first white paper illustrates this conclusion by research done on a ship in service. The research shows that 30-40% of the loss in performance is caused by the propeller. Meaning that this same propeller is responsible for a large part of the increasing fuel consumption due to decreasing propulsion performance. That is quite a percentage for such a small part of the ship. Especially when you bear in mind that it is rather ‘easy’ to keep a propeller in an optimal state by in-time maintenance and repair, compared to e.g. the hull.
When hull and propeller performance are measured individually and monitored closely, it enables for a.o. condition based maintenance or optimisation, both leading to savings on the fuel bill.
Retrofitting for performance
The second white paper shows the actual effect of a retrofit on the propulsion performance of a ship in service. The effect of both a new propeller and bulbous bow design have been measured and compared to the calculated ROI. The conclusion of which is interesting.
Retrofitting a ship is often a high investment and in this case the new bulbous bow design was a multi million operation. In addition the new propeller was a smaller part of the total investment. Initial calculations showed that both adjustments on the ship’s design would be important factors for performance increase. But when we compare the individually measured propeller performance before the retrofit with the measured performance after the installation of the new propeller it shows that the impact of this new propeller is much bigger than expected. Especially when it turned out that the actual result of the new bulbous bow design stayed behind.
Where the expensive new bulbous bow design showed an improvement of 5% on the propulsion performance (instead of the calculated 30%), the ‘low investment’ new propeller reduced the fuel bill by 10-15%. It may be clear to say that these results will surely effect future decisions for retrofits on other ships of the fleet.
Accurate trim routines
Another benefit of measuring thrust is that it directly measures changes in hull resistance, due to trim for example. When fuel consumption or shaft power are used to optimise trim, the efficiencies of the engine or propeller are also involved. This can interfere with finding the optimal trim. By measuring the individual effects on hull and propeller of trim adjustments the optimisation routines become more accurate. Therefore, measuring thrust is useful in finding the optimal trim, especially in combination with a Controllable Pitch Propeller (CPP) with varying efficiency. Find out more in the third white paper.
Even more accurate…
The accuracy of ship performance analysis is very much dependent on the accuracy of speed through water data. Ships are commonly equipped with doppler speed logs that can be influenced by seawater temperature. In the presence of large temperature differences this heavily affects the outcome of performance analysis.
Through the use of thrust and rpm measurements propeller a virtual speed sensor is designed. The measurement of thrust can therefore be used to provide an improved speed through water. In the presented case study the method reduced bias and temperature dependency compared to speed log data.
The case study has shown that thrust measurements can be used to improve ship performance analysis in two ways. Firstly, thrust measurements can be used to correct the signal of a deviating speed log. Secondly, thrust measurements allow the effects of a propeller cleaning and a hull cleaning to be separately determined. Read more about this in the fourth white paper which will be available soon.
All the previously mentioned white papers focus on the benefits offered via the full scale measurement of propeller thrust (and torque), in relation to propulsion performance optimisation, fuel saving potentials and emission reductions. Want to be te first to receive the next white papers? Sign up for the monthly VAF Insights Newsletter to get your monthly dose of Insights into Propulsion Performance, Fuel Efficiency and Environmental Compliance.