VAF Instruments
20Pages

EXPERIENCE WITH FULL SCALE THRUST MEASUREMENTS IN DYNAMIC TRIM OPTIMISATION A scientific paper by VAF Instruments Research & Development Erik van Ballegooijen, VAF Instruments, Dordrecht/Netherlands, evballegooijen@vaf.nl Torben Helsloot, VAF Instruments, Dordrecht/Netherlands, thelsloot@vaf.nl Marc Timmer, VAF Instruments, Dordrecht/Netherlands, mtimmer@vaf.nl

Abstract This paper presents an explorative study on the role that thrust measurements can play in trim optimisation. Currently data-driven trim optimisation solutions rely on a shaft power meter, but it will be demonstrated that also thrust measurements are a valuable input. A better understanding of the subject is gained by discussing the effects of trim and assessing different approaches to trim optimisation. Possible improvements that thrust measurements can bring to the data-driven method are suggested and investigated with the help of a case study. In this case study continuously...

The trim of a ship is the difference between its forward and aft draught. Trim can be influenced relatively easily. Before the start of a voyage it can be altered by carefully choosing the weight distribution of cargo, and during the voyage by changing the levels in the ballast tanks. Fuel savings can be achieved by choosing the right trim in the right circumstances, and therefore the subject of trim optimisation is of high interest to the maritime industry Changing the trim changes a ship’s resistance, even when all other variables are kept the same. As such there is an optimal trim value,...

Trimming changes the shape of the submerged part of the hull and because of this different hull shape almost every resistance and propulsion related aspect is affected by trim. As discussed by Reichel etal. (2014), the most important influences on the required propulsive power are the change in residual resistance of the ship and the change in propulsive efficiency. The residual resistance changes primarily due to a change in the wave pattern that is generated by the ship. Especially when a bulbous bow is present the effect of trim on the wave pattern can be large. A favourable trim will...

To predict the effects of trim it is most important that the change in (residual) ship resistance and the change in propulsive efficiency are correctly determined. The resistance has to do with the hull, whereas propulsive efficiency is the domain of the propeller (forgetting for a moment about interaction effects). Therefore, to have a good prediction of the effects of trim, both the hull and propeller need to be correctly captured. In the next few paragraphs three methods of modelling that were mentioned in the introduction will be briefly discussed, addressing the uncertainties involved...

Another weakness that the data-driven approach arguably has, is the problem of data scarcity. When conducting experiments or computations a predetermined matrix of draught, speed and trim can be accounted for such that there is a knowledge base covering all operational conditions of a ship, even though it rarely encounters. A trim optimisation model that learns ‘on the job’ from data obtained in service may not accurately predict in newly encountered conditions simply because it does not yet have the data to do so. These considerations are more elaborately discussed by Bertram (2014). To...

4. ADVANTAGES OF USING THRUST MEASUREMENTS IN TRIM OPTIMISATION In the previous sections it was discussed that trimming can be used to optimise the resistance of the hull as well as the performance of the propeller. It was also addressed that when using a power meter, the data-driven trim optimisation methodology only optimises the system as a whole (i.e. total propulsive power) without giving any insight into the contribution of the separate components. In addition it was mentioned that it is favourable to reduce the number of relevant variables that are used as an input for...

Some ships are equipped with a propeller of which the blade angles (pitch) can be adjusted during sailing. This type of propeller is called a controllable pitch propeller (CPP). In contrast to a conventional propeller, a CPP can deliver the same amount of thrust at different rotation rates by using a different pitch. The efficiency of the propeller will depend on the chosen pitch and will therefore affect the power needed to propel the ship, see Figure 3. operating curves per pitch angle Figure 3: Schematic graph of propeller efficiency and pitch (CPP) To illustrate how this can affect trim...

The previous sections introduced the concept of data-driven trim optimisation, and discussed the possibility of using thrust measurements instead of or in addition to power meter readings. To demonstrate that thrust measurements are indeed a useful input, some full-scale TT-Sense® data is presented within this case study. Mounting ring Power transmission foil In this section data is shown in order to compare the effect of trim on power with the effect that trim has on thrust. The case study encompasses a month of continuously monitored data that, to protect the interests of our clients, is...

5.2 Data preparation In order to isolate the effect of trim on power and thrust, the effects of external influences should be minimal. To ensure this is the case the data is filtered. Firstly data is only used from those periods in time during which the ship was sailing at a near constant speed. In other words, data from periods of time during which the ship was accelerating and not in physical equilibrium are removed from the data set. Moreover, the data set is filtered for deep water and low wind speeds so that the influences of shallow water effects, waves and wind are small. For the...

Figure 5: Side by side display of thrust and power at 14 knots Thrust and Power comparison (14kn) Figure 6: Direct comparison of thrust and power at 14 knots

Figure 7: Side by side display of thrust and power at 14.5 knots Thrust and Power comparison (14.5kn) Figure 8: Direct comparison of thrust and power at 14.5 knots