Effect of change of propulsion system on M/S Midnatsol

Abstract

In 2016 the costal liner M/S Midnatsol had her main thruster system changed from the twin fixed pitch contrarotating thruster system Contaz to the twin controllable pitch Azipull thruster system. The purpose of this thesis is to investigate the effect of this change. A Literature review was done and discovered benefits and drawbacks for contra-rotating propellers (CRP) and controllable pitch (CP) propellers. The main benefits with CRP is higher efficiency and lower propeller loadings. The main drawbacks are complex design, and weight. For CP the benefits are high flexibility, and the drawback is efficiency. The crew of sister ship M/S Trollfjord who did the same transformation in 2017 was interviewed. They reported higher course stability, lower fuel consumption, more power available for manoeuvring, more possibilities for manoeuvring and more noise when manoeuvring. The data analysed in this thesis are mainly from three sources; on board HCX, AIS from Marine Traffic and weather data from the NorKyst-800 model. They were pre-processed and used for different analysis. A cavitation check was performed during manoeuvring, and it is found that the Azipull propeller is likely to cavitate when the ship is docking. This is probably the main source of the noise during manoeuvring. The ships performance were also analysed using a speed power diagram plotting the power required to maintain different speeds. Compared to the speed trail and model test the power required was higher for the Azipull thruster system. Compared to manual reading from the crew when the ships had Contaz installed the power required were similar or higher for Contaz. The speed through water (STW) were used to correct for the current. The STW was found from both on board HCX data and NorKyst-800 data. This showed that STW from HCX were more stabile, and that it could be used for current correction. The NorKyst-800 also gave good results. The Admiralty coefficient was investigated and used to make a new prediction line of the required power. The effect of propeller pitch on engine loading in manoeuvre mode did not exceed the capacity of the engine, and were never above a 1600 kW for one thruster. vi The overall conclusion is that the ships performance is improved, especially with regards to operational problems. The exception is the noise during manoeuvring. The power required seem to be more dependent on the condition of the hull than the difference in performance of the two thruster systems.