Trim optimization of reefer vessels

Abstract: The influence of trim on the fuel consumption of reefer ships have been investigated. The project have been carried out at NYKCool AB, a ship operator of reefer vessels.  In the speed range 17-19 knots and deadweight percentage of approximately 70% the smallest calculated resistance is obtained for bow trims which are between -1 m and -2 m depending on the speed.   Stored voyage data from the reefer ships shows that the majority of the voyages are made with positive trims. The data also shows that the mean fuel consumption in ton per hour is smallest for the even keel condition in comparison with aft (positive) trims. The conclusions have been made for deadweight conditions of 70-80 % DWT. Savings of bunker are in the order of magnitude of 3-4% and have been seen from the data analysis.   The average fuel consumption in traffic is approximately 37 tons of heavy fuel oil per day. Due to factors such as different docking schedules the ships have different performance. The tests shows a correlation between the fuel consumption and optimal trims. Bow trims are always more favorable from a saving perspective.   The largest bow trim that was possible and which ensured proper propeller immersion was approximately -1 m and was achieved by planning the stowage together with burning bunker from the fuel tanks in the right order.   The stability is affected due to less buoyancy in the bow area compared to the mid ship sections, but always above the minimum criteria of GM0 <o 0.15 m with enough margins. GM0 remains in the interval of 2-3 m depending on the deadweight percentage and trim condition.   Economical savings will be seen over longer periods of time by operating the reefer vessels with the optimal trims.