pressure distribution round the nozzle. resulting in a force in the forward direction. To extend the application of ducted propellers towards higher ship speeds in combination with a larger bollard pull,Wartsila has introduced the high efficiency nozzle (HR-nozzle).The HR-nozzle has a curved exterior and interior,leading to absence of flow separation and consequently a higher efficiency. Replacing an open propeller with an HR ducted propeller,the bollard pull can be increased with about 25%,while the free running efficiency can be increased with 10% to 15%.This mainly depends on the power density of the propeller and the sailing speed of the vessel. A number of conducted retrofits were improvement device.Each type of device reviewed with respect to investment and fuel has a wide field of application,depending ECONOMICS OF RETROFITS savings.Data for the fuel consumption were on ship type,size,engine,type of fuel etc. To judge the profitability of a propulsion supplied by the ship operator,or otherwise Therefore an envelope is sketched in the improvement,the increase in hydrodynamic estimated based on the mission profile. chart,identifying a range of hydrodynamic efficiency needs to be known in relation with Figure below shows the estimated Return improvements and pay back time. investment or total costs. On Investment of each discussed propulsion Efficiency rudder 5 Less interesting Modern design propeller More interesting Modern design propeller diameter and rpm adapted Ducted propellers Grinding and repai Propeller-engine interaction 10 Efficiency improvement [% Return On Investments of propulsion improvement devices.0 1 2 3 4 5 6 0 5 10 15 Efficiency improvement [%] Return on investment [year] Grinding and repair Modern design propeller Efficiency rudder Modern design propeller diameter and rpm adapted Propeller-engine interaction Ducted propellers More interesting Less interesting Return On Investments of propulsion improvement devices. pressure distribution round the nozzle, resulting in a force in the forward direction. To extend the application of ducted propellers towards higher ship speeds in combination with a larger bollard pull, Wärtsilä has introduced the high effi ciency nozzle (HR-nozzle). The HR-nozzle has a curved exterior and interior, leading to absence of fl ow separation and consequently a higher effi ciency. Replacing an open propeller with an HR ducted propeller, the bollard pull can be increased with about 25%, while the free running effi ciency can be increased with 10% to 15%. This mainly depends on the power density of the propeller and the sailing speed of the vessel. ECONOMICS OF RETROFITS To judge the profi tability of a propulsion improvement, the increase in hydrodynamic effi ciency needs to be known in relation with investment or total costs. A number of conducted retrofi ts were reviewed with respect to investment and fuel savings. Data for the fuel consumption were supplied by the ship operator, or otherwise estimated based on the mission profi le. Figure below shows the estimated Return On Investment of each discussed propulsion improvement device. Each type of device has a wide fi eld of application, depending on ship type, size, engine, type of fuel etc. Therefore an envelope is sketched in the chart, identifying a range of hydrodynamic improvements and pay back time