The Hydrodynamics of Elogrid

The ship hull hydrodynamics impact to global energy consumption is about quarter of all energy used in the world. The amount of fuel consumed in an average Panamax container is over 200t per day. The ship hydrodynamics is improved by careful design of the body, but also all the appendages needed in the ship are aligned with streamlines to avoid excess resistance. The bow thruster tunnels and propeller units providing transversal thrust forces for maneuvering purposes are typically adding ship total resistance 2-3% per tunnel. The additional resistance from the tunnels is commonly decreased by vertical grid bars mounted in the opening – those reduce the resistance to 1-2% per tunnel. However those grids typically decrease the maneuverability increasing the propeller thrust resistance 7-12% depending on grid density and design. Vertical grids density having best performances reducing tunnel additional resistance contribute highest thrust losses. Elogrids (pat.pend) A new design for the tunnel opening grids has been developed to maintain the impact decreasing additional resistance at same level with commonly used dense vertical grids, but instead of reducing the thrust - keep or improve the thrust forces from the tunnel thrusters when needed in maneuvering. Basic idea of the stator type Elogrids is with the bars and circles prevent water flow into the tunnels while ship steaming and improve the propeller performances with same components to concentrate propeller jet produced at propeller pressure side and improve the flow pattern into the propeller at suction side. Testing The Elogrid performances has been simulated by computational fluid dynamics, designed based on the dimensions optimized and manufactured. Now these first pilots of the Elogrids has been installed to a passenger ferry with two Dp=2.4m tunnels and also tested in full scale. The testing of Elogrids include bollard pull tests to find these impact on thrust forces, vibrations and noise – and comparison of ship fuel economy before and after installation of Elogrids. The fuel economy comparison need as similar circumstances as possible to detect the impact reliably. Main challenge is slightly different conditions each time the ferry operates. A huge amount of ship data is needed to get statistically reliable results. Conclusions The full scale testing of Elogrids show 1.5% reduction of additional resistance, simulated results are between 2 – 2.5%. Compensating the draught differences lead test results closer to simulated values. Bollard pull testing show in average 5% improvement to thrust when Elogrids were installed, simulated values were between 1.6-3.6%. The simulated values were proposing bit higher saving potential, possibly explained with draught differences in test and simulation data, and for pull test they were noticed to be slightly conservative when compared to test results. The vibration levels dropped 12% in average after grids were installed.