Image Credit: Wikipedia
The bulbous bow (bulb) is a very important part of the hull as it can potentially reduce the hull resistance considerably, or affect the resistance in a negative way.
According to ABS ‘Ship Energy Efficiency Measures Advisory’ , the properly designed bulbous bow reduces wave-making resistance by producing its own wave system that is out of phase with the bow wave from the hull, creating a canceling effect and overall reduction in wavemaking resistance. The flow is more horizontal, reducing eddy effects at the forward bilge. Physical factors considered in bulb optimization include volume, vertical extension of the center of volume, longitudinal extension and shape. A bulb with a reverse pearshaped section is primarily effective at the design condition, pear-shaped bulbs work best for drafts below the design draft (i.e. ballast draft or partial load draft) and cylindrical shaped bulbs offer a compromise solution
The characteristics of the bulbous bow must be carefully balanced with the shape of the entrance and the transition towards the forward shoulder and bilge. Bulbs are most effective at certain Froude number (speed-length ratio) and draft. Changes in speed and draft significantly change the wave created, such that reductions in draft or speed can actually lead to increases in wavemaking resistance. As few commercial vessels operate solely at a design draft, compromises in the bulb design are needed to provide good performance over the expected range of operating drafts and speeds.
Maersk Lines reports fuel savings of over 5 percent by modifying the bulbous bow of a shipyard design which was optimized to the design draft, so that it provided more favorable performance over the anticipated operating profile of drafts and ship speeds.
Last month, NYK Group made energy-saving adjustments to a containership’s bulbous bow resulting in a verified 23 percent reduction in carbon dioxide (CO2) emissions over half a year, greater than the amount estimated prior to the tests.
Wilh. Wilhelmsen group posted on its facebook page the following picture to announce that MV Tamesis is ‘getting a nose job’. Their study shows they can save bunker consumption by changing the vessels bulb with a more efficient design.
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