Bulbous bow works well for conditions it is designed for, but can bring added fuel consumption in off-design conditions. Nowhere is this more apparent than in container vessels, which often sail in off design regimes due to fluctuating fuel oil prices and shipping prices. Sailing at lower speed and shallower draft compared to the design condition is common, where the speed difference in particular can be significant. Ships designed for 25 knots will often sail at 13 or 16 knots at various drafts. For this reason, bulbous bow retrofit is an effective means of reducing fuel consumption for container vessels and is commonplace for vessels built prior to 2008.
Finding the optimum bulbous bow for actual operational profile of the vessel can be challenging. CFD, if properly automated, can give an advantage of testing numerous alternatives to the existing bulbous bow. Apart from CFD, a crucial component is geometry parameterisation, enabling a systematic variation in important geometric characteristics of the bulbous bow.
We conducted a small optimisation exercise for the KRISO Container Vessel, where four relevant operational points with corresponding relative frequency, or weights, are considered:
The optimisation process produced a bow shape that reduced the overall fuel consumption by more than 14%, mostly by reducing resistance at lower speeds, but also by not affecting powering at design speed significantly.
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