ROI on bulbous bow retrofit

Foreship recently quantified the gains available when a cruise ship is retrofitted with a bulbous bow based on latest designs and modern CFD hydrodynamics simulations. Rather than having to cut speed, an owner can secure fuel savings of up to more than 10% by reducing hull resistance.

At a time of intense focus on reducing carbon emissions from ships, Foreship recently established the gains available from retrofitting new bulbous bow sections on board two existing cruise vessels. Carried out on behalf of the same owner, two feasibility studies included a full risk assessment and weighed up the requirements and potential costs of the retrofit.    

The first considered replacing a surface piercing top and goose neck bow shape that is typical of ships delivered in the early 2000s. The second case considered modifying a ship delivered around the same time, but whose bow featured a slightly older ‘blunt’ design, in a retrofit project which also envisages a ducktail added behind the stern.

In both instances, analysis showed that retrofitting new bulbous bows would have negligible consequences for vessel stability.

In the case involving the bulbous bow replacement only, CFD analysis showed that the surface piercing top and goose neck shape is efficient at the design point (design speed, draught and trim), but that performance is highly sensitive to variations in speed and draught.

“CFD simulations available in 2023 allow bulbous bow designs to be optimized for the typical service speed range and loading conditions seen in reality,” says Janne Niittymäki, Head of Hydrodynamics, Foreship. “Overall, better hydrodynamic performance creates the potential for 9-2% in fuel savings in the speed range of 14-22kn and 13% at 12kn, based on the model testing of this specific case.”

The second study compared overall performance gains available from replacing the blunt goose neck shaped bow section with an optimised bulbous bow, together with adding one of three ducktail options behind the stern. Here, in addition to CFD, model testing was needed to refine the fuel saving estimate and optimize the trim wedge/interceptor configuration of the ducktail.

In this case, Niittymäki says that similar fuel savings can be expected and will be seen after the model testing.

Hydrodynamics performers

With the International Maritime Organization’s Carbon Intensity Indicator (CII) now part of owner considerations, as well as straightforward fuel economy, the Foreship Head of Hydrodynamics says the owner is looking favourably on taking up the retrofit option in both cases.

Niittymäki says that both studies indicate that the new bulbous bows would call for a slight stretching forward of the backmost part of the stem line, with the forward end of the bulbous bow also moderately extended.

“However, retrofitting an optimised bulbous bow has negligible consequences for stability: compliance with SOLAS regulations is also assured because modifications would be limited to an area ahead of the bow thruster and collision bulkhead.

“Ultimately, measuring return on investment comes down to comparing the potential fuel and emissions savings against the costs (materials, installation, time penalty, etc.) on a ship-by-ship basis,” he adds. “That calls for CFD modelling of the characteristics of an existing ship and of the optimised forms that will achieve the most significant performance gains.”