Hull retrofitting

Current operating profiles (speed-draught matrix) for many vessels deviate significantly from the profile or design point that determined the initial design of the vessel. Accordingly, the vessel’s hull profile is not optimized for current operations. For existing vessels, where the degrees of freedom in hull form optimization are limited compared to a newbuilding project, retrofitting of the bulbous bow can bring considerable fuel savings.

Applicability and assumptions

Bulbous bow modification, source: DNV GL

Applicable for tank, LNG carrier, bulk and container vessels operating a large part of the sailing time in off-design conditions, i.e. at other conditions than design draught and contract speed for which the existing bulbous bow is optimized.

It is also possible to optimize the position of the bilge keel and the shape of any thruster tunnels. The bilge keel positioning improves the resistance of the hull by optimizing the positions of the bilge keels. The largest savings potential is for hulls up to about 50,000 DWT and there is no penalty in any loading condition. Bow thruster tunnels can add significantly to the total resistance of the ship. Optimizing the thruster tunnels is relatively inexpensive to implement and may have a significant impact. If the thruster tunnel is redesigned (introducing a scallop) it is also recommended to perform a grid alignment.

Some vessels have fender bars that in some conditions create significant increased resistance. These may be changed for improved performance.

Cost of implementation

For bulbous bow an assumption of $100,000 (USD) fixed cost for the new bulbous bow with optimization, engineering and approval (economies of scale for sister vessels would reduce the fixed cost per vessel). Additional material cost of $250,000 to $700,000 (USD) depending on size. The other retrofit costs are as follows:

• Thruster tunnel optimizing: $10,000 (USD) + detail design and docking cost
• Bilge keel optimizing: $10,000 (USD) + detail design and docking cost

Reduction potential

Reduction potentials are dependent on vessel size, vessel segment and design speed. The following reduction potentials on main engine fuel consumption are expected:

• New bulbous bow: 3% to 5%. Depending on the difference between design speed and average speed according to current operating profile the fuel saving will vary from 3% with >80% of design speed and 5% with <75% of design speed
• Thruster tunnel optimizing: 0.5% to 1%
• Bilge keel optimizing: 0.25% to 1%

Other References

1. DNV GL ECO Retrofit service / How replacement of bulbous bow can increase a vessel’s energy efficiency
2. Force Technology Retrofit – a new bulbous bow / Retrofitting possibilities /