If an owner wants to design a new ship or modify an existing ship there are 2 different options to lower the fuel consumption: modify existing design or develop a new design. Thes two options involve optimization of the shape of the ship. This can be the modification of to the forebody, stern shape design and optimization of the vessel hull. Optimizing one or more of these options will lead to lower fuel consumption. It is important that these modifications happen with the operation conditions of the ship in mind.
While main particulars are generally well optimized across shipyards, there is significant variance in the degree of hull form and propeller optimization. Shipyards tend to optimize around the specified design draft and speed, while giving less attention to operational efficiency at the ballast draft, and little or no attention is paid to partial load conditions.
The reduction potential is dependent on vessel size, segment, operation profile and trading areas. A reduction of 4% to 8% on main engine fuel consumption is likely.
Different optimization options
To minimze the hull resistance,while improving the overall hull performance and the hull propeller interaction, there are three options:
- Fore body optimisation
- Aft body optimisation
- Appendage Resistance
Fore Body optimisation
Fore body optimisation includes development in the design of the forward region of the ship which includes consideration of the bulb design, forward shoulder, and waterline entrance.
The bulbous bow is designed in such a way that it reduces the wave making resistance. The bulbous bow produces a wave that is not in phase with the wavesytem acting on the ship. both waves act on the ship and cancel eachother out. This changes the pressure distribution along the hull, thereby reducing wave resistance.
A V-shape may be introduced at the base of the bulb to mitigate slamming impact loads. Fuller ships such as tankers and bulk carriers are often arranged with bulbs having a large section area and V-shaped entrance, such that they behave as a traditional bulb at loaded draft and acts to extend the waterline length at ballast draft.
Aft Body Optimisation
The biggest concerns while designing the aft part of the ship is to mitigate the stern waves and improve the flow into the propeller. By improving the flow around the stern of the ship the hull resistance can be reduced. Flow improving devices such as stern flaps can be attached to do the same. The other important thing to be considered while designing the stern is the type of stern whether a transom or a cruiser or an elliptical etc. Each of them has its own set of pros and cons therefore, only after a proper CFD analysis or model experiments the appropriate stern has to be chosen.
Appendage resistance contributes to about 2 – 3 percent of the total resistance for a cargo ship in calm water condition. Roughly about half the appendage resistance is attributed by the bilge keels and the other half to the rudder.
Resistance due to rudder is experienced usually on directionally unstable ships and can be controlled using skeg. The bow thruster tunnel can also contribute significantly to the overall resistance of the ship, roughly in the range of 1 – 2 percent. Grid bars are frequently placed over the opening perpendicular to the flow direction. They serve to break up laminar flow and reduce vortices. Sometimes anti suction tunnels are used to reduce the pressure variation across the bow thruster tunnel.
Air lubrication is a technique where the energy consumption of vessel is reduced by reducing the drag resistance. This reduction is achieved by bubbles or a sheet of air between the water and the vessel.
This method works best for vessels with large flat bottom hulls, were friction resistance can be reduced for a large area. Because this method aims to reduce drag it is most efficient in situations where drag is the major resistance component.
The two main types of air lubrication on the market rigth now are described below.
The first type is the air cushion. This type creates a sheet underneath the vessel preventing a large area to come in contact with water while moving through water. Air cushions are created within cavities in the bottoms of the vessels hull preventing air from escaping. The design of these cavities varies between the suppliers and prototypes. For this type of air lubrication a flat bottom is important to be able to create sufficient area where the cavities can be positioned.
The second type works with micro bubbles or film layers. The most important difference is the fact that it does not need to have cavities in the hull. By air outlets in the bottom a layer of bubbles is created that will move along the hull because of the vessels speed. Another option is creating a film layer along the ship’ s hull. A fill follows the vessels hull form better than bubbles that do mainly follow the water.
For the installation of an air lubrication system a ship will have to go into dry‐dock for approximately 14 days.
The savings that the manufactures expect are significant. The companies providing the data that is presented have commercial interests with respect to the data. The reported savings differ significantly between manufactures. Some independent research institutes like MARIN reported that in towing tank test there where no measurable improvements on the fuel consumption. Below some examples are given for systems that are on the market at the moment.
- The Airmax system provided by Stena Bulk (20-25%)
- Air Cavity System designed by the DK Group savings up to 15% are claimed for tankers and bulkers.
- The Mitsubishi Air Lubrication System (MALS) have reported over 5% and 7% on installations of the system on ships.
- Damen shipyards developed a system for inland ships. Damen claims that the system has showed savings between 5 and 40%, dependent on vessel speed.
The weight of a ship has a big impact on the required power of a ship. This is especially the case for faster vessels like ferries. On the other side larger slow vessels like large cargo carriers can also have an advantage to reduce the structural weight. If the structural weight of a large cargo carrier is lower there is more deadweight available while not increasing the size of the vessel. This improves the transport efficiency and the EEDI of the ship. This does not directly improve the fuel consumption and absolute emissions of the ship.
Weight Savings from the Use of HTS
If High tensile steel is used on tankers substantial weight savings are realized up through 60 percent HTS. Above 60 percent HTS, the benefits of using HTS diminish. For the remaining mild steel plate, strength is no longer the dominant factor governing scantlings with buckling and corrosion margins mitigating the benefits of HTS application.
– Application of Simulation Technology to Mitsubishi Air Lubrication System (CHIHARU KAWAKITA 2015)