5. Hull modifications

One of the four fishing catamarans in Sète, France, that are used for longline fishing or small purse seine (lamparo), was equipped (in Summer 2021) with a retractable T-foil. This technology allows better stability of the vessel, reduced fuel consumption and a gain in speed to move to the fishing areas. Fuel consumption was reduced by up to 25% (depending on weather conditions).

A bulbous bow, which is a protruding bulb at the bow of a ship just below the waterline, is not only an effective solution to reduce the total resistance or required power of the ships, but also enhances safety and economic-techno efficiency for seagoing ships, such as increasing speed, decreasing fuel consumption, improving stability and seakeeping.

Typically used on larger ocean-going vessels, there is nevertheless analysis for the design of a bulbous bow on fishing vessels⁴. There are several examples of newly-built fishing vessels fitted with a bulbous bow⁵, as well as retrofitted ones⁶, with two examples illustrated on this page.

The inverted bow is essentially the reverse of a conventional bow, with the top of the bow behind the bottom. They maximise the length of waterline and hence the hull speed and often have better hydrodynamic drag than ordinary bows. The shipbuilding company, Ulstein, has developed this concept further with its X-BOW®. Through their design, instead of rising on the waves and then dropping, a vessel with an X-BOW® is able to distribute the force more evenly across its surface – enabling the ship to remain more stable during poor weather conditions. Since it uses less fuel to get through the waves, it also helps to save energy.

An example of a fishing vessel being constructed with an inverted bow is the Nuevo Argos. The Nodosa shipyard, located in the port of Marín, Spain, is constructing this freezer trawler for operations in the South Atlantic. The trawler will have a length of 85m and will be used for the fishing and processing of Patagonian squid. The design of the ship has prioritised improving the safety and quality of life of the crew on board, respect for the environment and energy efficiency. It will also incorporate various elements on board to mitigate incidental mortality of birds and other marine species and the ship’s refrigeration plant will use ammonia as the main refrigerant, eliminating the use of fluorinated gases, thereby reducing the environmental impact. The vessel is expected to be completed by the end of 2024, with then its first fishing campaign early the following year.

⁷The Horizon 2020 project Air Induced friction Reducing ship COATing (AirCoat)⁸ aimed to develop a disruptive hull coating that reduces the frictional resistance of ships. By creating a permanent air layer, ship drag is reduced and it also acts as a physical barrier between water and the hull surface. In this way, in addition to reducing fuel consumption and thus ship emissions, the air barrier reduces the attachment of marine organisms (biofouling); the release of biocides from traditional coatings into the water; and mitigates ship noise pollution. Initial laboratory experiments with small samples indicated drag reduction in the range of 10-30%.

The company Silverstream Technologies⁹ has installed their air lubrication system on the vessel of Norwegian Cruise Line¹⁰. It is the first commercial installation for improving operational and environmental efficiencies as a means of reducing emissions, fuel costs, and improving the sustainability of their operations.

The Silverstream System produces a thin layer of microbubbles that creates a single ‘air carpet’ along the hull of the vessel. This reduces the frictional resistance between the water and hull and improves the vessel’s operational efficiency, reducing fuel consumption and associated emissions. The technology can be added to new build design or retrofitted to an existing ship within just 14 days. With the right ship hull design, the air lubrication system is expected to achieve up to 10-15% reduction of CO2 emissions, along with significant savings of fuel.

A number of these are currently on the market or under development¹¹. For example, the company Wärtsilä has developed a Controllable Pitch propeller system which allows operating at optimum pitch settings for various operating conditions, when both trawling and free-sailing performance are important. Key benefits are high propeller efficiency, reduced fuel consumption, minimum noise and vibration levels and reliability.

Such propellors were also used by the Horizon 2020 LeanShips project, which aimed to demonstrate the effectiveness and reliability of these energy-saving and emission-reduction technologies at full scale. The project developed an energy-saving device that is suitable for use by ships with controllable pitch propellers which helped to achieve a 3.5% fuel saving for ships during sea trials.

Similarly, the shipping company Hapag-Lloyd is making its existing fleet more efficient, including by retrofitting new propellors. They state that this results in the ship saving between 10 - 13% fuel and CO2 emissions, depending on the sailing condition.

Elsewhere, and under the Horizon 2020 project GATERS¹², a Gate Rudder System was developed as an energy saving concept with excellent manoeuvrability. The project will explore retrofitting and its application to different kinds of vessels.

One example with a retrofitted high-efficiency propeller, along with upgrades to the engine and reduction gear, is the Anna PZ-657. A 4 200 mm diameter high-efficiency propeller was fitted which, with the other modifications, gave a 25-28% saving in fuel consumption translating to a saving of around 125-130 litres per towing hour.

The Horizon 2020 SeaTech project aims to develop a renewable-energy-based propulsion innovation consisting of a bio-mimetic dynamic wing mounted at the ship bow to augment ship propulsion in moderate and higher sea states, capturing wave energy, producing extra thrust and damping ship motions. This would be combined with power generation based on the idea of achieving ultra-high energy conversion efficiency by precisely controlling the engine for achieving radically reduced emissions. It is expected to offer shipowners a return-on-investment of 400% due to fuel and operational cost savings. The project estimates CO2 savings of 32.5 million tonnes annually if just 10% all EU short-sea vessels were retrofitted with this device.

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