VESSEL REVIEW | Mobula 8 – Compact oil and debris collecting boat for French NGO

The SeaCleaners, an NGO based on France's Brittany coast, recently welcomed a new vessel to its fleet of oil and debris collecting boats. Designed and built by the Efinor Sea Cleaner division of local shipyard Efinor, Mobula 8 was developed to possess the attributes of compact size, high cleaning capacity, and ease of operation and maintenance.

The craft was designed to help mitigate floating waste pollution in ports, marinas, rivers, lakes, and deltas. It is a relatively small vessel of only eight metres in length but with a payload of over 2.4 tonnes. The vessel is also equipped with a mobile motor pump of 50m³/h capacity, allowing flushing of river beds, rocks, and quays affected by oil spills. The pump may also be used for limited firefighting operations.

The vessel itself may also be used as a cargo transport and harbour support tender.

"The owner required that the vessel be small enough to fit in a 40-foot container," Efinor told Baird Maritime. "At the same time, it needed to be capable of hauling a significant amount of waste on each trip."

Other requirements included a forward-mounted console, aft platforms for facilitating maintenance of the propeller, a single lifting point, and a sorting table to enable the removal of organic waste by hand. Mounted forward are two deployable arms that will create a funnel, thus widening the vessel's cleaning coverage.

Like other pollution response boats designed by Efinor, Mobula 8 is equipped for collecting floating waste and cleaning up oil spills. However, the newbuild has an innovative feature not found on earlier craft developed by the same yard.

"On either side of the hull is a 6.5-metre-long float with a foldable platform. This feature maximises the available surface space on the deck, which will then allow the vessel to carry as much collected waste as possible without compromising stability."

The ability of the craft to extend its beam will enable it to operate in "sea mode" in deeper waters farther off the coast. Also, since the float-mounted platforms are foldable, it is possible to limit the beam to less than 2.3 metres for use in shallower waters and to allow for transport within a 40-foot container. Switching between the two configurations can be done while the craft is afloat, and without the need for divers to be deployed into the water to install the necessary components onto the hull.

Efinor said it decided upon this solution after considering a number of possible options, including the use of aluminium floats and inflatable hypalon floats. These other options presented some disadvantages such as increased weight, a greater number of moving parts (which in turn would generate increased underwater noise and vibrations), and higher risk of damage caused by sharp debris.

Achieving the desired level of operational flexibility presented some challenges for the design team.

"The first part of the project was to determine the optimal solution in order to collect both macro and micro waste," Efinor told Baird Maritime. "In order to do so, two solutions were tested on a smaller ship for the macro waste and three for the micro waste. After this series of tests, it was decided to keep the basket solution and implement the net for the micro waste.

"However, the results for the second solution considered to collect the macro waste – a conveyor belt – were unconvincing. This solution should be tested on a ship of equivalent size to the one it will be implemented on. Therefore, Mobula 8 was designed with this in mind. It can receive a conveyor belt instead of the basket solution."

Efinor added that a sorting table was added at the rear. This brought a series of changes to the basket that needed to drop the waste above the table without the "arms" of the system interfering with the table and equipment.

"The software used was a new version that needed to be implemented according to the company's needs. This project was used as a base for the software and a reference for future newbuildings."

The shape of the hull was also seen as difficult to adjust as stability was a key element that needed to be balanced by the consideration of the water flow within the structure. In addition, because of the ability to operate in two modes, cruising and cleaning, the hydrodynamics of the vessel are very particular.

Even with the incorporation of a range of features, the deck still needed to possess as much free space as possible to maximise payload and to enable the crew to move around easily.

"The 'front door' was problematic to assemble on the ship. This system requires precision, but welding the door together will cause it to deform slightly due to the heat. Also, the tubes going through the transverse and longitudinal watertight bulkheads for electric and hydraulic cables were difficult to put in place. They are, however, a necessary since it simplified the work for the electricians and protected the cables from moving elements."

Efinor added that the lifting point also proved challenging to incorporate since it was a requirement that came from the owners after the hull was already built. In order to put it in place, some parts of the hull needed to be removed.

The hydrocarbon tank also has some limitations, necessitating its eventual replacement. Efinor said the next iteration will have a bigger tank with more access. The same will apply to the diameters of the rails for the inflatable floats, which will later be increased to 40 milllimetres from their current 30 millimetres.

Power is provided by a single Suzuki outboard mounted on a specially developed support. This will allow the propeller to be moved from underneath the hull and up into a tunnel to create the suction flow for collecting waste. This configuration allows navigation and waste collection to be carried out using only one engine.

The base of the outboard is also fitted with a nozzle and a bucket. The nozzle guides the flow from the tunnel and the bucket makes it possible to reverse the flow forward. This makes it possible for the vessel to sail in reverse.

Mobula 8 is designed to operate from Manta, The SeaCleaners' larger, ocean-going waste collecting vessel. The smaller craft will be used in shallower waters that Manta cannot easily access.

See more stories from this month's Pollution Control Week here.