Despite the wide design variety of vessels afloat on the world’s seas and waterways, most of the marine industry wants the same things from its power and drivetrain systems: efficiency and reliability. Assuring these outcomes, however, takes careful consideration at the purchase decision phase when it comes to system-critical items such as motors and drives.
Take fishing vessels, for example. Due to the highly competitive nature of the industry and strict environmental regulations, modern fishing boats must be highly efficient, both in terms of fuel consumption and emissions (CO2/NOx). Opting for a diesel-electric propulsion system with variable-speed generator and DC bus, for instance, presents the opportunity to run the generator at a very high efficiency, at speeds that typically range from 800 to 1,200 rpm.
This system also means that fishing boats can be equipped with two generators (one running), instead of three in the case of an AC system (where two would normally be in operation). A larger generator (say 500kW) could be used during transit and fishing, while a smaller one (circa 100kW) could be retained mainly as an emergency generator, capable of bringing the boat back to shore at a lower speed. Such would be the efficiency of this system configuration that the fishing crew would be able to make money even when the price of fuel is high.
Such a vessel could leverage the benefits of a water-cooled permanent magnet motor for the main propeller in place of a conventional diesel engine. Along with higher efficiency, further advantages would include its small size, eliminating the need for an in-line gearbox with a transmission. Moreover, the fact that impact loads could be absorbed electrically by the engine controller makes the generator more stable and energy-efficient, with less engine noise, less vibration and virtually no need for maintenance.
Modular, three-phase variable speed drives are the ideal partner to work in tandem with these propulsion engines. Similar units could also drive the rotary transformer, hydraulic pump, flushing pump, engine room ventilation system, fish hold cooling system, and pulse fishing windlasses. Windlasses have to pay an electric cable in and out with a stable tensile force (without breaking), but with a variable swell.
Away from the fishing industry, other marine vessels able to benefit from modern motor and drive technologies include motor yachts and pleasure craft. For instance, using the latest modular drives as part of a specially designed power management system could deliver reductions of up to 20% in fuel consumption and, subsequently, greater yacht range between refuelling.
Again, diesel-electric propulsion is a growing trend among a number of today’s manufacturers of superyachts. Here, each engine can be connected to an alternator to produce sufficient power to drive the thrusters. Furthermore, generated power can be stored in battery banks, allowing the yacht to cruise completely under electric power.
Variable speed generators can produce electric power for the yacht, with diesel engine speed controlled by a propulsion management system featuring modular drives. The drives ensure that the engines run at best efficiency in any cruising conditions to optimise overall fuel consumption and maximise comfort for those sailing on the vessel. A DC variable voltage switchboard could be employed to distribute the generated electrical power. Here, power levels would be adapted instantly to the requirements of the system with the support of a battery pack.
This type of concept compares extremely favourably with traditional yachts, which run generators at a constant speed and produce a fixed output. This is often too much or too little for the yacht’s needs and, as a result, the generators are in frequent need of services and overhauls. The same is true of a yacht’s engines – these run at varying speeds according to requirements, consuming vast amounts of fuel and demanding regular attention and service.
A diesel-electric system is different. By connecting engines to alternators they can be employed as variable speed generators, producing only as much power as necessary at any given time; no more, no less. This results in greater efficiency from the engines – fuel savings of up to 20% (depending on usage) are estimated compared with the same boat using traditional twin-screw propulsion.
Ultimately it is the drives that help make this possible, with power from the generators or the battery banks converted into what is required at the time. This always assures the best performance in terms of consumption and efficiency, taking into account the overall demand for power on board.
Another important and beneficial feature of this design is that the drives convert power from AC at the generators, to DC at the battery banks, and back to AC again when it powers the thrusters or appliances on board. This guarantees the greatest possible efficiency for the propulsion system, but also acts as a voltage stabiliser, thus eliminating harmful voltage spikes.