With the notable exception of a small number of twin-propeller tankers and container vessels, most two-stroke engines are installed as the sole prime mover on the vessel. By contrast, on ships with highly variable power demands such as a cruise ship or offshore vessel, it would be quite common to find four, six or occasionally more four-stroke engines installed.
Because all engines have an optimum load at which they operate most efficiently, operating below this will increase fuel consumption. In such cases, the power required will be provided by an appropriate number of engines operating at near to optimum speed with perhaps another engine operating at low load as a spinning reserve. A multiple engine arrangement also means that failure of a single engine will rarely have disastrous consequences.
The power arrangements on multiple-engined vessels will normally mean that engines of different outputs are available. This can be achieved by having engines of the same type but with different cylinder numbers or larger bore engines supplemented by smaller bore types.
The modular design of engines and common parts across a range mean that ships can benefit from carrying lower numbers of spares. Even when vessels are mechanically driven, often an owner will choose to have the same basic engine for propulsion and smaller version of the same type as a genset for electrical power.
The electric option
Unlike the mechanical power that is delivered directly to a propeller or through a gearbox, electric power produced by four-stroke gensets needs to be managed to allow for safe and efficient distribution to all the consuming systems. Opting for a diesel-electric propulsion system does mean that electric motors must be used to power any propulsors.
This has advantages and disadvantages. Unlike the mechanical drive systems, the cables from the electric distribution system to the electric motors can be routed in any direction without any problems and with far less space needed than for mechanical drives. This can allow engines and motors to be isolated from each other and permit power to be generated from any genset on the ship. The most obvious disadvantage is that electric motors are not cheap and they are an additional point of potential failure. Recent developments in electric distribution have focussed on the benefits of direct current (DC) rather than alternating current (AC) systems
Another major advantage of diesel-electric is the potential for integration of energy storage sources such as batteries. The energy storage sources reduce the transient loads on the Diesel engines and give much better system dynamic response times. Also, emission-free propulsion can be realised when running on batteries especially when they are topped up using power from solar, recovered from waste heat onboard or even when charged in port using a local grid. The footprint of such a propulsion plant is up to 30% less compared with a classic diesel-electric propulsion plant.