Power and Propulsion

A choice of three transmission types

Paul Gunton
Paul Gunton

05 December 2018

A choice of three transmission types

Between the engine and the final propulsor, three basic means are available to transmit the power from one to the other(s): direct mechanical drive, drive through a gearbox or electric power from a generator to an electric propulsion motor.

The simplest system is the direct mechanical drive of low-speed diesels to a fixed- pitch propeller and is found in the vast majority of ships. Low-speed engines rotate at a speed that matches the design speed of the propeller and so need no reduction in revolutions, unlike medium- and high-speed engines with mechanical drives.

In a low-speed situation, the propeller is connected to the engine flywheel by the propeller shaft and an intermediate shaft with usually a damper to reduce vibration. Bearings at various points along the propeller shaft help maintain alignment and prevent the shaft whipping. At its aft end, the shaft passes through the stern tube and out of the vessel where the propeller is attached.

Classification rules require propeller shafts to be withdrawn for inspection at regular intervals as deformation can have disastrous consequences. Modern ship design with engines aft also means shafts are shorter and stiffer than used to be the case. This makes correct alignment critical otherwise damage to bearings and fatigue of the shaft can result. The stern tube bearings are vital components in the propulsion system and traditionally have been oil- lubricated. Leakage from stern tube bearings is a pollution issue and to overcome it manufacturers have come up with some ingenious designs including versions lubricated by seawater rather than oil.

The issue of shaft seal leakage is addressed in the ShipInsight Environmental Technology Guide especially as it relates to both MARPOL and the US VGP system. Early in 2013, Lloyd’s Register introduced new rules for water-lubricated propeller shaft bearings. If certain monitoring requirements are met, LR will give the ship owner the SCM (Screwshaft Condition Monitoring) notation allowing no shaft withdrawal for 18 years. This puts the use of water-lubricated propeller shaft bearings for commercial ships in the same envelope as oil-lubricated propeller shaft lines.

Gearboxes for higher speed systems

In any mechanical drive involving a higher- revving engine, a reduction gearbox will be needed to ensure that the final drive rotates at a speed suited to the propeller. In a simple layout, the gearbox will be installed between a single engine and the propeller with the drive otherwise being similar to that of a two-stroke engine.

Where multiple engines are installed, the gearbox arrangement will need to be more complex to deal with inputs from two engines. Clutches will allow the drive to come from one or both engines as required. Incorporating gearboxes introduces vibration into the drive above that emanating from the engines themselves so flexible couplings and dampers are used to counteract this.

Gearboxes are often supplied by the engine maker as part of a package but there are also independent gearbox makers whose products may be specified by customers or the yard. Reintjes, Flender (part of the Siemens group) and ZF are well known names, as is Renk in which MAN Energy Solutions has a majority stake. Vulcan and Geislinger Couplings are probably the most well-known couplings maker but it is not the only player in the market.

While two-stroke engines are invariably linked to conventional propellers, the same is not true of medium- and high-speed engines. Certainly the majority of ships employing faster- revving engines are also propelled by propellers, but some make use of alternatives. For example, pods and thrusters are found in both mechanical and electric drive versions. In a mechanical drive, the shaft from the gearbox will connect to another gear system transferring the drive to the propeller of the pod or thruster. Waterjets are a further option employed in a small number of fast ferries.

Drive by wire: the flexible choice

In a diesel-electric propulsion system the need for direct mechanical connections to propellers and gearboxes is removed and replaced by cable connections allowing for much more flexibility in location of the various components. Since these systems commonly involve multiple engines, it is usual for them to be split over two or more engine rooms that in modern ships are isolated from one and another, conferring redundancy in the case of one engine room being put out of action by some cause. The multiple engine set-up also means that each engine can be smaller in size and therefore the requirement for a very large space to accommodate a single engine is removed allowing designers more freedom.

Electric drive is not a new invention by any means even though the number of ships employing it is increasing at an accelerating pace. Whether the electricity is produced by turbine or a diesel generator is immaterial as regards its distribution to the propulsors.

In a typical system, the electric power is routed through a switchboard, a transformer, a frequency converter and finally to the propulsion motor. The propulsion motor may be a separate component of the transmission or it may be incorporated into the propulsor as is done with pods and some thrusters. Beyond the propulsion motor, the drive may be mechanical and may be taken direct to the propeller or via a gearbox if the motor speed is high. In some cases, the speed of the motor is managed and the final drive may be connected directly on the propulsion motor shaft.

There are power losses associated with each of these steps making diesel-electric slightly less efficient than a mechanical drive when all engines are operating at optimum load. However, the system is more flexible when power demand is lower as one or more engines can be taken off line leaving the remainder to operate at maximum efficiency. For this reason, diesel-electric systems are most common in ships with widely varying power demand, such as offshore vessels and cruise ships.