In a ship diesel-electric power and propulsion system wit shaft propeller, the propellers are normally driven by variable-speed electric motors. The horizontal ship motors may be directly connected to the shaft, which results in a simple and mechanically robust solution, or via a gear coupling, which allows for increased rotational speed of the motor and results in a much more compact motor. The disadvantage is increased mechanical complexity and increased mechanical power losses.
In diesel electric vessels, shaft lines are used in applications typically where the propulsion power is higher than available for azimuthing thrusters, or where the ability to produce transverse thrust, e.g. in station keeping and maneuvering is not needed – or can be produced cheaper by tunnel thrusters. Typically this applies for shuttle tankers, research vessels, larger anchor handler vessels, cable layers, etc.
The ship shaft line propulsion will always be combined with rudders, one rudder per ship propeller. By use of high-lift rudders, ship shaft propellers may also be used to provide a certain degree of transverse thrust. If additional transversal thrust is needed for ship maneuvering or station keeping, there will normally be required to install additional tunnel thrusters also in the aft of the vessel.
The ship propeller is normally speed controlled FPP (Fixed Pitch Propeller) type, which gives a simple and robust propeller design. In some applications, the ship propeller may be CPP (Controllable Pitch Propeller) type, even if it is speed controlled. To a certain degree, speed and pitch can be optimized for higher efficiency, and faster response than with only one control parameter. These benefits normally do not justify the additional investments in order to obtain combined speed and pitch control.
Shaft line ship drive configurations
Some typical drive configurations for ship shaft line propulsion system. These can be installed in single shaft propeller designs, or dual shaft designs.