The invention relates to marine vessel propulsion system according to the preamble of claim 1. The invention relates also to method of operating a marine vessel propulsion system according to the preamble of claim 8.
The general aims, just to mention a few, of designing propulsion system to a marine vessel are good efficiency, reliability, durability and good low speed manoeuvring capabilities. Emphasis of the features is depending on an application in question.
For example, in so called ROPAX (Roll On/Roll Off Passenger) vessels there is a need to provide a propulsion system, which is simultaneously efficient at cruise speeds, say over 25 knots, and which also provides for good harbour manoeuvring capabilities.
It has been suggested to use azimuthing propulsion drives, such as so called pod drives, instead of conventional mechanical shaft propulsion. A reference is made to a publication EP 590867, in which a propulsion arrangement is shown, which consists of a turnable drive unit, inside which there is an electrical motor acting as the ship's propulsion motor and being connected to a propeller at the end of the drive unit. The pod drives are favourable in a sense that they are flexible having azimuthing control possibility, and they also provide freedom in location of main engines. However, the pod drives are of high cost and the electric power transmission cause considerable transmission losses.
Mechanical propeller drive as such has clear advantages being of low cost, simple and having substantially low power transmission losses. However, mechanical propeller has disadvantages as well. Propulsion efficiency is not as good as desired and particularly at low speed conditions there is a risk for pressure side cavitation when controllable pitch propeller is used. This occurrence is caused when driving the propeller at low pitch, high rotational speed conditions resulting in decreasing of local pressure at the surface of the propeller blade. Also manoeuvrability is substantially poor.
A combination of pod drives and mechanical propeller drive in general level as such has been suggested in a paper "Hydrodynamics of fast ropax vessel" by Raimo Hämäläinen. The suggested combination has been considered to have several benefits. However, it has been found that the solution does not result in optimum solution as such, particularly at low speed / manoeuvring operation. Although the proposal in general seems to be promising, there are still practical issues to be solved.
In Japanese patent publication JP 914 2391 there is shown a marine vessel propulsion system comprising steerable azimuthing propellers arranged on both sides of main propeller at stern section of the vessel. This kind of system has a negative effect on the propulsion efficiency particularly in low speed / manoeuvring operation.
It is an object of the invention to provide marine vessel propulsion system and method of operating a marine vessel propulsion system in which the shortcomings of the prior art have been minimised. Particularly it is an object of the invention to provide a marine vessel hybrid propulsion system and method of operating a marine vessel propulsion system, which result in good overall and particularly low speed / manoeuvring operation efficiency and operation, and also reduced propulsion vibration and noise level.
Objects of the invention are met substantially as is disclosed in the claim 1, 8 and as is more clearly disclosed in the other claims.
The objects of the invention are met by marine vessel propulsion system, which comprises engine system for producing propulsion power and several propulsion means including at least two propulsion arrangements capable of steering action and at least one shaft driven propeller drive, at its stern section, The invention is characterised by a feature that the at least one shaft driven propeller drive is provided with a controllable pitch propeller being adjustable to substantially feathering conditions. According to the invention the at least one shaft driven propeller drive is provided with a controllable pitch propeller being adjustable to substantially feathering conditions.
According to a preferred embodiment of the invention the at least two propulsion arrangements capable of steering are azimuthing propulsion arrangements, which may be provided with either variable or constant pitch propeller systems. The azimuthing propulsion arrangements are preferably powered by electric motor, the power to which being supplied by piston engine generator set. The shaft driven propeller drive is mechanically connected to a piston engine by means of a gear system or alike. In some cases also water jet propulsion systems may be used as propulsion arrangements capable of steering.
The propulsion system according to invention is arranged to be driven differently at different operation modes, namely specifically at normal/cruise speed operation mode and at low speed / harbour manoeuvring operation mode. At normal/cruise speed operation mode both the at least two azimuthing propulsion arrangements and the at least one shaft driven propeller drive are adapted to at least cause thrust in order to move the vessel. At low speed /harbour manoeuvring operation mode only the at least two azimuthing propulsion arrangements are adapted to cause thrust in order to move the vessel. This way the manoeuvring behaviour is more advantageous. The steering of the vessel is accomplished always by means of the at least two propulsion arrangements capable of steering action i.e. the azimuthing propulsion arrangements.
The pitch angle of blades in the propeller of shaft driven propeller drive is adjustable to be at angle ± 80° - 100° in respect of the normal of the shaft of the shaft driven propeller drive. This means in practise that the blades of the propeller are rotated to either direction until they are substantially parallel to longitudinal axis of the vessel. The direction of rotation may be selected according to the shape of the blades so that the flow resistance is minimised. Preferably the engine system comprises a gear and a coupling means connecting the propeller drive shaft to an engine and being shiftable to disengaged position. This result in a benefit of stopping the transmission of power to the drive shaft before and while the propeller is at feathering conditions. Providing the propeller drive with coupling means also results in a possibility of keeping the engine running while the power transmission is disengaged. However, it is advantageous to stop the engine e.g. in order to reduce unnecessary emissions.
According to the method of operating a marine vessel propulsion system comprising at its stern section at least two propulsion arrangements capable of steering action and at least one shaft driven propeller drive, at normal/cruise speed the propulsion thrust is provided by the shaft driven propeller drive and the propulsion arrangements capable of steering action. Steering thrust is provided by the propulsion arrangements capable of steering action. The propulsion system further comprising at least one shaft driven propeller drive having a controllable pitch propeller, and at normal/cruise speeds the thrust of the shaft driven propeller drive is mainly adjusted by adjusting pitch angle of propeller. Further at low speed and/or harbour manoeuvring operation the shaft driven propeller drive is adjusted to be feathering and that power transmission to the propeller is stopped. This is accomplished by adjusting the pitch angle of the shaft driven propeller.
While the power supply to the propeller drive is stopped from the engine system, the at least two propulsion arrangements capable of steering action are maintained in operation. Power transmission to the shaft driven propeller drive is stopped by disengaging the engine from the shaft of the shaft driven propeller drive, or in case of single engine without a gear system, by stopping the engine before adjusting the propeller to be feathering.
The pitch angle of propeller of the shaft driven propeller drive is adjusted to be advantageously either + 80-100° or - 80-100°. The at least two propulsion arrangements capable of steering action are preferably powered by electric motors, the power to which being supplied by one or more piston engine generator sets. During low speed and/or harbour manoeuvring operation piston engines of the piston engine generator sets are operated at greater speed than idle speed, substantially at their most efficient constant speed, and preferably substantially independently from the power consumption of said propulsion arrangements. This manner the engines may run at optimum circumstances regardless of the power demand and prevailing speed of the vessel.
The invention provides several advantages over prior art. First of all the propulsion system causes lower investment costs compared to conventional diesel-electric propulsion system. It also provides excellent manoeuvring characteristics as well as excellent propulsion efficiency for high speed ships. With the present invention, at the low speed / manoeuvring operation the engines may operate at optimum conditions and there is no risk of cavitation of the shaft driven propeller drive. Also one of the advantage of the present invention is that the number of diesel generator sets connected to the network can be varied according to the load demand i.e. the engines may be started and stopped. This way the load of the engine may be kept doser to the optimum.
In the following the invention will be described by the way of example, with the reference to the appending drawings in which
In the figures with the reference number 1 it is referred to a hull of the vessel, only the stern section of which is shown. The vessel is provided with engine system 2 having a number of piston engine generator sets 2.1 and number of piston engines 2.2 mechanically connected to a shaft driven mechanical propeller drive 3. The shaft driven propeller drive 3 comprises a gear and a clutch 4 through which the engines 2.2 are connected to a drive shaft 5 of the propeller drive 3. At the outer end of the drive shaft 5 there is a propeller 6 connected thereto. The shaft driven propeller system drive 3 comprises a controllable pitch propeller 6 and an arrangement 7 for adjusting its pitch angle A, which is shown herein very schematically. The arrangement 7 for adjusting the pitch angle may be hydraulic or mechanically operated commercially available system known as such. However, the pitch angle adjusting arrangement 7 is such that the pitch of the propeller is adjustable to substantially feathering conditions. In practise this means that the mechanism for turning the propeller blades allows the blades to turn over to greater angle than in normal pitch adjusting. This will be described later in more detailed manner with the reference to figure 2.
The piston engine generator sets 2.1 produce electric power and supply it to a network 8, through which power may be transmitted to the propulsion arrangements capable of steering action, such as pod drive units 9. The pod drive unit 9 is turnable about its vertical axis as depicted by the arrows in the figures. This feature is utilised in the present invention so that the pod units 9 operate as steering devices and the shaft driven propeller drive 3 is without any rudder system.
The shaft driven propeller drive 3 is positioned at centre line of the hull. In case there would be several shaft driven propeller drives it is desired to position them symmetrically in respect of the centre line of the hull 1. In the figures the two propulsion arrangements capable of steering action i.e. the pod drives 9 are positioned at both sides of the shaft driven propeller drive 3 and substantially same longitudinal position as the propeller 6 of the shaft driven propeller drive 3.
At normal cruise speed conditions the shaft driven propeller drive 3 and the pod drives 9 are used for producing thrust force for moving the vessel. The pod drives 9 are provided with variable speed propeller systems and the thrust force of the pod drives 9 is adjusted by controlling the rotational speed of their propellers. The thrust force may also be controlled by controlling propeller pitch angle, in the case they are provided with controllable pitch propellers. The thrust force of the shaft driven propeller drive 3 is mainly adjusted by adjusting propeller pitch angle. Naturally, it is possible to adjust rotational speed by adjusting the engine speed.
In such operation in which better manoeuvrability is desired and the speed is lower, only the pod drives 9 are used for propulsion. At this mode of operation the pitch angle of the shaft driven propeller drive is adjusted such that its flow resistance is substantially decreased. This situation is shown in figure 2. This way the overall efficiency may be increased. Preferably the pitch angle (A) of the shaft driven propeller drive 3 is adjusted such that the propeller is feathering. By the term feathering or feathering conditions it is meant here that the angle (A) of the blades is positioned so that the chords, straight line between the leading and the trailing edge of a blade, become approximately parallel to the water streamline or longitudinal axis of the vessel. The blade angle (A) means an angle between normal of the drive shaft axis and a mean direction of straight line joining the leading and trailing edges of a blade.
When changing over to the low speed / harbour manoeuvring operation first the power supply to the shaft driven propeller drive 3 is stopped, which may be accomplished by shifting the clutch 4 to disengaged position. After stopping the power transmission, the propeller of the drive 3 is adjusted to feathering conditions. In this manner it is possible to obtain advantageous properties to the propulsion system for low speed / manoeuvring operation. The pod drives 9 are maintained in operation in such a manner that the piston engines of the piston engine generator sets 2.1 are operated at greater speed than idle speed, substantially at their most efficient constant speed substantially independently from the power consumption of said propulsion arrangements 9. This means that the total power production may be greater than actual demand of the pod drives 9. It is also advantageous to shut down the engine system 2.2. According to the invention, in this operation mode the engine system 2 is operated in most advantageous manner resulting in low emissions and vibrations to the vessels hull constructions.
The invention is not limited to the embodiments shown but several modifications of the invention are reasonable within the scope of the attached claims.