TECHNICAL FIELD

This disclosure relates generally to marine propulsion systems. According to particular aspects the disclosure relates to diagnostics systems for multi-propeller drive units. The disclosure can be applied in various types of marine vessels such as marine leisure craft and in, e.g., ferries and other smaller commercial vessels. Although the disclosure may be described with respect to particular types of vessels herein, the disclosure is not restricted to any particular vessel.

BACKGROUND

A dual propeller system uses two propellers for propulsion. These propellers are often counterrotating which provides an increased efficiency.

Marine drivelines are relatively robust mechanical systems, but may still suffer from malfunction due to, e.g., impact by external objects and component wear.

The efficiency of a marine driveline, including its transmission and its propeller or propellers, is a significant factor in the overall power efficiency of the marine vessel. It is important that the driveline and the overall propulsion system of the marine vessel is serviced in a timely manner in order to avoid decreased in efficiency.

It may also be desired to optimize the operation of the marine vessel driveline during operation, in order to increase its power efficiency.

SUMMARY

According to some aspects of the present disclosure, there is presented a control unit for monitoring a multi-propeller drive unit of a marine vessel. The drive unit comprises a first propeller, a second propeller and a drive shaft arranged to be driven by a motor, where the drive shaft is arranged to drive the first propeller and the second propeller via a differential gear arrangement. The control unit is arranged to obtain data indicative of a rotation speed of the first propeller and also data indicative of a rotation speed of the second propeller. The control unit is furthermore arranged to obtain data indicative of a torque applied to the drive shaft by the motor and to determine respective propeller accelerations in response to the torque based on the rotation speeds. The control unit is arranged to determine a difference between the determined propeller accelerations and expected propeller accelerations in response to the torque, and to detect anomaly in the multi-propeller drive unit in case the difference does not meet an acceptance criterion. This means that the control unit can detect when something is amiss with the propeller system and/or with the transmission system between motor and propeller. For instance, if the wrong type of propeller is used, this propeller often has a different inertia and/or a geometry giving rise to differences in resistance to acceleration in water compared to the correct propeller, or a different blade geometry, which gives a different response to the applied torque compared to the expected response. The motor may, for instance be an electric machine in which case the data indicative of the rotation speeds of the first and second propellers can be obtained from a propeller speed sensor arranged to measure the rotation speed of the first propeller or the rotation speed of the second propeller and from the electric machine as an axle rotation speed. This means that only a single propeller speed sensor is needed, which is an advantage. The axle speed data obtained from the electric machine is in most cases already available as part of the control of the electric machine. This axle speed data is normally quite accurate, which is an advantage. According to some other aspects, the data indicative of the rotation speeds of the first and second propellers is obtained from first and second propeller speed sensors arranged to measure the rotation speed of the first propeller and the rotation speed of the second propeller, respectively. Consequently, the herein proposed multi-propeller monitoring systems can be used also with other types of motors, such as combustion engines that lack internal axle speed data. It is, however, appreciated that many combustion engines comprise axle speed sensors as an integral part of the combustion engine system. Two or more propeller speed sensors can of course also be used with an electric machine power source.

According to some aspects, the differential gear arrangement comprises a planetary gear arrangement with a known gear ratio between an input shaft of the differential gear arrangement and output shafts of the differential gear arrangement. This means that the torque distribution from the input shaft to the propellers is known, at least in an approximative sense. In case the motor is an electric machine, the torque applied to the drive shaft by the motor can be obtained digitally from a control unit of the electric machine. In case no such information is available directly from the motor, a torque sensor can be arranged in connection to the drive shaft, or elsewhere along the transmission from motor to propellers. The applied torque can also be estimated indirectly from, e.g., throttle position and other operation parameters. According to some aspects, the control unit is arranged to store the expected propeller accelerations in response to the torque. This is a relatively straight forward way to implement the techniques described herein. The measured or estimated propeller accelerations can then be directly compared to the stored expected accelerations and the difference determined in a straight-forward manner. Alternatively, or as a complement to storing expected propeller accelerations, the control unit can be arranged to determine the expected propeller accelerations in response to the torque, based on data indicative of propeller inertias and/or geometrical properties of the first propeller and of the second propeller. In other words, the control unit can be arranged to calculate or predict an expected acceleration given propeller data such as inertia, weight, blade design, and the like. The control unit can also be configured to measure the expected propeller accelerations in response to the torque during a calibration procedure, and to store the measured reference acceleration in memory.

According to some aspects, the control unit is configured to obtain data associated with propeller inertias and/or geometrical properties of the first propeller and of the second propeller, and to determine the expected propeller accelerations in response to the torque based on the propeller inertias and/or geometrical properties. Thus, it is understood that both inertia as well as propeller geometry can be considered, such as propeller weight, propeller dimension, the number of blades of the propeller, the blade angles, and other geometrical parameters of the propeller. The control unit may for instance determine the relevant propeller properties based on a measured acceleration of the first propeller and of the second propeller in response to a reference torque applied to the drive shaft. This measurement will account for both inertia and propeller geometry, which is an advantage.

The control unit may also be arranged to determine a steady-state relation between the rotation speed of the first propeller and the rotation speed of the second propeller, after a settling time duration, and to determine a difference between the steady-state relation and an expected steady-state relation, and to detect anomaly in the multi-propeller drive unit in case the difference does not meet an acceptance criterion. This additional test can serve as a complement to the acceleration-based tests, which can be used as the vessel is travelling at constant speed, when there is only a little or no acceleration upon which the acceleration-based test can be based.

There are also disclosed herein systems, vessels, computer readable media, and computer program products associated with the above discussed technical benefits. The above aspects, accompanying claims, and/or examples disclosed herein above and later below may be suitably combined with each other as would be apparent to anyone of ordinary skill in the art.

Additional features and advantages are disclosed in the following description, claims, and drawings, as will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of aspects of the disclosure cited as examples.

Figure 1 illustrates an example marine vessel with a drive unit;

Figure 2 illustrates an example drive unit for a marine vessel;

Figure 3 illustrates another example drive unit for a marine vessel;

Figure 4 is a graph illustrating propeller speed vs. time;

Figure 5 is a graph illustrating applied torque vs. time;

Figure 6 is a flow chart illustrating methods;

Figure 7 schematically illustrates a control unit; and

Figure 8 shows an example computer program product;

DETAILED DESCRIPTION

Aspects set forth below represent the necessary information to enable those skilled in the art to practice the disclosure.

The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.

Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Figure 1 illustrates an example marine vessel 100 with a drive unit 110. This particular drive unit 110 extends down from a bottom portion of the vessel hull 120, which is common if the power source is an inboard engine. Other types of drive units are instead attached to the transom 130 of the marine vessel. The drive unit 110 is a dual-propeller (duo-prop) drive unit with a first and a second propeller arranged coaxially. The two propellers in a duo-prop arrangement are often counter-rotating since this increases the efficiency of the propulsion arrangement. Duo-prop arrangements are generally known and will therefore not be discussed in more detail herein. Figure 2 illustrates an example multi-propeller drive unit 200 with first and second propellers 210, 220. Figure 2 also schematically illustrates a control unit 230. Details of an example realization 700 of this control unit 230 will be discussed below in connection to Figure 7. The control unit 230 is arranged to obtained input signals which indicate how fast the propellers accelerate in response to an applied torque by a power source such as a combustion engine or an electric machine. This information is then used in a diagnostics procedure by the control unit 230 to identify anomalies in the drive unit 110. The diagnostics methods disclosed herein are based on the control unit 230 monitoring the accelerations by the first propeller 210 and by the second propeller 220 in response to a torque applied to the drive shaft of the drive unit. In case the propellers accelerate unexpectedly, i.e., with different magnitudes, either too fast or too slow, then an anomaly is declared. The specific type of anomaly can often also be determined based on the response by the propellers 210, 220 to an applied torque. Different types of anomalies which can be detected by the control unit 230 will be discussed in more detail below.

One type of anomaly that can be detected by the control unit 230 is if an unsuitable propeller has been mounted on the drive unit, i.e., a propeller which the drive unit was not designed to support. A duo-prop drive unit of the sort illustrated in the drawings is often designed for a specific type of propeller, having a specific weight and dimension, and a specific geometry in terms of, e.g., number of blades and/or blade pitch. If a propeller with a specification that differs from the intended is mounted on the drive unit, significant inefficiency may result, and potentially also risk of drive unit breakdown. This may for instance happen if the wrong type of propeller is inadvertently mounted to the drive unit, e.g., one with an incorrect pitch, or if a counterfeit propeller with the correct dimensions but the wrong weight is mounted.

Another type of anomaly that can be detected by the control unit 230 is if one or both propellers 210, 220 are damaged, e.g., due to impact with some foreign object like an underwater rock or the like. Such an event may dent the propeller blades, leave marks on the blades, or even result in cracks and lost propeller blades. It is of course desired to quickly detect when this happens, such that the damage can be repaired or at least investigated in a timely manner.

Unexpected component wear is also considered an anomaly herein, and it may be detected by the control unit 230. Bearings or transmission components such as gears and shafts are of course subject to natural wear, but as long as the drive unit is properly serviced this wear should not have a significant effect on the overall operation of the drive unit. A bearing which malfunctions introduces friction to the drive unit, which of course has a degrading effect on the efficiency of the drive unit. It is desired to detect when this type of unexpected component wear occurs, such that the component can be serviced or replaced.

Figure 3 illustrates another multi-propeller drive unit 300 which illustrates some of the main concepts of the diagnostics systems described herein. The drive unit comprises a first propeller 210, a second propeller 220 and a drive shaft 310 arranged to be driven by some form of motor 320, 330 (schematically illustrated by the dashed boxes in Figure 3), such as a combustion engine or an electric machine. A combination of combustion engine and electric machine is also possible, i.e., a hybrid drive system. An electric machine 330 may advantageously be submerged and integrated in the drive leg as schematically illustrated in Figure 3, while combustion engines are normally mounted as inboard power sources or as outboard engines mounted to the transom 130 of the vessel 100.

The drive shaft 310 is arranged to drive the first propeller 210 and the second propeller 220 via a differential gear arrangement 370. A differential is a gear mechanism with three drive shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others, or a fixed multiple of that average. The control unit 230 can be configured with information about the gear ratio in the planetary gear. Thus, if two of the axle speeds are known then the other of the three can be calculated in a straight-forward manner using the gear ratio of the differential drive arrangement. An open (non-locking) differential arrangement always supplies torques at a known relationship to each output axle, i.e., an input torque M applied to the drive shaft 310 of the drive unit is distributed between the first and the second propeller as respective known drive torques M ¹ and M ² .

The differential gear arrangement 370 may, e.g., be implemented by a planetary gear arrangement, where the drive shaft 310 drives the sun gear and where the propellers are driven by the planet gear carrier and by the outer ring gear, respectively. An advantage of using a planetary gear in this manner is that the rotation speed of the drive shaft 310 can be kept relatively high, and then reduced by the planetary gear to a lower propeller average speed to obtain a more efficient high power propulsion system. A high drive shaft speed is an advantage when using electric machines since a higher power can be obtained more easily in this manner. Planetary gear arrangements are differential drive arrangements which like all differential drive arrangements can be configured to distribute torque from the input shaft over the two output shafts according to a deterministically known torque ratio. Planetary gear arrangements are generally known and will therefore not be discussed in more detail herein.

When positive torque is applied to a propeller, the propeller starts to rotate. When negative torque is applied to a propeller, the propeller ceases to rotate (is braked), and then starts to rotate in the opposite direction. The amount of acceleration (positive or negative acceleration) achieved by a given applied torque is a function of the inertia of the rotating system and the time during which the torque is applied. There is a well-known relationship between the inertia of a rotating system, its acceleration and applied torque. Various models and approximative relationships can be used to relate applied torque and time period to speed change and inertia at different degrees of approximation. For instance, given information about the inertia J of a propeller, the expected angular mechanical impulse M(t _x — t ₀ ) that is required to change the speed of the propeller from one propeller speed m ₀ to another propeller speed from time t ₀ to time can be determined from the expression where M is the applied torque, M _o represents friction losses and the like, J is the propeller inertia and J _o summarizes contributions to system inertia from other rotating components in the system, i.e.,/ + J _o is the total system inertia. The applied torque M is in most cases significantly larger than M _o . In case J is also significantly larger than / ₀ , the expression simplifies to:

/(tOi - m ₀ ) « M(t _x - t ₀ ) where M is a constant torque. If needed, M _o and J _o can be handled as calibration parameters. I.e., if either M _o and/or J _o are large enough to have a significant effect on the rotating system, then they can be included in the model used to describe the system. Their values can be preconfigured in the control unit 230 or measured during a calibration operation involving, e.g., a reference propeller having known inertia. M _o may be a function of the difference between propeller speed and the speed off the water passing the propeller, i.e., the speed of the vessel 100 through water. Suitable values for M _o and J _o can be configured in the control unit 230, where it is appreciated that these calibration parameters may be functions of state variables such as speed through water.

As illustrated by the graph 400 in Figure 4, with everything normal (no drive unit anomaly present) and with a known propeller inertia /, the propeller will accelerate 410 from a rotation speed ) _Q at time t ₀ to a rotation speed at time in response to an applied torque M during the time period — t ₀ , since

/(tOi - m ₀ ) « M(t _x - t ₀ ) at least according to an approximation. Remember that the torque M applied to each propeller 210, 220 during normal operating conditions is a known fraction of the input torque since the differential arrangement distributes torque between the two propellers according to a known ratio.

In case of an anomaly where the propeller inertia J is smaller than expected, e.g., because the propeller has the wrong specification or is damaged or even missing, the time to accelerate the corresponding propeller with the same torque M will be smaller than — t ₀ , as illustrated by the dashed line 420, since the resistance to acceleration by the propeller and drive arrangement is smaller. It is appreciated that the relationships between inertia, axle speeds, applied torque and time which are used herein to detect anomaly in the drive unit are normally approximations which leave out smaller contributions from insignificant systems components. Such discrepancies can be handled by introduction of detection margins or by calibration. Some more advanced versions of the diagnostic methods discussed herein may comprise calibration parameters that can be configured to account for approximation errors incurred by modelling errors.

Generally, effort can be measured in terms of the acceleration which results when a given torque is applied to a propeller, where a higher acceleration for a given torque means that it takes less effort to accelerate the propeller compared to some reference operation. If a propeller suddenly starts to accelerate more rapidly or more slowly than expected, then an anomaly is likely to be present in the drive unit.

Effort can also be measured in terms of the torque required to accelerate a propeller at a given acceleration. The more torque it takes to maintain a given acceleration, the more effort it takes to accelerate the propeller. If all of a sudden it takes much less torque than it normally does to accelerate a propeller at a given acceleration, then an anomaly may be suspected. A counterfeit propeller of too low weight would for instance cause too high acceleration, and so would a propeller missing one or more blades.

Effort can furthermore be measured in terms of the angular mechanical impulse M(t _x — t ₀ ) required for a given speed change oq — m ₀ at some inertia J. If the required angular mechanical impulse for accelerating a propeller from a speed m ₀ to a speed a) _± suddenly decreased from an expected amount of angular mechanical impulse, then an anomaly is likely to be present.

Referring to the graph 400 in Figure 4 again, if the assumed propeller inertia J used in the calculations is smaller than the true inertia of the physical system, then the time duration to accelerate the propeller for the same applied torque will be longer, as illustrated by the dashed line 430. This could for instance be caused by excessive biofouling on one or both propellers 210, 220. A difference between the assumed applied torque used in calculations and the actual torque applied to the propeller can also give a difference between the predicted time to accelerate the propeller and the actual time to accelerate the propeller.

Figure 5 illustrates an example of a torque pulse which can be used to detect anomaly and/or to determine one or more system parameters such as the propeller inertia J in case negative torque can be applied to a propeller, i.e., a braking torque. A first torque is first applied at time t ₀ , the area under the curve is A _t . A second torque of opposite sign is then applied at time up to time t ₂ , with an area under the curve of d ₂ . Now, if the two areas are the same, then the expected response by the system is terms of propeller speed change should be zero, since

This can be used in an anomaly detection routine, by configuring the pulse with a difference in magnitude torques as shown in Figure 5, and then checking if the propeller speed after the pulse has changed by some amount greater than a margin. This type of pulse may advantageously be generated by an electric machine, which can be used to provide both positive and negative amounts of torque in an accurate and repeatable manner. The type of pulse illustrated in Figure 5, or some other pulse comprising an increase in torque followed by a decrease in torque may also be used to calibrate the control unit. Suppose for instance that a known correct propeller arrangement has been mounted which is brand new and known to be fault-free, then a pulse like this can be applied and the resulting acceleration by the system can be monitored. This “baseline” acceleration profile can then be used as reference for the anomaly detection procedures discussed herein.

The actual speed change of the propeller can also be compared to an expected speed change, i.e., by using the relationship

Anomaly can be detected if the actual speed change differs from an expected speed change by more than a margin.

To summarize, with reference to Figure 3, the control unit 230 is arranged to obtain data indicative of a rotation speed of the first propeller a> ^pl and of a rotation speed of the second propeller a> ^p2 . This data can be obtained in a number of different ways. According to one example, as discussed above, the motor 320, 330 is an electric machine which maintains accurate information about the current drive shaft speed (since this is needed for control of the electric machine by its inverter). Thus, the control unit 230 can obtain axle speed information directly from the electric machine control unit. In fact, the control unit 230 and the control unit of the electric machine may be one and the same control unit. The data indicative of the rotation speeds of the first and second propellers m ^pl , a> ^p2 can then be obtained from a propeller speed sensor 340, 350 arranged to measure the rotation speed of the first propeller 210 or the rotation speed of the second propeller 220 and from the electric machine as an axle rotation speed a) ⁱⁿ . I.e., if the input axle speed is known from the electric machine used to drive the drive shaft 310, and one of the propeller speeds is known to the control unit 230, then it is possible to determine the rotation speed of the other propeller using information about the transmission, such as the gear ratio of the differential gear 370. According to other aspects, the data indicative of the rotation speeds of the first and second propellers m ^pl , a> ^p2 is obtained from first and second propeller speed sensors 340, 350 arranged to measure the rotation speed of the first propeller 210 and the rotation speed of the second propeller 220, respectively. This set-up of course works with any power source, such as with a combustion engine where the actual drive shaft speed may not be available with sufficient accuracy.

The control unit 230, 700 is also arranged to obtain data indicative of a torque M applied to the drive shaft 310 by the motor 320, 330 and to determine respective propeller accelerations a ^pl , a ^p2 in response to the torque M based on the rotation speeds m ^pl , a> ^p2 . In case the motor 320, 330 is an electric machine, then the torque M applied to the drive shaft 310 by the motor 320, 330 can be obtained from a control unit of the electric machine. However, the torque M applied to the drive shaft 310 by the motor 320, 330 can also be obtained at least partly from a torque sensor 360 arranged in connection to the drive shaft 310. The control unit 230 may be arranged to determine a rotation speed a> ⁱⁿ of the drive shaft 310 based on one or more characteristics of an electric current drawn over an interface to an electric motor, such as the frequency of the current on the motor interface or phase relationship over time. The control unit 230 may in fact advantageously be integrated with a control unit for controlling an electric machine of the drive unit, which then already knows the drive shaft speed with high accuracy since this is an integral part of the electric motor control.

In case the control unit 230 is also arranged to control an electric motor of the drive unit to generate an amount of torque M applied to the drive shaft 310, then the control unit 230 knows both drive shaft speed and applied torque, which allows it to perform the diagnostics methods disclosed herein with only a single additional propeller speed sensor 340, 350.

The control unit may also alternatively, or as a complement, determine a rotation speed of the drive shaft 310 based on an output signal from a speed sensor 340 such as a rotary encoder arranged in connection to the drive shaft. This rotary encoder may comprise a Hall sensor, or other form of transducer. The rotary encoder may also be realized from an existing generator or ignition module of a combustion engine.

The control unit 230, having obtained information about the two propeller speeds, may now determine a difference between the determined propeller accelerations a ^pl , a ^p2 and corresponding expected propeller accelerations m ^pl , a> ^p2 in response to the torque M. Given data about the propellers which should have been mounted on the drive unit 300, it is possible to predict what the accelerations of the two propellers should have been in response to a given amount of applied torque M the drive shaft 310, in a normal operating condition where the propellers are undamaged and where the other driveline components are without error. The control unit 230 may therefore be arranged to detect anomaly in the multi-propeller drive unit 110, 200, 300 in case the difference does not meet an acceptance criterion.

The control unit 230 may be arranged to trigger a notification or warning to a user of the vessel 100 and/or external device in response to detecting anomaly in the drive unit 300. This way the user receives information that the drive unit is not functioning optimally and can take action to resolve the issue. The notification can, e.g., comprise a message on a display, a buzzer signal, a light signal, and/or a message transmitted via radio to an external device such as a remote wireless device or server. The control unit 230 may, according to an example, also be arranged to schedule maintenance in response to detecting a difference beyond the acceptance criterion. The vessel owner may then be prompted to deliver the boat to a workshop for investigation, such that the root cause of the difference from expected acceleration behavior can be determined and, in most cases, also remedied before more serious damage to the propulsion system results.

Control units implementing the drive unit diagnostic methods discussed herein can be configured to perform a calibration routine, in which calibration routine data indicative of a propeller inertia J and/or resistance to acceleration in water, of a propeller arrangement 210, 220 arranged to be driven by the electric motor 320, 330 or combustion engine is determined and stored in a memory 730 of the control unit. This calibration routine can, for instance, comprise mounting of a reference propeller to the propeller axle of known inertia. The first and the second propellers can then be carefully accelerated, while making sure that no anomaly is present. It is also possible to perform other calibration routines where data indicative of a reference amount of acceleration achieved for a set of propellers 210, 220 with a given respective propeller inertias and applied torques is determined and stored in a memory 730 of the control unit.

The control unit 230 is, according to an example, arranged to store the expected propeller accelerations in response to the torque M. This data on expected propeller accelerations may, e.g., be stored as a database, such as a look-up table, which can be indexed using the applied torque. The contents of this database is an example of the above-mentioned acceptance criterion. Alternatively, or as a complement, the control unit 230 may be arranged to determine the expected propeller accelerations in response to the torque M, based on data indicative of propeller inertias of the first propeller 210 and of the second propeller 220.

According to some other aspects, the control unit 230 is also arranged to determine a steadystate relation between the rotation speed of the first propeller m ^pl and the rotation speed of the second propeller m ^p2 , after a settling time duration, and to determine a difference between the steady-state relation and an expected steady-state relation, and to detect anomaly in the multipropeller drive unit 110, 200, 300 in case the difference does not meet an acceptance criterion. In this case the control unit 230 monitors the speeds of the propellers and looks for unexpected differences in steady state propeller speed. This steady state propeller speed relation is of course a function of the torque distribution by the differential drive arrangement, and the propeller characteristics such as the propeller dimension and pitch. A reference database of expected steady-state propeller speed relations in different operating conditions can be maintained and used for detecting anomaly in the drive unit. The contents of this database is also an example of the above-mentioned acceptance criterion.

The discussion above can be summarized in terms of a computer-implemented method, which is illustrated by the flow chart in Figure 6. This flow chart shows a computer-implemented method for monitoring a multi-propeller drive unit 110, 200, 300 of a marine vessel 100, where the drive unit 110, 200, 300 comprises a first propeller 210, a second propeller 220 and a drive shaft 310 arranged to be driven by a motor 320, 330, and where the drive shaft 310 is arranged to drive the first propeller 210 and the second propeller 220 via a differential gear arrangement 370. The method comprises obtaining SI, by a control unit 230, 700, data indicative of a rotation speed of the first propeller m ^pl and of a rotation speed of the second propeller m ^p2 , as well as data indicative of a torque M applied to the drive shaft 310 by the motor 320, 330, determining S2, by the control unit 230, 700, respective propeller accelerations a ^pl , a ^p2 in response to the torque M based on the rotation speeds a> ^pl , a> ^p2 , determining S3, by the control unit 230, 700, a difference between the determined propeller accelerations a ^pl , a ^p2 and expected propeller accelerations in response to the torque M, and detecting S4, by the control unit 230, 700, anomaly in the multi-propeller drive unit 110, 200, 300 in case the difference does not meet an acceptance criterion.

Figure 7 schematically illustrates, in terms of a number of functional units, the components of a control unit 700 according to embodiments of the discussions herein, such as any of the versions of the control unit 230. Processing circuitry 710 is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g., in the form of a storage medium 730. The processing circuitry 710 may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA. Particularly, the processing circuitry 710 is configured to cause the control unit 700 to perform a set of operations, or steps, such as the methods discussed in connection to Figure 6 and generally herein. For example, the storage medium 730 may store the set of operations, and the processing circuitry 710 may be configured to retrieve the set of operations from the storage medium 730 to cause the control unit 700 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 710 is thereby arranged to execute methods as herein disclosed. The storage medium 730 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The control unit 700 may further comprise an interface 720 for communications with at least one external device. As such the interface 720 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication.

The processing circuitry 710 controls the general operation of the control unit 700, e.g., by sending data and control signals to the interface 720 and the storage medium 730, by receiving data and reports from the interface 720, and by retrieving data and instructions from the storage medium 730. Other components, as well as the related functionality, of the control node are omitted in order not to obscure the concepts presented herein.

Figure 8 illustrates a computer readable medium 810 carrying a computer program comprising program code means 820 for performing the methods illustrated in Figure 6 and the techniques discussed herein, when said program product is run on a computer. The computer readable medium and the code means may together form a computer program product 800.

It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the inventive concepts being set forth in the following claims.