Marine vessel propulsion system and method for controlling same

Abstract

A marine propulsion system and method includes a large two-stroke internal combustion engine and a shaft system (42) connecting a propeller (44) with a crankshaft (22) of the engine. During engine operation, the system exhibits torsional vibrations having a first torsional vibration mode frequency. The system includes an engine speed sensor (52) that measures the speed of the marine propulsion system and generates a signal representative of the measured speed. A controller (50) calculates a first torsional vibration mode frequency from a signal from a sensor associated with the marine propulsion system. The controller (50) includes a computer model (56) of the marine propulsion system. The controller (50) estimates a weighted speed of the engine using the calculated first torsional vibration mode frequency in a computer model (56). The controller (50) adjusts the speed of the engine using the weighted engine speed.

Description

Technical Field

[0001] The present disclosure relates to a marine propulsion system comprising a large two-stroke uniflow internal combustion engine driving a propeller through a shaft line, and to a method for controlling such a marine vessel in a marine propulsion system. Background Art

[0002] Large two-stroke, uniflow-type internal combustion engines are commonly used as prime movers in marine propulsion systems of large ocean-going vessels, such as container ships. Torsional vibrations in marine propulsion systems can be difficult to handle when operating on ocean-going vessels. Such torsional vibrations occur because the propeller shaft, which connects the engine to the propeller, is relatively torsionally flexible.

[0003] Representative and accurate speed measurements are essential for the stability of the governor of the engine control system (ECS). The main purpose of the engine governing system is to control the amount of fuel injected into the engine cylinders to thereby control the speed of the engine. The governor must contain certain basic components, including a speed sensing component, a speed setting or reference component, and a regulating component. The governor uses the speed sensing component to sense changes in engine load or demand and compensates for large two-stroke internal combustion engines by regulating the so-called fuel index (ranging from 0 to 100). The fuel index (the amount of fuel injected into the engine cylinders) in turn keeps the engine speed at the desired set value. The speed sensing component of a large two-stroke engine includes an encoder located at the crankshaft. This unit is responsible for measuring the speed of the engine crankshaft. However, the speed measurement of the shafting on the engine side may be different from the speed measurement obtained on the propeller side. This is due to torsional vibration, which is an oscillating twist of the shafting caused by, for example, fluid dynamics on the propeller.

[0004] On some marine vessels, the frequency of the first torsional vibration mode of the slender shaft is low enough that it may interfere with the crankshaft speed measurement used by the regulator and may therefore affect the reliability of the fuel index calculation. This interference with the fuel index may even amplify the vibration itself, because the fuel index is directly related to the engine torque and cause severe engine hunting. Another example is the use of a large power take-off (PTO) (shaft generator). If the PTO solution includes a variable frequency drive (VFD), the PTO may cause the engine speed to be unstable due to the constant power output requirement. If the destabilizing effect is large, the engine regulator will not be able to compensate and stabilize the engine speed, and this may also cause engine hunting.

[0005] Currently, in order to avoid this problem, an additional speed measurement is performed using a second shaft speed sensing unit located below the shafting and closer to the propeller. The engine control system will then combine these measurements using an algorithm to obtain a reliable weighted mean rotational engine speed value that is not affected by vibrations. The regulator will then use this weighted measurement to calculate the fuel index. However, this structure using two speed sensors has material costs. In addition, it cannot be determined whether a second speed sensor unit needs to be added until after torsional vibration measurements and sea trials of the marine vessel. This is due to the fact that the shafting size and the optional presence of a PTO will affect whether the first torsional mode is in the frequency region that affects the regulator, which requires additional regulator stability calculations and evaluations.

[0006] JP2019007482 discloses a large two-stroke internal combustion engine according to the preamble of claim 1 . Summary of the invention

[0007] It is an object to provide a marine propulsion system which overcomes or at least reduces the above mentioned problems.

[0008] The above-mentioned objects and other objects are achieved by the features of the independent claims. Further implementations are apparent from the dependent claims, the description and the drawings.

[0009] According to a first aspect, there is provided a marine propulsion system, the marine propulsion system comprising:

[0010] - a large two-stroke internal combustion engine having a crankshaft,

[0011] - a shafting which operatively connects the propeller to the crankshaft,

[0012] During engine operation, the marine propulsion system exhibits torsional vibrations having a first torsional vibration mode frequency,

[0013] - an engine speed sensor, the engine speed sensor being configured to:

[0014] measuring a speed of the marine propulsion system at a location on the marine propulsion system, and

[0015] generating a velocity signal representative of the measured velocity,

[0016] - a controller receiving the speed signal,

[0017] The controller is configured to calculate a first torsional vibration mode frequency based on a signal from a sensor associated with the marine propulsion system,

[0018] The controller includes a computer model of the marine propulsion system,

[0019] The controller is configured to use the calculated first torsional vibration mode frequency in the computer model to estimate a weighted average engine speed of the engine,

[0020] The controller is configured to use the weighted average engine speed to regulate the speed of the engine.

[0021] By calculating the weighted average engine speed from an existing single engine speed sensor or, for example, from an axial vibration sensor which is usually already present in engines of this type, the need for a relatively expensive second speed sensing unit can be avoided. The absence of a dual speed signal eliminates additional computational load on the controller.

[0022] In a possible implementation of the first aspect, the marine propulsion system comprises only one single engine speed sensor.

[0023] In a possible implementation of the first aspect, the computer model is an inertial model of the marine propulsion system.

[0024] In a possible implementation of the first aspect, the computer model is a lumped parameter model of the marine propulsion system.

[0025] In a possible implementation of the first aspect, the lumped parameter model includes a first inertia and a second inertia corresponding to the engine side and the propeller side respectively, and the first inertia and the second inertia are connected by a spring representing the shaft system with negligible inertia contribution.

[0026] In a possible implementation of the first aspect, the marine propulsion system comprises a shaft generator, wherein the lumped parameter model comprises a third inertia corresponding to the inertia of the shaft generator.

[0027] In a possible implementation manner of the first aspect, the signal representing the measured speed is a signal used by the controller to calculate the frequency of the first torsional vibration mode.

[0028] In a possible implementation of the first aspect, the controller is configured to calculate the frequency of the first torsional vibration mode by applying Fourier analysis to the signal of the measured speed.

[0029] In a possible implementation of the first aspect, the signal is a signal from a vibration sensor configured to generate a signal representative of an axial vibration of the shaft system.

[0030] In a possible implementation manner of the first aspect, the controller is configured to perform a frequency analysis on a signal generated by the vibration sensor to calculate a frequency of the first torsional vibration mode.

[0031] In a possible implementation of the first aspect, the controller receives a signal representing a desired engine speed, and wherein the controller is configured to regulate the amount of fuel supplied to the engine according to the desired engine speed and a weighted average engine speed.

[0032] In a possible implementation of the first aspect, the engine comprises a flywheel, and wherein the engine speed sensor measures the speed of the marine propulsion system, preferably at the flywheel or on a shafting located close to the flywheel.

[0033] According to a second aspect, there is provided a marine vessel comprising the marine propulsion system of the first aspect and any possible implementation thereof.

[0034] According to a third aspect, there is provided a method for controlling a marine propulsion system, the marine propulsion system comprising:

[0035] - a large two-stroke internal combustion engine having a crankshaft,

[0036] - a shafting which operatively connects the propeller to the crankshaft,

[0037] During engine operation, the marine propulsion system exhibits torsional vibrations having a first torsional vibration mode frequency,

[0038] - an engine speed sensor, the engine speed sensor being configured to:

[0039] measuring a speed of the marine propulsion system at a location on the marine propulsion system, and

[0040] generating a signal representative of the measured speed,

[0041] - a controller receiving the speed signal,

[0042] The controller includes a computer model of the ship's propulsion system.

[0043] The method includes:

[0044] The controller calculates a first torsional vibration mode frequency based on a signal from a sensor associated with the marine propulsion system,

[0045] The controller uses the calculated first torsional vibration mode frequency in the computer model to estimate the weighted average engine speed.

[0046] The controller uses the weighted average engine speed to regulate the speed of the engine.

[0047] In a possible implementation of the third aspect, the signal representing the measured speed is a signal used by the controller to calculate the frequency of the first torsional vibration mode, and wherein the controller calculates the frequency of the first torsional vibration mode by applying a Fourier transform to the signal of the measured speed.

[0048] In a possible implementation of the third aspect, the signal from the vibration sensor is the signal used by the controller to calculate the frequency of the first torsional vibration mode, and the vibration sensor is configured to generate a signal representing the axial vibration of the shaft system, and wherein the controller is configured to: perform frequency analysis on the signal generated by the vibration sensor to calculate the frequency of the first torsional vibration mode.

[0049] In a possible implementation of the third aspect, the controller receives a signal representing a desired engine speed, and wherein the controller regulates the amount of fuel supplied to the engine based on the desired engine speed and a weighted average engine speed.

[0050] These and other aspects will become apparent from one or more examples and one or more implementations described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In the following detailed description of the present disclosure, the present invention will be described in more detail with reference to exemplary embodiments shown in the accompanying drawings, in which:

[0052] Figure 1 is an elevation view showing a front end portion and a side portion of a large two-stroke compression ignition turbocharged engine according to an exemplary embodiment,

[0053] Figure 2 It shows Figure 1 An elevation view of the rear end and another side of the engine,

[0054] Figure 3 is based on Figure 1 A schematic diagram of an engine and an intake system and an exhaust system of the engine,

[0055] Figure 4 Is set up to include Figures 1 to 3 A partially cutaway side view of a marine vessel of an embodiment of an engine marine propulsion system,

[0056] Figure 5 yes Figure 4 an enlarged view of a portion of a marine propulsion system showing an embodiment of the marine propulsion system in more detail,

[0057] Figure 6 yes Figure 4 an enlarged view of a portion of a marine propulsion system showing in more detail various embodiments of the marine propulsion system,

[0058] Figure 7 is a diagram showing torsional vibrations in a prior art marine propulsion system,

[0059] Figure 8 is a schematic diagram of an embodiment of a controller for a marine propulsion system, and

[0060] Fig. 9 is a schematic diagram of another embodiment of a controller for a marine propulsion system. DETAILED DESCRIPTION

[0061] In the following detailed description, a large two-stroke internal combustion engine and a method for operating a large two-stroke internal combustion engine will be described by way of exemplary embodiments. Figures 1 to 3 A large, low speed, turbocharged, two-stroke internal combustion engine is shown having a crankshaft 22, connecting rods, a crosshead 23 and piston rods. Figure 3 A schematic diagram of a large, low-speed, turbocharged, two-stroke internal combustion engine and the intake and exhaust systems of the engine is shown. In this exemplary embodiment, the engine has six cylinders 1 in line. Large, turbocharged, two-stroke diesel engines typically have between five and sixteen cylinders in line carried by an engine frame 24. The total output of the engine may, for example, range from 5,000 kW to 110,000 kW.

[0062] The engine can be a two-stroke single-flow diesel (compression ignition) engine or an Otto (premixed) engine, which has a scavenging port 19 in the form of a piston control port ring located at the lower area of ​​the cylinder 1 and an exhaust valve 4 located at the top of the cylinder 1. Therefore, the flow in the combustion chamber is always from bottom to top, and the engine is therefore of the so-called single-flow type. The scavenging air is delivered from the scavenging air receiver 2 to the scavenging air port 19 of each cylinder 1. The reciprocating piston 21 in the cylinder 1 compresses the scavenging air in the combustion chamber 14. The fuel is injected into the combustion chamber 14 via two or three fuel valves 30 arranged in the cylinder head 26 (in an Otto (premixed) engine, a fuel valve may also be present approximately centrally in the cylinder liner). The timing and amount of fuel injection are controlled by an electronic control unit 50, which is connected via a signal line (such as Figure 31 ) is connected to a fuel valve 30. Combustion then takes place and exhaust gas is produced. When the exhaust valve 4 is open, the exhaust gas flows through the exhaust line 20 associated with the relevant cylinder 1 to the exhaust receiver 3 and continues to flow through the first exhaust duct 18 to the turbine 6 of the turbocharger 5, from which the exhaust gas flows out through the second exhaust duct 7. The turbine 6 drives the compressor 9 supplied via the air inlet 10 via the shaft 8.

[0063] The compressor 9 delivers pressurized charge air to a charge air duct 11 leading to a charge air receiver 2. The scavenging air in the duct 11 passes through an intercooler 12 to cool the charge air. The cooled charge air is transferred to the charge air receiver 2 by an auxiliary blower 16 driven by an electric motor 17, which pressurizes the charge air flow under low or part load conditions. At higher loads, the compressor 9 of the turbocharger delivers sufficient compressed scavenging air, and then the auxiliary blower 16 is bypassed via a non-return valve 15.

[0064] The cylinder 1 is formed in a cylinder liner 1. The cylinder liner 1 is carried by a cylinder frame 25, which is supported by an engine frame 24.

[0065] In a reciprocating engine, dead center is the position where the piston is farthest or closest to the crankshaft 22. The position farthest from the crankshaft 22 is called the top dead center (TDC), and the position closest to the crankshaft 22 is called the bottom dead center (BDC). A rotating wheel (also called a flywheel) 41 is mounted on the rear end of the crankshaft, and a tuning wheel 49 is optionally mounted on the front end of the crankshaft.

[0066] The controller 50 includes a regulating function, the main purpose of which is to control the fuel used for the engine cylinders and thus the speed of the engine. The regulating function includes a speed sensing component, a speed setting or reference component, and a control component. The regulator uses the speed sensing component to sense changes in the load or demand of the engine and compensates the diesel engine by regulating the so-called fuel index (ranging from 0 to 100). The fuel index in turn keeps the speed at the desired setting. The speed set point is the desired speed of the marine propulsion system, for example, determined by the operator (crew) of the marine vessel 40. The regulator index, also known as the "fuel index", ranges from 0 to 100 and represents the amount of fuel supplied to the engine.

[0067] Figure 4 The installation in a large marine vessel 40 is shown. Figures 1 to 34. The engine 1 is a nine-cylinder engine in a container ship in this exemplary embodiment. However, it will be appreciated that the engine may have any other number of cylinders between 4 and 14 and that the marine vessel 40 may be of another type. The engine 1 is mounted in an engine room closer to the stern than the bow of a large marine vessel 40. A propeller shaft forming most of the shafting 42 connects the engine to a propeller 44 mounted at the stern.

[0068] Figure 5 yes Figure 4 A detailed description view of a part of Figure 6 In the embodiment where a shaft generator 48 has been added to the system Figure 4 Such a shaft generator 48 will generally increase the risk of engine oscillations due to the additional load and inertia.

[0069] During engine operation, the marine propulsion system exhibits torsional vibrations having a first torsional vibration mode frequency determined by, for example, the torsional stiffness of the shafting 42 , the (rotational) inertia of the engine and the (rotational) inertia of the propeller 44 .

[0070] Figure 7 is a graphical representation of torsional vibrations measured in a prior art marine propulsion system. The graphical representation shows engine speed in revolutions per minute (RPM), a regulator index in %, and a speed set point in RPM. The first node torsional vibration of the marine propulsion system has a relatively low natural frequency (e.g., below 5 Hz), and therefore the vibrations may be amplified by the regulator control loop of the controller 50. The controller 50 changes fuel injection due to the speed changes produced by the vibrations, and the fuel injection changes amplify the vibrations. This type of instability is a normal control loop instability, unrelated to excitation from the engine, and in the prior art this type of instability is addressed by obtaining a weighted average engine speed from both a speed sensor on the engine and another speed sensor near the propeller.

[0071] Generally, the first node torsional vibration frequency of the marine propulsion system is lower when there is a shaft generator 48, a controllable pitch propeller 44 and a slender shafting 42. However, this problem is also observed with a fixed propeller and without a shaft generator 48, so this problem is not completely limited to the system with a shaft generator 48 and a controllable pitch propeller 44.

[0072] Figure 8is a schematic diagram of an embodiment of a propulsion system and a controller 50. An engine speed sensor 52 is arranged on the engine, for example at the flywheel 41, or at a location close to the engine, for example on the shafting 42 at a location close to the flywheel 41. The engine speed sensor 52 is configured to measure the speed of the marine propulsion and generate a signal representative of the measured speed.

[0073] In this embodiment, the signal from the engine speed sensor is used to calculate or estimate the frequency of the first torsional vibration mode of the marine propulsion system. Here, the controller 50 is provided with a frequency analysis module 54, which receives the speed measured by the engine speed sensor 52. The Fourier analysis module 54 performs a Fourier transform to calculate or estimate the frequency of the first torsional vibration mode of the marine propulsion system. The Fourier analysis module preferably uses a Fourier transform to convert the signal representing the measured speed into a form that describes the frequencies present in the original signal. The output of the transform is a complex-valued function of the frequency, thereby representing the first torsional vibration mode frequency.

[0074] The controller 50 is provided with a computer model 56 of the marine propulsion system in the form of an inertial model of the marine propulsion system. The inertial model may be a lumped parameter model of the marine propulsion system.

[0075] The controller 50 uses the first torsional vibration mode frequency in the computer model 56 to obtain an estimate of a weighted average engine speed value that is not affected by torsional vibrations. The controller 50 uses this weighted average engine speed value in the regulator function instead of using the raw measured speed. Therefore, the engine oscillation problem is avoided or at least reduced.

[0076] In an embodiment, the controller uses the following lumped parameter model of two inertias Ie and Ip, which correspond to the engine inertia and propeller inertia [kg·m 2 ], the engine inertia and propeller inertia are connected by an elastic member, which represents a stiffness with negligible inertia contribution Axis system:

[0077]

[0078] Let θ = βcos(ωt), where β is the angular displacement (which is a dimensionless value measured in radians) and ω is the angular frequency t is the time [s], and substitute into the above parameter model:

[0079]

[0080] For nontrivial solutions, the determinant of the algebraic equation is and The following must be zero:

[0081]

[0082] By expanding the above equation, we can get:

[0083]

[0084] The above formula is simplified to:

[0085] [I e I p ω 2 -K θ (I e +I p )]ω 2 =0 (5)

[0086] A torsional system model of any complexity will usually have a zero-frequency mode, which corresponds to a rigid-body rotation of the entire shaft system including the mass, i.e., no shaft torsion. This can be seen from the fact that the first eigenvalue of the above determinant is ω 1 = 0. The second eigenvalue ω 2 From the brackets:

[0087]

[0088] Among them, the torsional natural frequency ω n It can usually be interpreted as:

[0089] use

[0090] If the torsional frequency is known, the torsional stiffness can be calculated from the inversion term:

[0091]

[0092] Substituting equation 8 above into the determinant, the change in angular displacement between the propeller and the engine can be calculated:

[0093]

[0094] The above formula can be used to find the current engine speed measurement value ω e The combined weighted 1 ,p 2 ) Average engine speed

[0095]

[0096] Knowing the speed difference between the propeller and the engine allows a reliable weighted average engine speed to be determined directly.

[0097] If the propulsion system is provided with a shaft generator 48, the lumped parameter model is provided with an additional third inertia (Is) corresponding to the shaft generator inertia.

[0098] Fig. 9 is a schematic diagram of another embodiment of a propulsion system and controller 50. In this embodiment, for simplicity, structures and features that are the same or similar to corresponding structures and features previously described or shown in this document are represented by the same reference numerals as the previously used reference numerals. In this embodiment, the signal used to calculate the frequency of the first torsional vibration mode of the marine propulsion system is a signal from a vibration sensor 53, which is configured to generate a signal representing the axial vibration of the shaft system 42. Such a vibration sensor 53 is typically found in large two-stroke turbocharged internal combustion engines and therefore does not represent an additional cost. The axial direction of the shaft system 42 is excited by a forced coupled response of torsional vibration. Therefore, the engine crankshaft is forced to deflect axially when it is twisted. The deflection is measured by the vibration sensor 53.

[0099] The controller 50 uses the signal generated by the vibration sensor 53 in the frequency analysis module 54 and uses the frequency of the first torsional vibration mode of the marine propulsion system in the computer model 56, as for Figure 8 Likewise, the controller 50 uses the weighted average engine speed generated by the computer model for the regulation function. The computer model 56 of this embodiment is used in conjunction with the computer model 56 for Figure 8 The computer model of the implementation scheme is the same.

[0100] In all embodiments, the controller 50 may be an electronic control unit comprising one or more processors. The one or more processors may be microprocessors, and the electronic control unit may also include memory and software in the form of one or more computer programs.

[0101] In an embodiment, the speed sensor 53 is a pick-up for speed measurement, and the speed sensor 53 may include a proximity sensor that measures the speed of the flywheel 41. The proximity sensor senses movement of the teeth of the flywheel 41 when the flywheel 41 rotates.

[0102] In an embodiment, the marine propulsion system includes only a single engine speed sensor 53 .

[0103] Various aspects and implementations have been described herein in conjunction with various embodiments. However, other variations of the disclosed embodiments may be understood and implemented by those skilled in the art in practicing the claimed subject matter, by studying the drawings, the present disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor, controller or other unit may perform the functions of multiple items recited in a claim. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A marine propulsion system, comprising: - a large two-stroke internal combustion engine, said engine having a crankshaft (22), - a shaft system (42) operatively connecting a propeller (44) to the crankshaft (22), During engine operation, the marine propulsion system exhibits torsional vibrations having a first torsional vibration mode frequency, - an engine speed sensor (52), the engine speed sensor (52) being configured to: measuring a speed of the marine propulsion system at a location on the marine propulsion system, and generating a signal representative of the measured speed, - a controller (50), the controller (50) receiving the speed signal, The marine propulsion system is characterized in that the controller (50) is configured to calculate the first torsional vibration mode frequency based on a signal from a sensor associated with the marine propulsion system, The controller (50) includes a computer model (56) of the marine propulsion system, The controller (50) is configured to use the calculated first torsional vibration mode frequency in the computer model (56) to estimate a weighted average engine speed, and the controller (50) is configured to use the weighted average engine speed to regulate the speed of the engine.

2. The marine propulsion system according to claim 1, wherein: The computer model (56) is an inertial model of the marine propulsion system.

3. The marine propulsion system according to claim 1 or 2, wherein: The computer model (56) is a lumped parameter model of the marine propulsion system.

4. The marine propulsion system according to claim 3, wherein: The lumped parameter model includes a first inertia (Ie) and a second inertia (Ip) corresponding to the engine side and the propeller side respectively, and the first inertia (Ie) and the second inertia (Ip) are connected by an elastic member whose inertia contribution can be ignored, and the elastic member represents the shaft system (42).

5. The marine propulsion system of claim 4, comprising a shaft generator (48), wherein: The lumped parameter model includes a third inertia (Is) corresponding to the inertia of the shaft generator.

6. A marine propulsion system according to any one of the preceding claims, wherein: The signal representative of the measured speed is the signal used by the controller (50) to calculate the frequency of the first torsional vibration mode.

7. The marine propulsion system according to claim 6, wherein: The controller (50) is configured to calculate the first torsional vibration mode frequency by applying a Fourier transform to the measured velocity signal.

8. A marine propulsion system according to any one of claims 1 to 5, wherein: The signal is a signal from a vibration sensor (53) configured to generate a signal representative of an axial vibration of the shafting (42).

9. The marine propulsion system of claim 8, wherein: The controller (50) is configured to perform a frequency analysis on the signal generated by the vibration sensor (53) to calculate the first torsional vibration mode frequency.

10. A marine propulsion system according to any one of the preceding claims, wherein: The controller (50) receives a signal indicative of a desired engine speed, and wherein the controller (50) is configured to regulate an amount of fuel supplied to the engine as a function of the desired engine speed and the weighted average engine speed.

11. A marine propulsion system according to any one of the preceding claims, wherein: The engine comprises a flywheel (41), and wherein, preferably, the engine speed sensor (52) measures the speed of the marine propulsion system at the flywheel (41) or on the shafting (42) at a position close to the flywheel (41).

12. A marine vessel (40) comprising a marine propulsion system according to any one of the preceding claims.

13. A method for controlling a marine propulsion system, the marine propulsion system comprising: - a large two-stroke internal combustion engine, said engine having a crankshaft (22), - a shaft system (42) operatively connecting a propeller (44) to the crankshaft (22), During engine operation, the marine propulsion system exhibits torsional vibrations having a first torsional vibration mode frequency, - an engine speed sensor (52), the engine speed sensor (52) being configured to: measuring a speed of the marine propulsion system at a location on the marine propulsion system, and generating a signal representative of the measured speed, - a controller (50), the controller (50) receiving the speed signal, The method is characterized in that The controller (50) includes a computer model (56) of the marine propulsion system, The method comprises: The controller (50) calculates the first torsional vibration mode frequency based on a signal from a sensor associated with the marine propulsion system, The controller (50) uses the calculated first torsional vibration mode frequency in the computer model (56) to estimate a weighted average engine speed, The controller (50) uses the weighted average engine speed to regulate the speed of the engine.

14. The method according to claim 13, wherein: The signal representing the measured speed is a signal used by the controller (50) to calculate the first torsional vibration mode frequency, and wherein the controller (50) calculates the first torsional vibration mode frequency by applying a Fourier transform to the signal representing the measured speed.

15. The method according to claim 13, wherein: The signal from the vibration sensor (53) is the signal used by the controller (50) to calculate the frequency of the first torsional vibration mode, the vibration sensor (53) is configured to generate a signal representing the axial vibration of the shaft system (42), and wherein the controller (50) is configured to: perform frequency analysis on the signal generated by the vibration sensor (53) to calculate the frequency of the first torsional vibration mode.

16. The method according to any one of claims 13 to 15, wherein: The controller (50) receives a signal indicative of a desired engine speed, and wherein the controller (50) regulates the amount of fuel supplied to the engine as a function of the desired engine speed and the weighted average engine speed.

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