A method for controlling a rotational speed (w) of an output shaft of a propulsion unit, said method comprising: - determining a speed setpoint value (ω s (k)), indicative of a rotational speed setpoint for said output shaft, and an actual speed value (w(k)), indicative of an actual rotational speed of said output shaft, - determining an acceleration value 〈( ω̇ s (k)〉) using an acceleration value determination procedure comprising employing an acceleration conversion function ( f ( ω s ( k), ω ( k), asm)) that uses said speed setpoint value (ω s (k)), said actual speed value (w(k)) and an acceleration setting member (asm) as inputs and which produces a resulting value 〈 ω̇ s (k)〉 to be used for determining said acceleration value said acceleration conversion function ( f ( ω s ( k), ω ( k), asm)) being such that different resulting values 〈 ω̇ s (k)〉 can be obtained for the same set of said speed setpoint value (ω s (k)) and said actual speed value (w(k)) but for different choices of said acceleration setting member (asm), - determining a torque request value ( T s ( k)) using a torque conversion operation that uses said acceleration value (〈 ω̇ s ( k)〉), wherein said torque conversion operation comprises forming a torque request sum (T(k)) by multiplying said acceleration value 〈( ω̇ s (k)〉) by a moment of inertia value ( Ĵ ( k)), indicative of a moment of inertia associated with said propulsion unit, and adding thereto a resistive torque value ( T̂ r ( k)), indicative of a resistive torque associated with said propulsion unit and wherein said torque conversion operation further comprises adding a torque disturbance value ( T̂ d ( k)), indicative of torque disturbance associated with said propulsion unit, to said torque request sum (T(k)), wherein said torque disturbance value ( T̂ d ( k)) is determined using values determined for at least one previous time instant for each one of an actual torque value ( T a ( k - 1)), indicative of an actual torque of said output shaft, said moment of inertia value ( Ĵ ( k - 1)), said resistive torque value ( T̂ r ( k - 1)), said torque disturbance value ( T̂ d (k - 1)) and an actual acceleration value ( ω̇ (k)), indicative of an actual acceleration of said output shaft, and - controlling said propulsion unit using said torque request value. - determining a speed setpoint value (ω s (k)), indicative of a rotational speed setpoint for said output shaft, and an actual speed value (w(k)), indicative of an actual rotational speed of said output shaft, - determining an acceleration value 〈( ω̇ s (k)〉) using an acceleration value determination procedure comprising employing an acceleration conversion function ( f ( ω s ( k), ω ( k), asm)) that uses said speed setpoint value (ω s (k)), said actual speed value (w(k)) and an acceleration setting member (asm) as inputs and which produces a resulting value 〈 ω̇ s (k)〉 to be used for determining said acceleration value said acceleration conversion function ( f ( ω s ( k), ω ( k), asm)) being such that different resulting values 〈 ω̇ s (k)〉 can be obtained for the same set of said speed setpoint value (ω s (k)) and said actual speed value (w(k)) but for different choices of said acceleration setting member (asm), - determining a torque request value ( T s ( k)) using a torque conversion operation that uses said acceleration value (〈 ω̇ s ( k)〉), wherein said torque conversion operation comprises forming a torque request sum (T(k)) by multiplying said acceleration value 〈( ω̇ s (k)〉) by a moment of inertia value ( Ĵ ( k)), indicative of a moment of inertia associated with said propulsion unit, and adding thereto a resistive torque value ( T̂ r ( k)), indicative of a resistive torque associated with said propulsion unit and wherein said torque conversion operation further comprises adding a torque disturbance value ( T̂ d ( k)), indicative of torque disturbance associated with said propulsion unit, to said torque request sum (T(k)), wherein said torque disturbance value ( T̂ d ( k)) is determined using values determined for at least one previous time instant for each one of an actual torque value ( T a ( k - 1)), indicative of an actual torque of said output shaft, said moment of inertia value ( Ĵ ( k - 1)), said resistive torque value ( T̂ r ( k - 1)), said torque disturbance value ( T̂ d (k - 1)) and an actual acceleration value ( ω̇ (k)), indicative of an actual acceleration of said output shaft, and - controlling said propulsion unit using said torque request value.