The present invention concerns a method and a means for achieving optimum utilization of the propulsion engine of a vessel, and more specifically, optimum operation of a ship's propeller in relation to an economic utilization of fuel, in relation to the cavitation problem (formation of bubbles in metal) on the propeller surface, as well as in relation to increased manoeuvering safety at maximum utilization of the propeller performance.

This principle can be exploited in different manners, e.g. in connection with ships in regular service between harbours, where it is possible to achieve a more precise timing regarding arrival and quay bookings, since an elec¬ tronically controlled propeller capacity provides the ability for calculating the precise time of arrival (slot time) .

In other cases, it may be desirable from a charterer's view that a closer specified "cost price", i.e. average velo¬ city possibly can be determined.

In time chartering, the freight rate will be determined by speed and consumption, i.e. when tonnage can be regarded as equal in other respects, that tonnage will be preferred which can guarantee the lowest consumption of bunker fuel at a certain stated velocity.

In situations of crisis in the form of risk of running aground or danger of collision with other vessels, it is very important to be able to stop the ship as fast as possible. Lately, high speed sea buses appear in increasingly larger numbers in narrow fiords and closed waters where the traffic of ships and small boats is substantial, and it is therefore important to be able to stop the vessel rapidly.

In all cases, the present invention is based upon an optimum utilization of the action of the single propeller blade during motion in water. Under stormy conditions with high waves, the ship will experience a constantly varying resistance to its motion through the water, sometimes with the

propeller more or less freely rotating in the air, and with a subsequent variation of the propeller power. The consequence thereof is an engine load with unefficient utilization of the propeller, with a subsequent reduction in speed and possible cavitation of the propeller blade surface on the lee side.

The optimum utilization of the propeller will therefore be closely connected with the ability of the ship to overcome the water resistance during motion.

Traditionally, most ship's propellers in larger ships are moulded in one piece, without any possibility of turning the attack angle of each propeller blade. Smaller vessels have in many cases variable-pitch propellers, however, the development now shows that increasingly larger ships find advantages in using such propeller types with twistable blades.

The construction of a ship is often made on the basis of a predetermined normal velocity, and it is left to the desig¬ ner in the shipyard to find the most favourable shape of hull and propeller in order to satisfy such a requirement.

The penetration ability of the hull through the water, or expressed inversely, the resistance to the ship's motion, will vary with draught and load. The attack angle of the propeller or the propeller blade in order to achieve optimum efficiency will therefore also vary, so that a fixed, i.e. not variable- pitch propeller must be chosen using an average consideration. Outside this average, the propeller will not provide optimum efficiency. It is therefore clear that a variable-pitch solution is preferable, however, this has clearly been diffi¬ cult in cases of large dimensions, partly due to cost savings, partly due to causes connected with technological development.

Parameters influencing the propulsion of a ship in water are draught, wave resistance, induced resistance, wind and weather. Of these parameters, draught and induced resistance are given for one single voyage. (In another voyage, another draught may be present) . The other parameters, like wave resistance, wind and weather, will vary all the time.

If one takes as a starting point a situation with a given draught and a given velocity, which may represent an optimum

working situation, then both an increase and a decrease of the velocity will imply increased total expenses. In the first case, disproportionate amounts of fuel are used (unlinear relation between fuel consumption and velocity) , and in addi¬ tion, engine wear is increased, and also the risk of propeller blade cavitation, with the consequences of increased costs as to maintenance and repair. In the second case (lowered velo¬ city) the results are prolonged time at sea with increased salary expenses, later time of arrival and the consequences due hereto regarding lower possibility of profits.

A variable-pitch propeller is that part of the solution which may be compared to driving a car with manual gear-shift, but a continuously manual "shifting of gears", e.g. in a storm, would be inconceivable. An automatic mechanism would be preferable, in the form of "measuring force in real time".

The best manner in which to utilize the propeller maxi¬ mally, is to find that balance point for the attack angle of the propeller blades which results in the best utilization of the applied force.

From Swedish laid-open publications no. 345.634 and 350.938 are previously known methods of controlling the load on ship engines in connection with variable-pitch propellers, where the propeller blade attack angle or "pitch" is con¬ trolled in relation to the sensed shaft torque, i.e. sensing of the torque on the propeller shaft, while one attempts to maintain the engine rp at a constant value. Mainly, these systems relate to an overload protection for the engine, and the main point is filtering and delaying of signals in order to avoid too rapid oscillations when adjusting the propeller pitch.

From Norwegian patent no. 152.968 is previously known a method of regulating the engine of a vessel with a variable- pitch propeller, however, in this case control is only effec¬ ted in relation to measured values of speed, fuel consumption and rpm. There is no direct measurement of the vessel's driving force.

Also, British patent no. 1.200.588 deals with the control of variable-pitch propellers, however, the parameter sensed in the regulating circuit, is only how large a current is de¬ livered to an electric drive motor.

None of these previous publications go to the core of the matter, namely a direct sensing of the force with which the propeller influences the ship at the present moment.

The present invention aims at providing an improved control system by providing a method and a system for ob¬ taining optimum propulsion of the ship. The invention is defined precisely by means of the appended patent claims.

One attempts with the system and method in accordance with the present invention to obtain the optimum propulsion power, F. , in relation to a predetermined intention.

This is done by finding the optimum rpm or number of revolutions in relation to the attack angle for each propeller blade.

At a given rpm for the propeller, under influence from a given engine power, there is a given attack angle for achie¬ ving a given propulsion force F, . Thus, there exists a precise balance between forces.

If an increase of F. is desirable, it is possible to increase the rpm, or to increase the attack angle with the same engine power, or both these measures can be taken simul¬ taneously.

The purpose of this arrangement is to achieve the best possible F- .

As can be visualized, there is a precise connection between applied engine power or rpm on the shaft, and the twist or the pitch of the propeller blades for finding the most favourable combination for achieving the best possible

^F h ^*

This force is most appropriately read directly in the propeller thrust bearing of the vessel. Usually, this bearing is located near the engine. Using the invention, there is achieved a self-tracking towards the optimum propulsion force

by measuring this force continuously, i.e. real time measure¬ ment of the force F. for control purposes.

A computer hunts continuously for the presence of a balance between delivered fuel/engine power and the efficiency of the propeller in the form of torque and number of revolu¬ tions, i.e. there is at all times an attempt to find an opti¬ mum yield of force for the propeller which is read and veri¬ fies that the propeller efficiency will be balanced against the ship's velocity and possible external influences. In other words, if it is desirable with the longest possible, or optionally the best possible covered distance per ton of bunker fuel, then the pressure force from the propeller must be controlled in such a manner that the ship moves within this range of optimum performance of propeller/engine.

It should be mentioned that e.g. a fully loaded 100.000 ton vessel with a speed of about 14 knots, consumes about 40 tons of fuel in 24 hours, and that the same ship needs 30/40 minutes or 5/7 nautical miles to stop from full speed with a traditional propeller system. Further, such a ship needs about 40 minutes or 4/6 nautical miles from standstill to full speed when a traditional system is used. This means that an optimum performance propeller can give a substantial contribu¬ tion both as to increasing safety regarding danger of colli¬ sion and running aground, and furthermore contributes to an improved fuel economy. The present invention also has the advantage that it will provide optimum operation at all speeds. A system in accordance with the present invention will be able to provide both optimum utilization of the pro¬ peller efficiency in economic cruise control, and optimum efficiency during breaking and acceleration.

The invention shall be described in more detail below, with reference to the drawings, where:

Fig. 1 shows an example of mounting of a force sensor at a ship's propeller shaft,

Fig. 2 shows schematically the location of various control elements in the ship, as well as signal paths within the control system, and

Fig. 3 shows the same as Fig. 2 in the form of a block diagram.

In Fig. 1 is shown the general principle for reading the propulsion force F. in accordance with the invention. The propeller 1 is visualized as a propeller of the variable-pitch type (but may also be of the type with fixed blades) . A load cell 3 (force sensor) reads F. against the thrust bearing 4 of the propeller shaft 2, forwards and backwards, possibly in the rear sleeve 5 ("stern tube") . The measurement signals from load cell 3 are applied to a computer 6 of the microprocessor type, which in principle executes the following operation, compare Figs. 2 and 3: a) successively and with short intervals, the present pressure force from propeller 1, F- . is read/measured, and is related to the ship velocity, b) the present pressure force, F. , is all the time compared to the engine 8 power P , and attempts are continu¬ ously made via a correction circuit to maintain the optimum attack angle (pitch angle) for each propellor blade, and optionally, the optimum speed of the propellor shaft 2 for achieving optimum pressure force (with variable-pitch propel¬ ler blades) . For propellers with fixed propeller blades, only the speed of the propeller shaft, i.e. the shaft rpm, will be the determining parameter (approximate optimum operation) . c) The present pressure force F, is compared to the desired F, (optimum or pre-set F, , Fig. 3, "Auto-optimum"). d) In accordance with a further defined programme, the detailed description of which should be unnecessary in this specification, the computer controls the fuel control valve 7 which in its turn controls the force output from engine 8, at the same time as the pitch angle (or in the case with fixed blades, the rpm) is varied to optimum or pre-set F, by means of the propeller pitch control 9. e) In the case of variable-pitch propeller blades, the computer 6 also controls the number of revolutions (rpm) of shaft 2, so that the propeller does not enter into a stalling

situation, with formation of a vacuum adjacent to the propel¬ ler blades. f) The strategy of the programme is as described in the following: As long as a measured F. is larger than the pre¬ determined value, the attack angle of the propeller blades is lowered (or possibly the rpm in the fixed blade case) , and in combination with a decreased fuel delivery, and when large variations occur, also the rpm, even if the propellor is of the variable-pitch type. When F. is less than the predeter¬ mined value, the attack angle of the propeller blades is increased (and possibly also the rpm is increased in the fixed blade case) , and in combination also the fuel delivery is increased, and when large variations occur, also the rpm, even if the propellor is of the variable-pitch type. This de¬ scribes a cycle which starts again and again.

In this manner, there is provided a rapid hunting of and "commuting around" the summit of interest for an optimum utilization of a ship's propeller.

The possibility of using a force sensor 3 mounted in the thrust bearing 5 both for forward and reverse propulsion, was mentioned above, and there should be no problem using "double action" force sensors or load cells. Suitable load cells 3 may exist in many embodiments. For example, strain gauges, semiconductor force sensors or piezoelectric sensors of per se known types may be used. g) The computer forming the basis of the control loop is adapted in such a manner that the operator, for example the ship's captain, can select a programme (Fig. 3, "Select/ control") . This may very well be connected with the ship's navigation system in order to comprise the route structure and the time aspect in addition to the optimum economical opera¬ tion. h) The computer has also been fed with all structural limitations, e.g. engine limitations, so that at certain programming choices, the computer limit values which corre¬ spond to the structural limitations, will override the opera¬ tor's programming. A warning system (Fig. 3, "Display") then

warns the operator, e.g. that this manoeuvre is not possible due to an excessive exhaust temperature, or that the manoeuvre is executed, however will be limited at maximum exhaust tem¬ perature.

There are clear possibilities of a better economic utili¬ zation of the fuel in relation to the covered distance. A large ship needs a considerable time and distance in order to reach its cruising speed. Until harmony between the ship's velocity and the propulsive force, F, , has been achieved, a traditionally driven propeller will work outside its optimum working range, and therefore, it will also be prone to cavita¬ tion. During braking (reversing) , this may happen as a conse¬ quence of an emergency stop, i.e. in connection with a risk of collision, running aground etc.

Particularly for high speed passenger vessels in closed waters with heavy traffic, the need of rapid stopping is essential to be able to solve a rapidly arisen situation of crisis. A quick reaction of reversing the propeller is a usual and natural reaction from the shipmaster. The risk of stalling the propeller is then imminent, and is a usual experience. The propeller will only spin without any effect, while the vessel moves rapidly further on.

Clear possibilities exist in this case for using a so- called "panic button" or a selected "Optimum reverse" in the form of a pre-set optimum programme in connection with the present invention.