DESCRIPTION

The present invention relates to an electric propulsion system for a boat.

As is known, modern boats, whether vessels or pleasure boats, have hulls that offer much higher dynamic performance (i.e. manoeuvering capability) than those of just a few years ago. This is due to the materials employed for building them and to the way in which they are designed and constructed, which make it possible to produce hulls having good performance at relatively low costs.

For these boats to reach the design-specified performance levels, they need to be equipped with propulsion units capable of producing maximum power outputs at the motor shaft of the order of tens of kilowatts.

When an electric propulsion unit is employed, it becomes apparent that, in order to obtain a maximum power output of the order of tens of kilowatts, the electric motor needs to be supplied with an electric current having an intensity of hundreds of Amperes and a voltage of tens of Volts.

In these operating conditions, the scattered magnetic field produced by the stator power supply or an electric leakage caused by a worn insulation of the windings of said stator will be able to produce such galvanic currents that would presently result in damages to the boat; in particular, damages would result in the metallic parts of the boat which are in direct contact with the water on which the boat is floating, such as, for example, the screw and/or the impeller of a water jet pump, the cooling water intake duct, etc. In fact, an electric current of a few Amperes flowing through a metallic part immersed in water will cause said part to wear out very quickly, because that part will act as an anode and will therefore be corroded/worn by an electrolytic process. It follows that, in a boat equipped with an electric propulsion system, the problem of the corrosion caused by galvanic currents is much more important than in a boat having a thermal propulsion system, since the intensity of such galvanic currents may be greater by two or more orders of magnitude than that of the galvanic currents generated in a boat equipped with a thermal propulsion system.

This increases the risk that a boat equipped with an electric propulsion system may suffer from propulsion problems (e.g. when the screw or the water jet pump are corroded) and/or floating problems (e.g. when the cooling water intake duct (also known as "seacock") , which is usually located under the water line, is corroded) , because the galvanic currents that can be generated may be so intense as to damage the boat in a few minutes.

The present invention aims at solving these and other problems by providing an electric propulsion system for a boat as set forth in the appended claim 1.

The basic idea of the present invention is to galvanically isolate a propelling impeller of a boat (e.g. a screw or an impeller of a water jet pump, also referred to as "turbine") from a rotor of an electric motor.

In this way, it is possible to extend the service life of a boat, in particular of the motor assembly of said boat, i.e. it is possible to increase the mean time between failures (MTBF) , the operating conditions being equal (e.g. the total time during which the motor is operated at maximum power, the degree of corrosion and/or the salinity of the water on which the boat is floating, etc.).

Further advantageous features of the present invention are set out in the appended claims .

These features as well as further advantages of the present invention will become more apparent from the following description of an embodiment thereof as shown in the annexed drawings, which are supplied by way of non-limiting example, wherein :

- Fig. 1 illustrates a first embodiment of an electric propulsion system for a boat according to the invention;

- Fig. 2 illustrates a first variant of the electric propulsion system shown in Fig. 1.

Any reference to "an embodiment" in this description will indicate that a particular configuration, structure or feature is comprised in at least one embodiment of the invention. Therefore, the phrase "in an embodiment" and other similar phrases, which may be present in different parts of this description, will not necessarily be all related to the same embodiment. Furthermore, any particular configuration, structure or feature may be combined in one or more embodiments as deemed appropriate. The references below are therefore used only for simplicity' s sake and do not limit the protection scope or extent of the various embodiments.

With reference to Fig. 1, the following will describe an electric propulsion system 1 according to the invention, which can be installed in a boat; one such propulsion system 1 preferably comprises the following parts:

- motor means 11 comprising an electric motor 111 and a speed regulator 112 (also known as "inverter") that supplies power to said motor 111, thus allowing the latter to turn at different revolution speeds;

- power supply means 12 (such as, for example a battery pack, one or more fuel cells, or the like) , which supply power to said motor means 11, preferably via a pair of electric cables or insulated bars; in particular, said power supply means preferably supply power to the speed regulator 112 in a direct manner, i.e. without the interposition of any electronic device;

- a heat exchanger 13 adapted to dissipate, preferably in the water on which the boat is floating, the heat generated by the operation of the motor means 11, i.e. the motor 111 and/or the speed regulator 112 _/

- an impeller 14 (e.g. a screw or a turbine of a water jet pump) coupled to the motor shaft of the electric motor 111 of the motor means 11, wherein said impeller 14 can generate, when driven by the motor means 11 (i.e. when it is turning) , a water flow capable of producing a thrust that can move the boat;

- an isolating joint 15 positioned in such a way as to galvanically isolate the impeller 14 from the motor means 11;

- a sacrificial anode 16 adapted to protect the impeller 14 against corrosion. This anode 16 must be in electric communication, preferably via a cable, with the impeller 16 and must be positioned in such a way as to stay immersed in the water on which the boat is floating; such sacrificial anode 16 is made of a material having higher electronegativity than the material used for manufacturing the impeller 14, e.g. zinc or an alloy thereof.

The heat exchanger 13 preferably comprises a coolant delivery duct 131 and a coolant intake duct 132, through which a (first) flow of cooling liquid, e.g. a mixture of demineralized water and ethylene glycol, is made to circulate by means of a pump (not shown in the annexed drawings) ; in addition, said heat exchanger 13 also comprises a water intake duct 133 and a water drain duct 134, through which a (second) flow of cooling liquid is made to circulate, which preferably consists of the water on which the boat is floating.

This cooling liquid puts the heat exchanger 13 in thermal communication with the motor means 11, which are advantageously prearranged for circulating the cooling liquid (preferably composed of demineralized water and ethylene glycol) in proximity to the areas where heat production is highest, such as, for example, the stator windings of the electric motor 111 and the MOSFET transistors (or other types of transistors) used by the speed regulator 112. In this manner, the first flow of cooling liquid carries the heat from the motor means 11 towards the exchanger 13, so that all or some of the heat produced by the motor means 11 is exchanged with the second flow of cooling liquid, which will dissipate such heat into the water volume around the boat.

As already mentioned, and more in detail, the material normally used for making the sacrificial anode 16 is zinc (when the boat is to be used in sea waters) or an alloy of zinc and magnesium (when the boat is to be used in "fresh waters", like those found in lakes and rivers) . The sacrificial anode 16 is useful to avoid that, should any galvanic currents be generated, this will result in wear of parts of the boat that are necessary for its operation (e.g. the screw) and/or for its buoyancy, i.e. all those metallic parts that lie under the water line and that must prevent water from entering the hull of the boat, such as, for example, the water intake duct 133. In fact, the high electronegativity of the material used for making the sacrificial anode 16 ensures, in the presence of galvanic currents, that the sacrificial anode 16 will wear first, thus safeguarding the other metallic parts of the boat.

It is clear that, although the sacrificial anode 16 can prevent damage to the boat for a few years in the presence of weak galvanic currents (i.e. of the order of milliamperes ) , or for a few hours if currents of a few Amperes are generated (e.g. caused by a short circuit in an electric appliance aboard the boat) , said anode 16 cannot safeguard the integrity of the boat in the event that galvanic currents of tens or hundreds of Amperes are generated, caused by the scattered magnetic field of an electric motor and/or by electric dispersion due to degradation of the insulating material (also called varnish) of the stator coils. In the presence of such intense galvanic currents, in fact, the sacrificial anode 16 will wear out within minutes, leaving the boat with no protection, and hence exposed to the effects of galvanic currents .

In its most essential embodiment, therefore, the electric propulsion system 1 according to the invention comprises the impeller 14 capable of generating a thrust that can move said boat and the electric motor 11 coupled to said impeller 14 and causing it to rotate, wherein said electric motor 111 is galvanically isolated from said impeller 14.

This makes it possible to reduce by several orders of magnitude the intensity of the galvanic electric currents induced by the scattered magnetic field generated when power is supplied to the stator windings of the motor 111, or which are due to a dispersion caused, for example, by wear of the insulating material that insulates the conductors used for making the stator windings of the motor 111.

It is thus possible to extend the service life of a boat, in particular of the motor assembly of said boat.

More in detail, the electric motor 111 comprises a rotary shaft 1111, which in turn comprises a first end 1112 and a second end 1113, wherein such ends 1112,1113 are galvanically isolated from each other.

This galvanic isolation can be accomplished in different ways, e.g. by making at least a portion of the motor shaft from an electrically insulating material. In the preferred embodiment, the rotary shaft 1111 may comprise an isolating joint 15 positioned between the first end 1112 and the second end 1113. This provides galvanic isolation between the motor 111 and the impeller 14, thereby extending the service life of a boat, in particular of the motor assembly of said boat.

The isolating joint 15 is preferably made by using a pair of flanges with insulating material (e.g. rubber) in between, preferably cylindrical in shape, wherein said insulating material is coupled to each one of the flanges by the friction generated when said flanges are pushed against said joint and/or by fastening means, e.g. screws, which can be engaged into threaded holes suitably arranged in the portions of insulating material in contact with said flanges.

It must be pointed out that said joint 15 must be selected as a function of the revolution speed of the motor 111, so as to ensure appropriate balancing and possibly compensate for any misalignment between the motor (e.g. the end 1112 of the motor shaft 1111) and the shaft portion coupled to the impeller 14 (i.e. the end 1113). Moreover, if the insulating material is rubber, the joint 15 also acts as a flexible coupling, thus protecting the integrity of the motor 111 and of the impeller 14.

Of course, the example described so far may be subject to many variations.

A first variant is shown in Fig. 2; for brevity, the following description will only highlight those parts which make this and the next variants different from the above- described main embodiment; for the same reason, wherever possible the same reference numerals, with the addition of one or more apostrophes, will be used for indicating structurally or functionally equivalent elements.

With reference to Fig. 2, the following will describe a first variant of the above-described electric propulsion system 1; such electric propulsion system 1' comprises, in addition to the elements already described, monitoring means 17 (e.g. a CPU, a microcontroller or the like) adapted to detect a risk situation that may jeopardize the integrity of the boat, in particular the integrity of the impeller 14. More in detail, the monitoring means 17 are configured for executing the following steps: - checking that the electric motor 111 is galvanically isolated from the rest of the boat, in particular from the impeller 14;

- signalling the occurrence of risk situations in the event that said electric motor 111 is not galvanically isolated from the rest of the boat (e.g. by activating signalling means, such as a warning light positioned on the dashboard, an audible alarm, a liquid crystal display displaying a message indicating the occurrence of the risk situation, or the like) .

This makes it possible to detect the occurrence of risk situations that, should a strong galvanic current be generated by the operation of the motor 111, would result in damage to the boat, in particular to the impeller 14 or other vital parts (e.g. the water intake duct 133) . In this way, it is possible to extend the service life of a boat, in particular of the motor assembly of said boat.

More in detail, the monitoring means 17 may comprise a first electric resistance sensor 171 adapted to sense a first electric resistance between the motor means 11 (i.e. the motor

111) and the heat exchanger 13. This permits the monitoring means 17 to detect, and hence indicate, the presence of a

(first) risk condition in the event that said electric resistance decreases to a first threshold, which is preferably a value comprised between 1 and 1.5 MegaOhms. It must be pointed out that the value of the resistance between the motor means 11 and the heat exchanger 13 is typically in the range of 1.5 to 10 MegaOhms when the isolation is correct.

In fact, a reduced electric resistance between the motor means 11 and the heat exchanger 1 may be indicative of

(anomalous) entrance of sea water into the cooling liquid circulating in the ducts 131,132, because the salts in solution in sea water will cause the specific electric resistance (i.e. the electric resistivity) of the solution of water and ethylene glycol to decrease. This situation may occur when the heat exchanger 13 is damaged by the corrosion undergone by a boat in the course of its life; the entrance of salt water into the ducts 131,132 is a risk situation that must be remedied because it leads to increased corrosion within said motor means 11 (thus disadvantageously reducing the service life of said motor means 11) and allows the establishment of galvanic currents that will result in wear of the water intake duct 133 (thus disadvantageously increasing the risk of shipwreck) and/or of the impeller 14 due to the presence of salts dissolved in the water on which the boat is floating .

It is thus possible to extend the service life of a boat, in particular of the motor assembly of said boat.

In combination with or as an alternative to the above, the monitoring means 17 may comprise a second electric resistance sensor 172 adapted to sense a second electric resistance between the motor means 11 (i.e. the electric motor 111) and the impeller 14. This permits the monitoring means 17 to detect, and hence indicate, the presence of a (second) risk condition in the event that said electric resistance decreases to a second threshold, which is preferably a value comprised between 5 and 10 MegaOhms. It must be pointed out that the value of the resistance between the motor means 11 and the impeller 14 is typically in the range of 10 to 100 MegaOhms when the isolation is correct. In fact, a reduced electric resistance between the motor means 11 and the impeller 14 may be due to the fact that the isolating joint 15 is wet or has been greased with grease containing metal particles (e.g. titanium) or graphite. This makes it possible to avoid the occurrence of conditions in which the impeller 14 could be damaged as a result of an electric dispersion in the motor means 11. It is thus possible to extend the service life of a boat, in particular of the motor assembly of said boat.

In combination with or as an alternative to the above, the monitoring means 17 may comprise a third electric resistance sensor 173 adapted to sense a third electric resistance between the impeller 14 and the sacrificial anode 16. This permits the monitoring means 17 to detect the presence of a (third) risk condition in the event that said electric resistance increases until it exceeds a third threshold, which is preferably a value in the range of 1 to 15 Ohms, preferably 10 Ohms. It must be pointed out that this system may require periodic calibration, e.g. when changes are made to the hull of the boat, such as replacement of the sacrificial anode 16 or replacement of an electric conductor (e.g. a twisted copper wire or the like) that connects the impeller 14 (or the second end 1113 of the transmission shaft 1111) to said sacrificial anode 16.

In fact, an increased electric resistance between the impeller 14 and the sacrificial anode 16 may be due to the fact that said sacrificial anode 16 is excessively worn, so that the user of the boat can be alerted (e.g. by a message displayed on a liquid crystal display, a lit warning light, or the like) that said anode 16 needs to be replaced during a maintenance session.

This avoids the occurrence of conditions in which the impeller 14 could be damaged because of an electric dispersion in the motor means 11.

It is thus possible to extend the service life of a boat, in particular of the motor assembly of said boat.

In combination with or as an alternative to the above, the monitoring means 17 may comprise a fourth electric resistance sensor (not shown in the annexed drawings) adapted to sense a fourth electric resistance between the sacrificial anode 16 and the exchanger 13, preferably between the sacrificial anode 16 and the water intake duct 133. This permits the monitoring means 17 to detect the presence of a (fourth) risk condition in the event that said electric resistance exceeds a fourth threshold, which is preferably a value in the range of 5 to 100 kiloOhms. It must be pointed out that the threshold value can be more effectively selected by taking into account the salinity in which the boat is immersed and/or the position of the electrode in the water. Therefore, this system may require periodic calibration, e.g. when seasons change (because of a different water temperature) , when the boat enters, from the sea, into a lagoon or a river, or vice versa, or when mechanical changes are made to the hull of the boat.

In fact, a high value of the electric resistance between the sacrificial anode 16 and the water intake duct 133 may be due to the use of an improper sacrificial anode 16, i.e. one made of materials unsuitable for the application, so that the user of the boat can be alerted (e.g. by a message displayed on a liquid crystal display, a lit warning light, or the like) that the anode 16 should be replaced as soon as possible, perhaps with one that is more suited to the type of water on which the boat is floating.

This also makes it possible to detect an incorrect operation of the sacrificial anode 16 due to improper maintenance; in fact, it may happen that sacrificial anodes installed during maintenance sessions have a wrong composition (e.g. contain a high percentage of lead) , which disadvantageously cancels their protective effect against corrosion. In fact, the anodes should have a composition with a high percentage of zinc, preferably higher than 95% in weight, but actually they often contain lead in significant percentages and, rarely, magnesium or aluminium.

When the monitoring means 171 comprise at least two electric resistance sensors (e.g. the sensors 171,172,173), each one adapted to sense an electric resistance between two parts of the boat, such monitoring means 17 can be configured for executing the following steps:

- sensing at least two electric resistance values by means of said at least two electric resistance sensors 171,172,173;

- determining a ratio (preferably an arithmetic ratio) between said at least two electric resistance values;

- signalling, on the basis of said ratio, the occurrence of at least one risk situation that may jeopardize the integrity of the boat, e.g. the occurrence of said at least one risk situation will be signalled if the ratio value is not within a predefined range.

In this way the occurrence of a risk situation can be detected in a more accurate manner than would be possible by using resistance values coming from one sensor only. For example, when the electric resistance measured by the first sensor 171 decreases and the electric resistance measured by the third sensor 173 increases, this means that a risk situation is arising wherein the anode is wearing out and the heat exchanger 13 is getting damaged (probably because of the effect of galvanic currents) ; or, when the electric resistance measured by the second sensor 172 decreases and the electric resistance measured by the third sensor 173 increases, this means that a risk situation is arising wherein the anode is wearing out and the rotary shaft 1111 is wet, probably because of a leak in the hull caused by galvanic currents due to an electric problem in the motor means 11.

Furthermore, this feature makes it possible to determine the occurrence of a risk situation without the need for specific calibrations of every single sensor 171,172,173, the characteristics of which may change over the service life. In fact, after putting the boat in water (preferably after a maintenance session) , it will be sufficient to determine the predefined interval on the basis of (the value of) an (arithmetic) ratio determined between two resistance values sensed by said at least two sensors 171,172,173 when the boat is in water, e.g. by determining the lower limit of the predefined interval as 90% of the value of said ratio and the upper limit of said predefined interval as 110% of the value of said ratio.

This simplifies the use of the system according to the invention, thereby extending the service life of a boat.

Although this description has tackled some of the possible variants of the invention, it will be apparent to those skilled in the art that other embodiments may also be implemented, wherein some elements may be replaced with other technically equivalent elements. The present invention is not therefore limited to the illustrative examples described herein, since it may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the basic inventive idea, as set out in the following claims.