Patent Description:
Marine fouling is a well-known problem for many marine applications. The build-up of marine organisms such as algae, mussels and barnacles on the exterior surfaces of the hulls and propulsion units of marine vessels will result in reduced performance, due to the increased resistance between the hull and water flowing past the hull. This will in turn result in increased fuel consumption. It is of particular interest to keep the propeller clean because of the increased drag effect from marine growth on propeller blades moving at high speed through the water. In severe cases, hull resistance and propeller drag might result in loss of maneuverability, which can become a safety concern. In addition, sea water is a corrosive environment for most metal parts used for marine propulsion units, which require cathodic protection not to corrode.

An efficient way of providing corrosion and marine growth protection is the use of a method termed impressed current cathodic protection (ICCP). ICCP systems are often used on cargo carrying ships, tankers and larger pleasure craft. <CIT> discloses the general principle for an ICCP system wherein a metal element and an anode are attached to a vessel and immersed in water. The metal element is connected to the positive terminal and the sacrificial anode is connected to the negative terminal of a source DC electrical power to provide an electric de-passivation current through an electrical circuit including the sacrificial anode, the metal element and the electrolyte. In this way, the anode provides corrosion protection for the metal part. <CIT> describes a propeller antifouling method and arrangement wherein to an existing ICCP system at the stern of a ship is added an antifouling mode by reversing the current between the propeller and an auxiliary anode, both connected to a power source.

A problem with a standard ICCP system is that they can be quite bulky. For larger vessels this is less of a problem, as the increase in drag caused by externally mounted ICCP units is small in relation to the drag of a relatively large hull. For relatively small vessels, however, the problem of added drag and/or limit available space on or near the transom can become an issue. For vessels used as pleasure craft, externally mounted ICCP units can also cause aesthetic issues.

A further problem is that many types of relatively smaller vessels equipped with, for instance, stern drives or outboard engines can have very limited physical space available on the transom or the hull where ICCP units could be fitted. Vessels of this type are usually provided with less efficient passive sacrificial anode protection.

The invention provides a marine propulsion system and a method for protecting a marine vessel aiming to solve the above-mentioned problems.

An object of the invention is to provide a cathodic protection and anti-fouling capabilities for a marine propulsion system, which solves the above-mentioned problems.

The object is achieved by a marine propulsion system and a method according to the appended claims.

In the subsequent text, the cathodic protection and anti-fouling arrangement is described for application to a marine propulsion system in the form of a stern drive mounted to a transom on the vessel. However, the inventive arrangement is also applicable to, for instance, azimuthing or pod drives and outboard drives. The cathodic protection and anti-fouling arrangement involves an impressed current cathodic protection (ICCP) arrangement which is operated using direct current (DC). In the subsequent text, the power source used for supplying DC power to the arrangement is not necessarily a battery, but can be any suitable source of electrical power such as a fuel cell or a source of alternating current (AC) provided with an AC/DC rectifier.

According to a first aspect of the invention, a marine propulsion system is provided with a cathodic protection and anti-fouling arrangement. The marine propulsion system comprises at least one driveline housing at least partially submerged in water, a torque transmitting drive shaft extending out of each driveline housing and at least one propeller mounted on the drive shaft. According to the invention, the at least one propeller is electrically isolated from its drive shaft and each electrically isolated propeller is connected to a positive terminal of a direct current power source. The vessel can comprise one or more driveline housings comprising a single drive shaft with a propeller, or counter-rotating propellers with coaxial drive shafts. The anti-fouling arrangement uses at least one or preferably all propellers making up the propulsion system. Simultaneously, the arrangement provides cathodic protection, wherein each metallic component to be protected against corrosion is connected to a negative terminal of the direct current power source. A control unit is arranged to regulate the voltage and current output from the direct current power source.

The cathodic protection and anti-fouling arrangement is an impressed current cathodic protection (ICCP) arrangement and at least one propeller is used as an anode. The at least one metallic component to be protected forms a cathode and can be the at least one driveline housing, at least one trim tab, seawater intake, swimming platform and/or at least a portion of the vessel hull. Note that this is a non-exclusive list of metallic components suitable for marine growth and corrosion protection. At the same time, the ICCP arrangement provides marine growth protection for the at least one anode.

According to one example, the at least one propeller is electrically isolated from its drive shaft by a torque transmitting electrically isolating component mounted between the at least one propeller and its respective drive shaft. The electrically isolating component is mounted in a gap formed by the outer surface of the drive shaft and the inner surface of the propeller hub. The torque transmitting electrically isolating component can be made from an elastic material, such as a natural or synthetic rubber. The at least one propeller is made from an inert anode material, such as titanium, niobium or a similar suitable metal or metal alloy.

According to a further example, a dielectric shield can be provided between the at least one propeller and the drive shaft on which the propeller is mounted. A dielectric shield is used as an electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. In this arrangement the dielectric shield is used to protect the surface of the drive shaft near the propeller hub from hydrogen embrittlement and local overprotection caused by unacceptably high potentials in areas adjacent the at least one propeller that is used as an anode. Local overprotection can cause adjacent surfaces of the drive shaft to become too negatively polarized, wherein a dielectric shield is provided to prevent high current densities on those surfaces.

The dielectric shield can comprise a layer of dielectric material extending along the drive shaft over at least the entire axial extension of the propeller hub. A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. A non-exclusive list of suitable materials for use in such a dielectric shield includes polymer or polymer-ceramic materials with suitable dielectric properties.

According to a further example, the propeller can be connected to the positive terminal of the direct current power source by wiring extending through a hollow portion of the drive shaft. For instance, an axially extending internal groove can be provided in the inner surface of the drive shaft and can be used for the electrical wiring. Alternatively an external groove in the outer surface of the drive shaft can be used for the electrical wiring to the at least one propeller. The electrical wiring can be electrically connected to the hub of the propeller by means of a wiping contact within the hub portion surrounding the drive shaft or a wiping contact located inside the transmission housing.

According to a further example, the cathodic protection and anti-fouling arrangement comprises a reference electrode that is at least partially submerged in water and is connected to the control unit in order to provide a ground reference value. The ground reference value is used to determine the effectiveness of the anti-fouling arrangement. In response to this determination, the control unit can regulate or fine tune the voltage and current output from the direct current power source.

According to a second aspect of the invention, the invention relates to a marine vessel that is protected by a cathodic protection and anti-fouling arrangement as described above. The cathodic protection and anti-fouling arrangement can be operated using an on-board source of DC power or using DC power supplied from a shore facility, in order to conserve the on-board power source.

According to a third aspect of the invention, the invention relates to a method for protecting a marine vessel with a marine propulsion system against corrosion and fouling. The propulsion system comprises at least one driveline housing at least partially submerged in water; a torque transmitting drive shaft extending out of the driveline housing; and at least one propeller mounted on the drive shaft. The method involves performing the steps of:.

According to a further example, the method involves controlling the direct current flow through said galvanic circuit using a reference electrode at least partially submerged in water to provide a ground reference value for the control unit.

The marine propulsion system according to the invention solves at least in part the problem of added drag caused by externally mounted ICCP units. By using an existing component, in this case a propeller, as the anode of an ICCP system, added drag from an externally mounted anode is avoided. Using a propeller as the anode also avoids any aesthetic issues caused by extra components mounted on the hull or transom. The invention also solves the problem of limited physical space available on the transom or the hull for vessels with stern drives or outboard, as the anode can be replaced by the at least one propeller. The arrangement provides protection against fouling for the propellers and simultaneous corrosion protection for metallic components connected to the arrangement.

<FIG> shows a schematically illustrated marine vessel <NUM> comprising an anti-fouling arrangement.

The vessel comprises a hull with a transom <NUM> to which a marine propulsion system is attached. The propulsion system in this example comprises a single driveline housing <NUM> at least partially submerged in water, a torque transmitting drive shaft <NUM> (not shown) extending out of the driveline housing <NUM>, and a pair of counter-rotating propellers <NUM>, <NUM> mounted on the drive shaft <NUM>. In the current example, both propellers <NUM>, <NUM> are electrically isolated from its drive shaft <NUM>. The drive shaft arrangement is shown in <FIG> and will be described in further detail below. Each electrically isolated propeller <NUM>, <NUM> to be protected against fouling is connected to a positive terminal <NUM> of a direct current (DC) power source <NUM>, such as a battery, in order to form an anode. Further, each metallic component <NUM>, <NUM>, <NUM> to be protected against corrosion is connected to a negative terminal <NUM> of the direct current power source <NUM>, in order to form cathodes. A control unit <NUM> is connected to the direct current power source <NUM> and distributes current to all component parts forming an electrical circuit. The control unit <NUM> is arranged to regulate the voltage and current output from the direct current power source <NUM>. In order to assist regulation of the voltage and current output a reference electrode <NUM> is mounted on the hull remote from the anode and connected to the control unit <NUM> via an electrical wire <NUM>. The reference electrode <NUM> measures a voltage difference between itself and the metallic components, which is directly related to the amount of protection received by the anode. The control unit <NUM> compares the voltage difference produced by the reference electrode <NUM> with a pre-set internal voltage. The output is then automatically adjusted to maintain the electrode voltage equal to the pre-set voltage.

Regulation of the voltage and current output from the direct current power source is controlled to automate the current output while the voltage output is varied. This allows the protection level to be maintained under changing conditions, e.g. variations in water resistivity or water velocity. In a sacrificial anode system, increases in the seawater resistivity can cause a decrease in the anode output and a decrease in the amount of protection provided, while a change from stagnant conditions results in an increase in current demand to maintain the required protection level. With ICCP systems protection does not decrease in the range of standard seawater nor does it change due to moderate variations in current demand. An advantage of ICCP systems is that they can provide constant monitoring of the electrical potential at the water/hull interface and can adjust the output to the anodes in relation to this. An ICCP system comprising a reference electrode is more effective and reliable than sacrificial anode systems where the level of protection is unknown and uncontrollable.

The anti-fouling arrangement is an impressed current cathodic protection (ICCP) arrangement using the propellers <NUM>, <NUM> as an anode <NUM>. In <FIG>, the metallic component to be protected against corrosion is the driveline housing <NUM>, the trim tabs <NUM> (one shown), and a metal portion of the hull, in this case the transom <NUM>. Note that this is a non-exclusive list of metallic components suitable for marine growth and corrosion protection. In order to achieve this, the positive terminal <NUM> and the negative terminal <NUM> of the battery <NUM> are connected to the control unit <NUM>. The control unit <NUM> is arranged to connect the positive terminal <NUM> to the propellers <NUM>, <NUM> via a first electrical wire <NUM>. The control unit <NUM> is further arranged to connect the negative terminal <NUM> to an electrical connector <NUM> on the driveline housing <NUM> via a second electrical wire <NUM>. The negative terminal <NUM> is also connected to an electrical connector <NUM> on the trim tab <NUM> via a third electrical wire <NUM>, and connected to an electrical connector <NUM> on the transom <NUM> via a fourth electrical wire <NUM>.

<FIG> shows a cross-section of the rear portion of the marine vessel <NUM> of <FIG>, through a transom <NUM> and a driveline housing <NUM>. The single driveline housing <NUM> is partially submerged in water and comprises torque transmitting drive shafts <NUM>, <NUM> extending out of the driveline housing <NUM>. A pair of counter-rotating propellers <NUM>, <NUM> is mounted on their respective drive shafts <NUM>, <NUM>. In this example, the drive shafts <NUM>, <NUM> are driven by an internal combustion engine ICE via a transmission <NUM>. Transmissions for driving counter-rotating propellers are well known in the art and will not be described in detail here. Alternative drive units for driving the propellers are possible within the scope of the invention. Both propellers <NUM>, <NUM> are electrically isolated from its respective drive shaft <NUM>, <NUM> (see <FIG>). As schematically indicated in <FIG>, each electrically isolated propeller <NUM>, <NUM> is connected to a positive terminal <NUM> of a direct current power source <NUM> at schematically indicated points <NUM> via electrical wiring <NUM>. The electrical connection of the propellers will be described in further detail below. Further, each metallic component <NUM>, <NUM>, <NUM> to be protected against fouling is connected to a negative terminal <NUM> of the direct current power source <NUM>. A control unit <NUM> is arranged to regulate the voltage and current output from the direct current power source <NUM>. As described above, the positive terminal <NUM> and the negative terminal <NUM> of the battery <NUM> are connected to the control unit <NUM>. The control unit <NUM> is arranged to connect the positive terminal <NUM> to the propellers <NUM>, <NUM> via a first electrical wire <NUM>. The control unit <NUM> is further arranged to connect the negative terminal <NUM> to an electrical connector <NUM> on the driveline housing <NUM> via a second electrical wire <NUM>. The negative terminal <NUM> is also connected to an electrical connector <NUM> on the trim tab <NUM> (one shown) via a third electrical wire <NUM>, and connected to an electrical connector <NUM> on the transom <NUM> via a fourth electrical wire <NUM>. A reference electrode <NUM> is mounted on the hull remote from the propellers <NUM>, <NUM> forming an anode and connected to the control unit <NUM> via an electrical wire <NUM>. Regulation of the voltage and current output from the direct current power source using the control unit <NUM> has been described above.

<FIG> show schematic cross-sections of propeller arrangements suitable for use with the invention. <FIG> shows a schematic propeller <NUM> that is electrically isolated from its drive shaft <NUM> by a torque transmitting electrically isolating component <NUM> mounted between the propeller <NUM> and the drive shaft <NUM>. The electrically isolating component is mounted in a gap formed by the outer surface of the drive shaft <NUM> and the inner surface of the propeller hub <NUM>. The torque transmitting electrically isolating component <NUM> can be made from an elastic material, such as a natural or synthetic rubber. The propeller <NUM> is connected to the positive terminal of the direct current power source (see <FIG>) by electrical wiring <NUM> extending through a hollow portion <NUM> of the drive shaft <NUM>. For instance, an axially extending internal groove can be provided in the inner surface of the drive shaft can be used for the electrical wiring. Alternatively an external groove in the outer surface of the drive shaft can be used for the electrical wiring to the propeller. The location of the wiring is dependent on factors such as whether a single propeller or a counter-rotating duo-prop arrangement is used. The electrical wiring <NUM> is electrically connected to the hub <NUM> of the propeller by means of a wiping contact <NUM> mounted between the drive shaft <NUM> and the hub <NUM>. A wiping contact mounted inside the transmission housing and an electrical wire in or along the drive shaft can be used for connecting the positive terminal of a power source to the propeller hub.

<FIG> shows a schematic propeller <NUM> that is electrically isolated from its drive shaft <NUM> by a torque transmitting electrically isolating component <NUM> mounted between the propeller <NUM> and the drive shaft <NUM>. As in <FIG>, the electrically isolating component is mounted in a gap formed by the outer surface of the drive shaft <NUM> and the inner surface of the propeller hub <NUM>. The torque transmitting electrically isolating component <NUM> can be made from an elastic material, such as a natural or synthetic rubber. The propeller <NUM> is connected to the positive terminal of the direct current power source (see <FIG>) by electrical wiring <NUM> extending through a hollow portion <NUM> of the drive shaft <NUM>. For instance, an axially extending internal groove can be provided in the inner surface of the drive shaft can be used for the electrical wiring. Alternatively an external groove in the outer surface of the drive shaft can be used for the electrical wiring to the propeller. The location of the wiring is dependent on factors such as whether a single propeller or a counter-rotating duo-prop arrangement is used. The electrical wiring <NUM> is electrically connected to the hub <NUM> of the propeller by means of a wiping contact <NUM> mounted between the drive shaft <NUM> and the hub <NUM>. Alternative solutions can include a wiping contact mounted inside the transmission housing and an electrical wire in or along the drive shaft for direct connection to the propeller hub.

The example in <FIG> differs from that of <FIG> in that a dielectric shield <NUM> is provided between the propeller <NUM> and the drive shaft <NUM> on which the propeller is mounted. The dielectric shield <NUM> is used as an electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. In this arrangement the dielectric shield <NUM> is used to protect the surface of the drive shaft <NUM> near the propeller hub <NUM> from hydrogen embrittlement caused by unacceptably high potentials in areas adjacent the propeller <NUM> that is used as an anode in the anti-fouling arrangement.

The dielectric shield <NUM> can comprise a layer of dielectric material extending along the drive shaft over at least the entire axial extension of the propeller hub <NUM>. The dielectric shield <NUM> is preferably arranged to extend a predetermined length L<NUM> and L<NUM> in front of and behind the propeller hub <NUM>, respectively, in order to ensure that the protection potential at the point of contact with the shaft does not become to electronegative. The lengths L<NUM> and L<NUM> will vary depending on anode area, propeller hub design and the protection current used for the actual application. A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. A non-exclusive list of suitable materials for use in such a dielectric shield includes polymer or polymer-ceramic materials with suitable dielectric properties.

<FIG> shows a schematic diagram illustrating a method of operating a cathodic protection and anti-fouling arrangement. In operation, the method comprises an initial step <NUM> when the arrangement is being operated for protecting a marine vessel with a marine propulsion system against corrosion of submerged metallic components and fouling of the propellers. The cathodic protection and anti-fouling arrangement can be operated using an on-board source of DC power, as described in connection with <FIG> and <FIG>, or using DC power supplied from a shore facility, in order to conserve the on-board power source.

As described above, the propulsion system comprises at least one driveline housing at least partially submerged in water; a torque transmitting drive shaft extending out of the driveline housing; and at least one propeller mounted on the drive shaft. In a first step <NUM>, the method involves providing electrical power from a direct current (DC) power source. In a second step <NUM>, the method involves causing at least one metallic component of the vessel to act as a cathode, by connecting the at least one metallic component to a negative terminal of the DC power source. In a third step <NUM>, the method involves causing the at least one propeller of said marine propulsion system to act as an anode, by connecting the at least one propeller to a positive terminal of the DC power source. The arrangement forms a galvanic circuit which comprises the DC power source, the at least one metallic component, the at least one propeller and water, in which water the metallic component and the propeller are at least partially submerged. In a fourth step <NUM>, the method involves electrically connecting said anode to the DC power source and directing a direct current flow through said galvanic circuit. In a fifth step <NUM>, the method involves controlling the direct current flow through said galvanic circuit by means of a control unit. In a sixth step <NUM>, which can be optional, the method involves connecting the control unit to a reference electrode which at least partially submerged in water. The reference electrode provides a ground reference value for the control unit. After a predetermined period of operation, the anti-fouling arrangement can be disconnected from the power source in a final step <NUM>. The cathodic protection and anti-fouling arrangement can be operated continuously or at least over extended periods of time, as long as shore power is provided. When an on-board source of power is used, the anti-fouling arrangement can be operated intermittently or over limited periods of time, while the power levels of the on-board power source allows.

Claim 1:
A marine propulsion system comprising a cathodic protection and anti-fouling arrangement configured to protect at least one metallic component (<NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>) in a marine vessel (<NUM>), the propulsion system comprising;
- at least one driveline housing (<NUM>; <NUM>) at least partially submerged in water;
- a torque transmitting drive shaft (<NUM>; <NUM>, <NUM>) extending out of the driveline housing (<NUM>; <NUM>);
- at least one propeller (<NUM>, <NUM>; <NUM>, <NUM>) mounted on the drive shaft (<NUM>; <NUM>, <NUM>); characterized in that
- the at least one propeller (<NUM>, <NUM>; <NUM>, <NUM>) is electrically isolated from its drive shaft (<NUM>; <NUM>, <NUM>);
- each electrically isolated propeller (<NUM>, <NUM>; <NUM>, <NUM>) to be protected against fouling is connected to a positive terminal (<NUM>; <NUM>) of a direct current power source (<NUM>; <NUM>);
- each metallic component (<NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>) to be protected against corrosion is connected to a negative terminal (<NUM>; <NUM>) of the direct current power source (<NUM>; <NUM>); and that
- a control unit (<NUM>; <NUM>) is arranged to regulate the voltage and current output from the direct current power source (<NUM>; <NUM>).