Abstract
The invention relates to a propeller arrangement in a cathodic protection system for a marine vessel with a marine propulsion system, which cathodic protection system comprises a direct current power source with a positive terminal. 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. According to the invention, the at least one propeller is electrically isolated from its drive shaft. Each electrically isolated propeller is electrically connected to a slip ring connector, which slip ring connector is in electrical connection with the positive terminal. The invention further relates to a vessel provided with such a propeller arrangement.
Claims
1. Propeller arrangement in a cathodic protection system for a marine vessel with a marine propulsion system, which cathodic protection system comprises a direct current power source; the propulsion system comprising; at least one driveline housing at least partially submerged in water; a torque transmitting drive shaft extending out of the driveline housing; at least one propeller mounted on the drive shaft; characterized in that the at least one propeller is electrically isolated from its drive shaft; each electrically isolated propeller is electrically connected to a slip ring connector; which slip ring connector is in electrical connection with a terminal of the direct current power source.
2. Propeller arrangement according to claim 1, characterized in that each propeller hub has a slip ring connector attached to the hub upstream in the power flow direction.
3. Propeller arrangement according to claim 1, characterized in that the slip ring connector is annular and arranged surrounding and spaced from the drive shaft.
4. Propeller arrangement according to claim 1, characterized in that the slip ring connector is mounted onto an internal surface at one end of the propeller hub.
5. Propeller arrangement according to claim 1, characterized in that the slip ring connector is mounted between a first propeller hub and the driveline housing.
6. Propeller arrangement according to claim 5, characterized in that a further slip ring connector is mounted between a first propeller hub and a second propeller hub.
7. Propeller arrangement according to claim 1, characterized in that each slip ring connector is made from an inert metallic material.
8. Propeller arrangement according to claim 1, characterized in that the at least one propeller is made from an inert metallic material.
9. Propeller arrangement according to claim 1, characterized in that a torque transmitting electrically isolating component is mounted between the at least one propeller and the drive shaft.
10. Propeller arrangement according to claim 1, characterized in that a dielectric shield is provided between the at least one propeller and the drive shaft.
11. Propeller arrangement according to claim 10, characterized in that the dielectric shield comprises a layer of dielectric material extending along the drive shaft over at least the entire axial extension of the propeller hub.
12. Propeller arrangement according to claim 1, characterized in that slip ring connector is in electrical connection with a positive terminal of the direct current power source, so that the at least one propeller forms an anode.
13. Propeller arrangement according to claim 1, characterized in that slip ring connector is in electrical connection with a negative terminal of the direct current power source so that the at least one propeller forms a cathode.
14. Marine vessel characterized in that the marine vessel is protected by a propeller arrangement according to claim 1.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings:
[0029] FIG. 1 shows a schematically illustrated vessel comprising a marine cathodic protection system/corrosion protection system according to the invention;
[0030] FIG. 2A-B show schematic cross-sections of the rear portion of the marine vessel;
[0031] FIG. 3 shows schematic cross-sections through a twin propeller arrangement; and
[0032] FIGS. 4A-B show cross-sectional views of slip ring connectors in FIG. 3.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION
[0033] FIG. 1 shows a schematically illustrated marine vessel 100 comprising a cathodic protection system according to the invention. The vessel comprises a hull with a transom 104 to which a marine propulsion system is attached. The propulsion system in this example comprises a single driveline housing 101 at least partially submerged in water, a torque transmitting drive shaft 106 (not shown) extending out of the driveline housing 101, and a pair of counter-rotating propellers 102, 103 mounted on the drive shaft 106. In the current example, both propellers 102, 103 are electrically isolated from its drive shaft 106. The drive shaft arrangement is shown in FIGS. 2A and 2B and will be described in further detail below. Each electrically isolated propeller 102, 103 to be protected against fouling is connected to a positive terminal 111 of a direct current (DC) power source 110, such as a battery, in order to form an anode. Further, each metallic component 101, 104, 105 to be protected against corrosion is connected to a negative terminal 112 of the direct current power source 110, in order to form cathodes. A control unit 113 is connected to the direct current power source 110 and distributes current to all component parts forming an electrical circuit. The control unit 113 is arranged to regulate the voltage and current output from the direct current power source 110. In order to assist regulation of the voltage and current output a reference electrode 124 is mounted on the hull remote from the anode and connected to the control unit 113 via an electrical wire 123. The reference electrode 124 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 113 compares the voltage difference produced by the reference electrode 124 with a pre-set internal voltage. The output is then automatically adjusted to maintain the electrode voltage equal to the pre-set voltage. Alternatively, the control unit can control the current to regulate the voltage to a desired potential.
[0034] Regulation of the voltage and current output from the direct current power source can be controlled to automate the current output while the voltage output is varied. Alternatively, the voltage and current output from the direct current power source can be controlled to automate the voltage output while the current 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.
[0035] The cathodic protection system is an impressed current cathodic protection (ICCP) arrangement using the propellers 102, 103 as an anode 115. In FIG. 1, the metallic component to be protected against corrosion is the driveline housing 101, the trim tabs 105 (one shown), and a metal portion of the hull, in this case the transom 104. 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 111 and the negative terminal 112 of the battery 110 are connected to the control unit 113. The control unit 113 is arranged to connect the positive terminal 111 to the propellers 102, 103 via a first electrical wire 114. The control unit 113 is further arranged to connect the negative terminal 112 to an electrical connector 117 on the driveline housing 101 via a second electrical wire 116. The negative terminal 112 is also connected to an electrical connector 119 on the trim tab 105 via a third electrical wire 118, and connected to an electrical connector 121 on the transom 104 via a fourth electrical wire 120.
[0036] FIG. 2A shows a cross-section of the rear portion of the marine vessel 100 of FIG. 1, through a transom 204 and a driveline housing 201. The single driveline housing 201 is partially submerged in water and comprises torque transmitting drive shafts 232, 233 extending out of the driveline housing 201. A pair of counter-rotating propellers 202, 203 is mounted on their respective drive shafts 233, 232. In this example, the drive shafts 232, 233 are driven by an internal combustion engine ICE via a transmission 231. 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 202, 203 are electrically isolated from its respective drive shaft 232, 233 (see FIG. 3). As schematically indicated in FIG. 2A, each electrically isolated propeller 202, 203 is connected to a positive terminal 211 of a direct current power source 210 at schematically indicated points 215 via electrical wiring 214. The electrical connection of the propellers will be described in further detail below. Further, each metallic component 201, 204, 205 to be protected against fouling is connected to a negative terminal 212 of the direct current power source 210. A control unit 213 is arranged to regulate the voltage and current output from the direct current power source 210. As described above, the positive terminal 211 and the negative terminal 212 of the battery 210 are connected to the control unit 213. The control unit 213 is arranged to connect the positive terminal 211 to the propellers 202, 203 via a first electrical wire 214. The control unit 213 is further arranged to connect the negative terminal 212 to an electrical connector 217 on the driveline housing 201 via a second electrical wire 216. The negative terminal 212 is also connected to an electrical connector 219 on the trim tab 205 (one shown) via a third electrical wire 218, and connected to an electrical connector 221 on the transom 204 via a fourth electrical wire 220. A reference electrode 224 is mounted on the hull remote from the propellers 202, 203 forming an anode and connected to the control unit 213 via an electrical wire 223. Regulation of the voltage and current output from the direct current power source using the control unit 213 has been described above.
[0037] FIG. 2B shows an alternative cross-section of the rear portion of the marine vessel 100 of FIG. 1, through a transom 204 and a driveline housing 201. As in FIG. 2A, the single driveline housing 201 is partially submerged in water and comprises torque transmitting drive shafts 232, 233 extending out of the driveline housing 201. A pair of counter-rotating propellers 202, 203 is mounted on their respective drive shafts 233, 232. In this example, the drive shafts 232, 233 are driven by an internal combustion engine ICE via a transmission 231. 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 202, 203 are electrically isolated from its respective drive shaft 232, 233 (see FIG. 3).
[0038] As schematically indicated in FIG. 2B, each electrically isolated propeller 202, 203 is connected to a negative terminal 211 of a direct current power source 210 at schematically indicated points 215 via electrical wire 214. This differs from the example in FIG. 2A in that a hull mounted active anode 226 is used, while the propellers 202, 203 form cathodes to be protected. The electrical connection of the propellers and the other metallic components will be described in further detail below. Each metallic component 201-205 form a cathode to be protected against corrosion if connected to a negative terminal 212 of the direct current power source 210. A control unit 213 is arranged to regulate the voltage and current output from the direct current power source 210 to provide a desired potential for each component. For instance, the control unit 213 is arranged to control the voltage to the propellers towards a different potential relative to the other metallic components, as the propellers can be made from a more noble alloy than most other metallic parts thus requiring a less electronegative protection potential, which can significantly reduce the required cathodic protection current. According to one example, the voltage supplied to the propellers can be −500 mV and the voltage supplied to the driveline housing can be −950 mV. As described above, the positive terminal 211 and the negative terminal 212 of the battery 210 are connected to the control unit 213. The control unit 213 is arranged to connect the negative terminal 211 to the propellers 202, 203 via a first electrical wire 214. The control unit 213 is further arranged to connect the negative terminal 212 to an electrical connector 217 on the driveline housing 201 via a second electrical wire 216. The negative terminal 212 is also connected to an electrical connector 219 on the trim tab 205 (one shown) via a third electrical wire 218, and connected to an electrical connector 221 on the transom 204 via a fourth electrical wire 220. The negative terminal 212 is also connected to the hull mounted active anode 226 via a fifth electrical wire 225. Finally, a reference electrode 224 is mounted on the hull remote from the propellers 202, 203 forming an anode and connected to the control unit 213 via a separate electrical wire 223. Regulation of the voltage and current output from the direct current power source using the control unit 213 has been described above.
[0039] FIG. 3 shows a schematic cross-section of a counter-rotating propeller arrangement suitable for use with the invention. FIG. 3 shows a pair of propellers 302a, 302b which are electrically isolated from their respective drive shafts 301a, 301b by a torque transmitting electrically isolating component 306a, 306b mounted between the respective propeller 302a, 302b and its drive shaft 301a, 301b. The electrically isolating component is mounted in a gap formed by the outer surfaces of the respective drive shaft 301a, 301b and the inner surface of the corresponding propeller hub 303a, 303b. The torque transmitting electrically isolating component 306a, 306b can be made from an elastic material, such as a natural or synthetic rubber. A dielectric shield 307a, 307b is provided between each propeller hub 303a, 303b and the drive shaft 301a, 301b on which the respective propeller is mounted. The 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 307a, 307b is used to protect the surface of the drive shafts 301a, 301b near the propeller hubs 303a, 303b from hydrogen embrittlement caused by unacceptably high potentials in areas adjacent each propeller 302a, 302b that is subjected to a sufficiently high electrical field in the cathodic protection system. 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 303a, 303b. 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. The dielectric shield is preferably arranged to extend a predetermined length in front of and behind the respective propeller hubs 303a, 303b, respectively, in order to ensure that the protection potential at the point of contact with the shaft does not become too electronegative. The longitudinal extension of the dielectric shield will vary depending on factors such as anode/cathode area, propeller hub design and the protection current used for the actual application.
[0040] In the example shown in FIG. 3, the driveline is a conventional duo-prop driveline, that comprises a drive shaft driving a transmission (see FIGS. 2A-B) located within a driveline housing 305. The duo-prop driveline allows the first propeller 302a mounted onto the outer first drive shaft 301a and a second propeller 302b mounted onto an inner second drive shaft 301b to be rotated in opposite directions. The figure also shows a cylindrical spacer part 304 fixed to the end of the inner second drive shaft 301b, which spacer part 304 allows the propeller hubs 303a, 303b to be located at the same radial distance from the axis of rotation X. Counter-rotating propeller arrangements of this type are well known in the art and will not be described in further detail.
[0041] The propellers 302a, 302b are connected to the positive terminal of a direct current power source (see FIGS. 2A-B) via slip ring connectors 313a, 313b. A first slip ring connector 313a is mounted between a tubular support 307 fixed to the driveline housing 305 and the first propeller hub 303a. The tubular support 307 extends out of the driveline housing 305 and provides support for bearings and sealing arrangements surrounding the second drive shaft 301a. The first slip ring connector 313a transfers electrical current from an electrical wire 314 connected to the positive terminal of the power source (see FIGS. 2A-B) to the first propeller hub 303a of the first propeller 302a. The first slip ring connector 313a is made up of an assembled unit comprising two annular components which are rotatable relative to each other. The first slip ring connector 313a comprises slip ring or conductor ring consisting of a stationary or rotating graphite or metal contact, e.g. a brush, which rubs against a facing surface of a rotating or stationary metal ring. In this example, the component mounted onto the driveline housing 305 is stationary, while the component mounted onto the first propeller hub 303a is rotatable. As one component turns, the electric current is conducted through the stationary brush to the metal ring making the connection. The brush and the metal ring are mounted onto a pair of annular components arranged to be rotatable relative to each other as shown in FIGS. 4A and 4B. According to one example, the annular components can have facing radial surfaces on which the mating contactors are located. According to a further example, the annular components can have facing circumferential surfaces on which the mating contactors are located. A second slip ring connector 313b is mounted between the first propeller hub 303a and the second propeller hub 303b. The second slip ring connector 313b transfers electrical current from the first propeller hub 303a to the second propeller hub 303b of the second propeller 302b. The second slip ring connector 313b is made up of an assembled unit comprising two annular components which are rotatable relative to each other, similar to the first slip ring connector 313a described above. In this example, the component mounted onto the first propeller hub 303a and the component mounted onto the second propeller hub 303b are rotatable in opposite directions. The electrical components, including the brush, conductor ring, and any electrical connectors are made of highly conductive materials. The materials are selected based on requirements such as current density, voltage drop, rotational speed, temperature, resistance variation and characteristic impedance. Power is generally transmitted through composite brushes of a carbon-graphite base and may have other metals such as copper or silver to increase current density.
[0042] A control unit (se FIGS. 1-2B) is arranged to monitor and regulate the voltage and current output from the direct current power source in order to maintain a desired voltage potential for the cathodic protection system. The control unit is arranged to compensate for any voltage drop caused by the first and second slip ring connectors.
[0043] The use of slip ring connectors 313a, 313b for supplying electrical power to one or more propellers eliminates the need for physical wiring within the rotating components. The elimination of wiring extending along or in the drive shafts avoids problems with vibrations and/or unbalance when the propellers rotate at relatively high speeds. Although FIG. 3 shows an embodiment for a duo-prop arrangement, the figure is also considered to show the corresponding layout for a single propeller arrangement, which would only comprise the first slip ring connector between the driveline housing and the first propeller.
[0044] FIGS. 4A and 4B show cross-sectional views of the slip ring connectors in FIG. 3. FIG. 4A shows the first slip ring connector 313a mounted between a tubular support 307 fixed to the driveline housing (see FIG. 3) and the first propeller hub 303a. The first slip ring connector 313a transfers electrical current from an electrical wire 314 connected the positive terminal of the power source to the first propeller hub 303a of the first propeller 302a. The first slip ring connector 313a is made up of an assembled unit comprising two annular components arranged concentrically and having facing circumferential surfaces on which mating contactors are located. The inner annular component mounted on the tubular support 307 must be electrically isolated from said tubular support 307, e.g. by a suitable coating or an intermediate component (not shown) made from an isolating material, such as natural or synthetic rubber.
[0045] FIG. 4B shows the second slip ring connector 313b mounted between the first propeller hub 303a and the second propeller hub 303b. The second slip ring connector 313b transfers electrical current from the first propeller hub 303a to the second propeller hub 303b of the second propeller 302b. The second slip ring connector 313b is made up of an assembled unit comprising two annular components having facing radial surfaces on which mating contactors are located. In this example, the annular components can be made from an inert metallic material, such as titanium, niobium or a similar suitable metal or metal alloy
[0046] It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.
