BACKGROUND

1. Technical Field

[0001] The present invention relates generally to marine propulsion, and more particularly to so-called "grim vane" propeller systems.

2. Description of Related Art

[0001] An increasing focus on reducing environmental impact raises the importance of energy efficiency. However, improvements in efficiency, emissions, and the like often add to production costs and reduce reliability of a product. To improve energy efficiency, it is desirable to minimize losses in a propulsion system (e.g., energy imparted to the water that does not yield useful thrust).

[0002] A so-called vane wheel or grim vane (as in US4623299) may increase propulsion efficiency. Typically, the grim vane is a second, free-spinning propeller located immediately aft of the drive propeller. The blades of the grim vane have a pitch that varies radially. An inner portion of the blades (the "turbine" portion) is shaped to "harvest" energy from the water passing through the inner portion. As the grim vane spins, an outer portion of the blades (the "thrust" portion) provides thrust. The combination of a drive propeller and grim vane may increase the efficiency of a propulsion system.

[0003] A grim vane may harvest non-motive momentum from the water that might otherwise be lost (e.g., axial and/or tangential momentum in the flow field of the water aft of the drive propeller). This harvesting may enable the use of an improved pitch and/or rotation rate for the drive propeller, such that the combination of drive propeller and grim vane offers increased efficiency. Various other documents describe the art, including DE3508203, and JPH03287488. Documents cited herein are

incorporated by reference. KR 20120136153 describes a ship having an energy recovery propeller that remains coaxial with the drive propeller. An independent rudder is used for steering. DE 3207398 describes a ship having propellers that remain coaxial. An independent rudder is used for steering. GB 1227354 describes a ducted propeller apparatus in which the propellers remain coaxial. The ability to effectively navigate is important, particularly as ship size increases.

SUMMARY

[0004] Various aspects describe a propulsion system for a boat or ship. A propulsion system may comprise a drive propeller coupled to a drive system (e.g., engine, motor and the like) and configured to impart a thrust to a ship. The propulsion system may comprise a non-driven propeller having a turbine portion and a thrust portion. The turbine portion may be shaped to harvest energy from fluid (e.g., water) passing through the turbine portion. The thrust portion may be shaped to generate a thrust in a thrust direction, such that energy harvested by the turbine portion is converted to thrust energy via the thrust portion. Inasmuch as the turbine portion is shaped to harvest momentum from the water, and the thrust portion is shaped to impart momentum to the water, the turbine and thrust portions may have different pitches, shapes, twists, curvatures, and/or other features. The non-driven propeller may be a contra-rotating propeller with respect to the main (drive) propeller, which may reduce aggregate swirl and other non-motive losses from the drive propeller. The non-driven propeller may corotate with the drive propeller.

[0005] Various embodiments include a nozzle or shroud proximate to and/or around one of, including both of, the drive propeller and non-driven propeller. A shroud may be ahead of or aft of a propeller. A shroud may be between propellers. A shroud may be configured to modify a flow of liquid into, out of, through, and/or around at least one propeller. A shroud may axially encompass at least one propeller (e.g., a drive propeller and/or a non-driven propeller). A shroud may axially encompass the non-driven propeller and at least a portion of the drive propeller. In an

embodiment, the shroud encompasses the non-driven propeller and substantially all of the drive propeller when the propeller planes are parallel. In an embodiment, the shroud encompasses the non-driven propeller and a portion of the drive propeller only when the propeller planes are not parallel (e.g., in a straight-ahead configuration, the shroud does not encompass the drive propeller). A shroud may be substantially cylindrical (e.g., for propellers having the same diameter). A shroud may be

conical/frustoconical (e.g., for one propeller having a larger diameter than the other). A frustoconical shroud may have an opening angle that is between 10 and 20 degrees, including between about 13 and 18 degrees. A shroud radius may be larger than that of the non-driven propeller. A shroud radius may match (e.g., be within 10% of) the radius of the interface between turbine and thrust portions of the non-driven propeller. A shroud radius may match or be larger than the radius of the drive propeller.

[0006] A transmission may be coupled to the non-driven propeller and configured to direct the thrust portion of the non-driven propeller. The thrust from the non-driven propeller may be directed in a direction independent of the thrust direction of the drive propeller (e.g., to provide for navigation). A non-driven propeller may be coupled to the ship (e.g., via the transmission) with a hub mount and/or a rim mount, each of which may comprise one or more bearings (e.g., roller bearings, ball bearings, journal bearings, magnetic bearings). A rim mount may be integrated with a shroud and/or a bracket (e.g., a ring-shaped bracket).

[0007] The transmission may be configured to vary the direction in which the non-driven propeller is directed. An angle between the thrust directions of the drive and non-driven propellers may be varied as needed (e.g., up to +/- 40 degrees from centered) to provide for navigational thrust. A transmission may direct a shroud (e.g., optionally in concert with a propeller within the shroud) to provide lateral thrust.

[0008] In some embodiments, a ship comprises a non-driven propeller whose thrust direction is fixed. In some cases, the non-driven propeller may be coupled to the ship via a rim mount (e.g., via a shroud). In some cases, the ship comprises a shroud, which may or may not be coupled to the ship via a transmission configured to direct the shroud. A ship may comprise a grim vane and a shroud, particularly wherein the grim vane is coupled to the shroud via a rim mount. A shroud may comprise a ring that does not entirely encompass a propeller.

[0009] A ship may have a large diameter propeller (including one or both of the drive propeller and the non-driven propeller) having a diameter that is greater than 50% of the draft of the ship, particularly greater than 75% of the draft, particularly greater than 90% of the draft, particularly greater than the draft. In some cases, the drive propeller has a first diameter, the turbine portion of the non-driven propeller has a second diameter smaller than the first diameter, and the thrust portion of the non-driven propeller has a third diameter larger than the first diameter.

[0010] In some embodiments, a drive propeller has a first diameter, and at least a part of the turbine portion, particularly greater than 90% of the turbine portion, particularly substantially all of the turbine portion, has a second diameter (e.g., smaller than the first diameter). At least a part of the thrust portion, particularly greater than 90% of the thrust portion, particularly substantially all of the thrust portion may have a third diameter (e.g., larger than the first diameter). In some embodiments, a shroud has an inner diameter (e.g., proximate to the non-driven propeller) that does not

substantially exceed the third diameter.

[0011] A propulsion system for a ship may comprise a drive propeller, a non-driven propeller, a shroud, and a rim-mount coupling the non-driven propeller to the shroud.

[0012] A method of navigating or otherwise changing an attitude of a ship may comprise receiving instruction data from a navigation console, the instruction data requesting a change in a thrust vector of the drive propeller and grim vane, identifying a change in orientation of the transmission that is expected to change the thrust vector, and sending navigation data comprising the change in orientation to the transmission to redirect a thrust portion of the steerable grim vane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 illustrates a side view of a ship having a propulsion system according to some embodiments.

[0014] FIG. 2A illustrates a steerable grim vane, according to some embodiments.

[0015] FIG. 2B is a schematic of the effect of steering axis location on shroud movement, according to some embodiments.

[0016] FIGS. 3 and 4 illustrate embodiments with a hub mount, according to some embodiments.

[0017] FIGS. 5A - C illustrate different steering configurations, according to some embodiments.

[0018] FIG. 6 illustrates a steerable grim vane, according to some embodiments.

[0019] FIGS. 7 and 8 illustrate exemplary propulsion systems, according to some embodiments.

[0020] FIG. 9 illustrates a ship, according to some embodiments.

DETAILED DESCRIPTION

[0021] A drive propeller may impart a significant amount of non-thrust momentum (e.g., rotational and/or tangential momentum) to the water it passes through, resulting in reduced efficiency. A non-driven propeller (e.g., a grim vane or vane wheel propeller) disposed behind the first propeller may "harvest" this

momentum, converting it to forward thrust. A drive propeller and non-driven propeller may be sized (e.g., with feedback from 3D finite element models of their flow fields at cruising speeds) such that a large portion of the rotational/tangential momentum of the drive propeller is substantially entirely harvested by the non-driven propeller.

Particularly, the flow field exiting the non-driven propeller has a minimum, or even no, rotational/tangential component. In such a design, efficiency may be increased (e.g., by at least 5%, including at least 10%, or even at least 15%, typically up to about 20%).

[0022] A non-driven propeller may be designed to "virtually" increase the diameter of the drive propeller, which may increase propulsion efficiency. A non- driven propeller may enable equivalent thrust at lower propeller speeds, which may increase efficiency.

[0023] In some embodiments, a non-driven propeller having turbine and thrust portions is coupled to the ship via a transmission. The transmission may be operated to direct the thrust of the non-driven propeller independently of the thrust direction of the drive propeller. The transmission may comprise one or more hydraulic cylinders, electric actuators, geared (e.g., worm gear) actuators, and the like.

[0024] FIG. 1 illustrates a side view of a ship having a propulsion system according to some embodiments. In exemplary FIG. 1, a ship 100 has a hull 101 having a draft 102. Various embodiments may be implemented with a large ship, which may have a draft greater than 2 meters, at least 5 meters, including greater than 10 meters, including greater than 15 meters. An exemplary ship may have a draft between 4 and 30 meters, including between 6 and 20 meters.

[0025] A drive propeller 110 may be coupled to a drive system 103 (e.g., an LNG engine, a diesel engine, a combustion turbine, an electric motor), and configured to impart a thrust to the ship (e.g., longitudinally). In some ships, the drive propeller may be a fixed propeller (e.g., coupled to drive system 103 via a rigid driveshaft) and only provide for forward or reverse thrust. A drive propeller may provide for lateral thrust in some embodiments (e.g., as part of a pod or azimuth thruster).

[0026] At least one propeller may have a large diameter relative to the size of the ship (e.g., the ratio of propeller diameter to draft). A large diameter propeller may provide for increased propulsion efficiency as compared to a smaller propeller (that rotates at higher speed to provide the same thrust). In some cases, a diameter 112 of a (e.g., drive) propeller is greater than 50% of, greater than 75% of, greater than 85% of, greater than 90% of, or even greater than draft 102. An exemplary propeller may be greater than 1.5 m in diameter, including greater than 2m in diameter. A propeller may be between 3 and 20 meters in diameter, including between 5 and 15 meters in diameter. Various embodiments may be implemented with smaller propellers (e.g., between 0.2 and 3 meters, including between 0.5 and 2 meters). A non-driven propeller may have a diameter larger than that of the drive propeller, including at least 5%, 10%, or even at least 20% larger.

[0027] Exemplary ship 100 comprises a steerable grim vane 120. A steerable grim vane may comprise a non-driven propeller (122) coupled to the ship via a transmission 130. The non-driven propeller may have a turbine portion (124) shaped to harvest energy from water flowing through the turbine portion and a thrust portion (126) shaped to impart this energy to the water, propelling the ship. Typically, the turbine and thrust portions have different pitches (e.g., opposite twist angles) and/or different shapes. The non-driven propeller may be designed to rotate at a different speed (e.g., more slowly) than the drive propeller.

[0028] Transmission 130 may be configured to direct the thrust of the non- driven propeller (e.g., about a steering axis 230). The direction of thrust of the non- driven propeller may be changed with respect to that of the drive propeller, such that the rotation axis of the non-driven propeller's blades is not aligned with that of the drive propeller. The ship may be steered by controlling a thrust direction of the steerable grim vane (e.g., to impart a lateral thrust to port or starboard). In a shrouded embodiment (with a shroud axially encompassing at least a portion of at least one, including both propellers), transmission 130 may rotate the non-driven propeller and the shroud about steering axis 230. A shroud may axially encompass a propeller in an aligned (parallel propeller planes) configuration yet only partially encompass a propeller in a nonaligned configuration (e.g., steering to port or starboard). The steering axis 230 may be located at a position that maximizes the hydrodynamic benefits of the shroud yet minimizes undesirable interaction between propellers (e.g., contact between the shroud and the driven propeller as the transmission rotates the non-driven propeller to change its thrust direction).

[0029] A shroud encompassing both propellers may be subject to a range of hydrodynamic and drive forces. Mechanical and/or hydrodynamic properties may be enhanced by using a shroud having a cross section that forms a hydrofoil. Typically, computer aided design tools may be used to simulate various flow, propulsion, loading, and navigation conditions. In some cases, a length 224 of the shroud (e.g., a long chord length of a cross section) may be comparable to a propeller diameter. Shroud length 224 may be at least 50% and up to 300% of propeller diameter, including from about 60% of the diameter of the drive propeller to about 200% of the diameter of the drive propeller. Length 224 may be at least 3 meters, including at least 4 meters, including above 5 meters, including above 7 meters. Length 224 may be larger than the diameter of the non-driven propeller, including 1.5 times larger, including more than twice as large.

[0030] A width 226 (e.g., a short chord of a cross section) of the shroud may be at least 10%, including at least 15%, including at least 20% of the length 224. A width 226 may be at least 0.4 meters, including at least 0.6 meters.

[0031] In some cases, at least a portion of the interior face of a shroud includes an annular concave portion 228 (e.g., a somewhat spherical portion) which may have a curvature to match the tip curvature of the drive propeller in combination with an expected radius over which that portion of the shroud moves. Such a concave portion may improve macroscopic flow through the propeller yet improve clearance around the blade tip, such that shroud can move around the drive propeller without contacting it. A propeller tip may be correspondingly concave with a convex shroud portion.

[0032] The centerlines of the propellers may be kept coplanar (e.g., for straightline steaming). In some embodiments, the steerable grim vane is configured such that the transmission directs the thrust of the grim vane in a vertical direction with respect to the drive propeller. In some cases, the steerable grim vane may be configured to impart a combination of vertical and horizontal thrust, such that the centerlines of the grim vane and drive propeller are not coplanar.

[0033] In some embodiments, the rotation axis of the steerable grim vane may be directed independently of the rotation axis of the drive propeller, providing for a controllable lateral thrust of the steerable grim vane with respect to the thrust direction of the drive propeller (regardless of the thrust direction of the drive propeller). An azimuth thruster may combine a rim-driven propeller (e.g., a drive propeller) and a center-driven propeller (e.g., a steerable grim vane). In some embodiments, an azimuth thruster combines a main drive propeller (that itself can impart lateral thrust) and a steerable grim vane (whose lateral thrust may be directed independently of that of the drive propeller).

[0034] A ship may comprise a shroud that guides, shapes, or otherwise improves a flow of water through and/or around one or more propellers. A shroud may axially encompass the propeller(s). In exemplary FIG. 1, ship 100 comprises a shroud 150. In this example, shroud 150 encompasses both drive propeller 110 and non-driven propeller 122, and transmission 130 rotates both the shroud and non-driven propeller around steering axis 230. In some cases, substantially the entire propeller is

encompassed by the shroud. In some cases (e.g., FIG. 6), a portion of a propeller may be within the flow field created by the shroud, and a portion of the propeller may be outside the flow field created by the shroud. At least a portion of a shroud may be slightly ahead of or behind a propeller.

[0035] FIG. 2A illustrates a steerable grim vane, according to some embodiments. Steerable grim vane 220 may comprise a non-driven propeller 222 coupled to the ship (not shown) via transmission 130. In this example, the diameter of the non-driven propeller is larger than that of the drive propeller, and at least a portion of (e.g., most of, >85%, >95%, or even substantially all of) the thrust portion 126 has a radius that is larger than the tip radius of the drive propeller. An interface between the turbine and thrust portions may be angled, as shown by the square features between these portions.

[0036] In this example, a shroud 250 axially encompasses the non-driven propeller 222 and at least a portion of the drive propeller 110. The shroud and non- driven propeller are integrated with transmission 130, which rotates the non-driven propeller and shroud about steering axis 230. In this example (non-driven propeller behind drive propeller) the trailing edge of the shroud is proximate to (slightly aft of) the non-driven propeller, axially encompassing the entire non-driven propeller.

Typically, in a "straight ahead" configuration, the shroud encompasses substantially the entire drive propeller. In a "turning" configuration, part of the shroud (e.g., toward the outside of the turn) may not encompass the drive propeller, or substantially the entire drive propeller may remain encompassed by the shroud.

[0037] In this example, non-driven propeller 222 is coupled to transmission 130 via a rim mount 240 (in this case, integrated with shroud 250). A rim mount may also be used to couple a propeller to the ship without a shroud (e.g., with a ring shaped bracket). A rim mount may comprise one or more bearings (e.g., distributed

circumferentially). The rim mount may allow the propeller to spin freely, and may support the tips of the propeller blades, reducing bending loads and/or fatigue.

[0038] In some implementations, a "nose" 223 of the non-driven propeller is extended close to the aft terminus of the drive propeller axis, (including into a corresponding recess into the axle body of the drive propeller). A nose may ensure uniform flow rotation into the contra-rotating propeller (e.g., directing flow into the blades of the contra-rotating propeller). Such a configuration may smooth flow (near the centers) from the drive propeller to the non-driven propeller. A steering axis may be disposed above this nose (e.g., within a horizontal distance from the nose that is less than 10%, including less than 5%, including less than 2% of the propeller diameter). A steering axis may be aligned with the nose of the non-driven propeller, particularly wherein a longitudinal distance from the nose (223) to a propeller plane of the non- driven propeller exceeds 50% of the radius, including exceeds the radius, including exceeds the diameter, including above 150% of the diameter, of the non-driven propeller. In an embodiment (e.g., with a propeller diameter above 3 meters, including above 5 meters) a steering axis may be located within 0.5 meters, including within 0.3 meters, of a propeller plane of the drive propeller.

[0039] FIG. 2B is a schematic of the effect of steering axis location on shroud movement, according to some embodiments. To rotate the shroud in concert with the (typically contra-rotating) non-driven propeller, various geometrical factors (shroud dimensions, location of steering axis, tolerances, and the like) may be chosen to achieve a desired hydrodynamic response. FIG. 2D is a schematic illustration of a longitudinal view (e.g., facing aft) of a planar cut of a shroud 250 at a location that is substantially coplanar with the drive propeller. Orientation 250' may correspond to a "straight ahead" shroud (and non-driven propeller) position. Orientation 250" may correspond to a "steering" shroud position (e.g., to direct the non-driven propeller's thrust to port or starboard). The center points of the respective positions 250' and 250" are shown.

[0040] For the steering axis location used in FIG. 2B, the center point of the shroud plane moves laterally as the shroud pivots. In some implementations, it may be advantageous to incorporate a concave annular portion (e.g., substantially "spherical" annulus) around the drive propeller to prevent the shroud from contacting the propeller (while still retaining a tight shroud/propeller tolerance).

[0041] For the geometry used in FIG. 2B, steering axis 230 has been located substantially coplanar with the drive propeller. By locating the steering axis close to the drive propeller (e.g., aligned with the plane of maximum radius of the drive propeller) the shroud/non-driven propeller may rotate "around" the drive propeller.

[0042] As shown by the coincident center points in FIG. 2B, the center point of the shroud plane (which is coplanar with the drive propeller in a "straight ahead" configuration) may be designed to remain coincident with the center point of the drive propeller plane as the shroud rotates around the steering axis. To an extent, the non- driven propeller and shroud rotate around the drive propeller via appropriate choice of the location of the steering axis. Such a configuration may be enhanced with the combination of a non-driven propeller with a rim mount, hub mount, and/or or other mechanism that provides for a horizontal offset between the propeller plane and the steering axis. In some embodiments, modest lateral motion of the non-driven propeller may be accommodated using a drive axle having an articulated (e.g., constant velocity) joint.

[0043] A propeller plane may be represented by a maximum diameter plane, a foremost point plane, an aftmost point plane, and/or another plane. Inasmuch as the propeller plane represents the "most likely contact points" between the propeller and shroud, the steering axis may be chosen such that rotation of the shroud does not cause contact with the propeller.

[0044] In an embodiment (e.g., non-driven propeller behind the drive propeller), the steering axis 230 (FIG. 2B) may be located, with respect to forward direction of the ship, ahead of a propeller plane of the non-driven propeller, including up to a distance ahead of the drive propeller that does not exeed 50% of the diameter, including 20% of the diameter of the drive propeller, particularly up to a distance that is not ahead of a leading edge of the drive propeller. In an embodiment (e.g., a non-driven propeller ahead of the drive propeller), the steering axis 230 may be located behind the propeller plane of the non-driven propeller (with optional corresponding drive propeller limitations). In an embodiment, the steering axis is located ahead of the leading edge of the drive propeller.

[0045] In some cases, steering axis (230) is located closer to a propeller plane of the drive propeller than to a corresponding propeller plane of the non-driven propeller when the propeller planes are parallel, particularly wherein the steering axis (230) is disposed between the propeller planes, particularly wherein the steering axis (230) is disposed within a distance from the propeller plane of the drive propeller that does not exceed 50% of the radius the drive propeller, particularly within 30% of the radius, particularly within 10% of the radius, particularly wherein the steering axis (230) is substantially coplanar with a propeller plane defined by the outermost tip of the drive propeller. A steering axis may be located between a longitudinal midpoint of the shroud and a leading edge of the shroud, particularly within the front 30% of the shroud.

[0046] FIGS. 3 and 4 illustrate embodiments with a hub mount, according to some embodiments. In FIG. 3, a hub mount 340 couples a hub of the non-driven propeller to a post 342, which (in this example) connects the hub to transmission 130. Hub mount 340 comprises one or more bearings, allowing the non-driven propeller to spin freely.

[0047] FIG. 3 illustrates a propulsion system in which hub mount 340 is combined with a shroud 350 (which in this case, encompasses both propellers). In FIG. 4, steerable grim vane 420 does not comprise a shroud.

[0048] FIGS. 5A - C illustrate different steering configurations, according to some embodiments. These figures show schematic illustrations as viewed from beneath the ship. An angle 500 between the thrust directions of the drive propeller and steerable non-driven propeller may be varied over a range of angles sufficient to provide for navigation (e.g., up to +/- 35 degrees, such as for a SOLAS compliant vessel). The range of angles may vary from straight ahead to at least 1 degree, including at least 5 degrees, including at least 10 degrees, including at least 20 degrees. In some cases, angle 500 is less than 85 degrees, including less than 75 degrees, including less than 65 degrees., including up to 55 degrees. A typical apparatus may be steered over a range of angles up to +/- 50 degrees, including up to +/- 40 degrees. Angle 500 may be increased to increase the non-aligned component of the thrust (e.g., to make a ship "turn more tightly"). Angle 500 may generally be limited by a requirement that the steerable grim vane not significantly degrade the performance of the drive propeller and/or interact with the hull. Angle 500 may be limited by mechanical interaction between the propellers and/or a shroud. In some cases, at least a portion of the interior face of a shroud is concave (e.g., substantially spherical) such that the shroud can move around the drive propeller without contacting it.

[0049] FIG. 5A illustrates an "aligned" configuration, in which the schematic thrust directions of the drive propeller and steerable grim vane 520 are the same (in this case, thrusting aftward to move the ship ahead). FIG. 5B illustrates a configuration in which steerable grim vane 520 is directing its thrust partially to starboard. FIG. 5C illustrates a configuration in which steerable grim vane 520 is directing its thrust partially to port. In these illustrations, point 502 represents the intersection between the propeller plane of the non-driven propeller and its axis of rotation. In FIG. 5A, point 502 is aligned with the drive propeller (in this case, on the centerline of the ship). As the shroud and non-driven propeller are pivoted (to generate lateral thrust) point 502 moves off the centerline. In FIG. 5B (steering the ship to starboard) point 502 is displaced laterally to starboard. In FIG. 5C (steering the ship to port) point 502 is displaced laterally to port.

[0050] A hull may comprise one or more pockets 510, which may be shaped to receive a shroud and/or propeller (e.g., as it moves over angles 500). A pocket or "divot" may comprise a hull shape (e.g., a concavity) that "receives" or fits around the propeller or shroud (e.g., as it is redirected/revolved among various positions). A pocket may allow a propeller or shroud to move freely, even though it is located close to the hull (e.g., with a propeller tip to hull distance below 25% of propeller diameter and/or shroud diameter). In exemplary FIGS. 5A-C, two pockets 510 are disposed on either side of the centerline, and are shaped to allow for movement of grim vane 520.

[0051] FIG. 6 illustrates a steerable grim vane, according to some

embodiments. Ship 600 may comprise a steerable grim vane 620 having a non-driven propeller 622. FIG. 6 illustrates a system in which a shroud 650 is disposed substantially between the propellers. In this example, the trailing edge of the shroud is located proximate to (and may be coupled with) an interface between the turbine portion and thrust portion of the non-driven propeller, such that the shroud directs flow into the turbine portion. Water flowing from drive propeller 110 may be guided substantially entirely into turbine portion 124, which may have a diameter smaller, larger, or substantially equivalent to that of drive propeller 110. Shroud 650 may minimize (or substantially prevent) the flow of water into the thrust portion 126 of the non-driven propeller. FIG. 6 illustrates exemplary representative propeller planes, including the plane 111 of maximum radius, leading edge plane 114, and trailing edge plane 113 (in this case, illustrated on the drive propeller).

[0052] A ship may comprise a drive propeller and a steering shroud (or nozzle) without a grim vane. The shroud may be configured (e.g., via a transmission) to direct the thrust of the drive propeller (e.g., over a range of angles). A steerable shroud may be used to navigate a ship. Various embodiments illustrated herein may be implemented without a grim vane (e.g., with just the shroud).

[0053] FIGS. 7 and 8 illustrate exemplary propulsion systems, according to some embodiments. FIGS. 7 and 8 illustrate a drive propeller 110 and a non-driven propeller 722 (FIG. 7) or 822 (FIG. 8). In these examples, the non-driven propellers are coupled to a shroud 750 configured to shape the flow of water exiting the drive propeller. In these examples, shroud 750 substantially encompasses the drive propeller, but does not encompass the non-driven propeller. Shroud 750 may be shaped to preferentially direct water into the turbine portion 124 of the non-driven propeller. FIG. 7 illustrates a steering axis 230 located between the drive and non-driven propellers (in this example, approximately midway). FIG. 8 illustrates a steering axis 230 located immediately behind the drive propeller. In FIGS. 7 and 8, nose 223 is aligned with steering axis 230. In FIG. 8, nose 223 is disposed in a matching recess (e.g., a concave cavity) in the rearmost part (e.g., the axle assembly) of the drive propeller.

[0054] FIG. 7 illustrates a configuration of a rim mount 740. FIG. 8 illustrates an alternate configuration of a rim mount 840. FIGS. 7 and 8 illustrate a propulsion system that does not include a transmission; these systems may also be combined with a transmission to form a steerable grim vane. A rim mount may be used to couple a shroud to a propeller (e.g., the non-driven propeller). In an embodiment, a rim mount couples the trailing edge of the shroud to a portion of the non-driven propeller that is near the interface between the turbine and thrust portions (e.g., such that the inner surface of the shroud is aligned with the leading edge of the turbine/thrust interface of the non-driven propeller) such that the shroud smoothly directs flow from the driven propeller into the turbine portion of the non-driven propeller.

[0055] FIG. 9 illustrates a ship, according to some embodiments. Ship 900 may comprise a propulsion system having a drive propeller 110 and a steerable grim vane 920. In this example, both the drive propeller 110 and non-driven propeller 122 are large area propellers. In this example, a shroud 950 substantially encompasses drive propeller 110.

[0056] In some embodiments, a turbine portion of the non-driven propeller may have a larger diameter than the diameter of drive propeller. The performance of certain embodiments may be enhanced with a shroud (e.g., that guides water into the turbine portion rather than the thrust portion). A shroud may guide the water into the thrust portion in some embodiments. A shroud may be conical, frustoconical, and/or otherwise shaped to accommodate varying propeller diameters, hull shapes, expected flow fields, and the like.

[0057] In some embodiments, the turbine portion of the non-driven propeller has a diameter that is 80%-105%, including 90%-100%, of the diameter of the drive propeller. In some embodiments, the thrust portion of the non-driven propeller has a minimum diameter that is at least 90%, including at least 95%, including at least 100%, including at least 105%, of the outer diameter of the drive propeller. An outer diameter of the thrust portion may be larger than that of the drive propeller, including 10-70% larger, including 20-50% larger.

[0058] A shroud may or may not be included. An embodiment having a shroud may have a shroud around a drive propeller, a grim vane, and/or both. A shroud may direct water preferentially into the turbine portion, the thrust portion, and/or both. In some embodiments, a grim vane comprises a transition region between the turbine and thrust portions (e.g., where the blade pitch and/or blade shape changes from a "momentum capturing" shape to a "momentum imparting" shape. The radius of the transition region may be approximately equal to (e.g., within 10% of) the outer radius of the drive propeller.

[0059] In some embodiments, a shroud has a terminal radius (at an aft end) that is different than a leading radius (at a forward end). The terminal radius may be larger or smaller than the leading radius. In some cases, the terminal radius is approximately equal to the radius of the transition region. In some cases, the terminal radius is slightly larger than the outer radius of the turbine portion. In an embodiment, the leading radius is larger than the terminal radius, the drive propeller has an outer radius within 10% of that of the outer radius of the turbine portion. The thrust portion may be "within" or "outside" the flow field of the shroud. An outer radius of the thrust portion may be larger or smaller than the terminal radius.

[0060] Various features described herein may be implemented

independently and/or in combination with each other. An explicit combination of features does not preclude the omission of any of these features from other

embodiments. The above description is illustrative and not restrictive. Many variations of the invention will become apparent to those of skill in the art upon review of this disclosure. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.