BACKGROUND

1. Technical Field

[0001] The present invention relates generally to propulsion, such as marine propulsion, and more particularly to contra-rotating propeller systems.

2. Description of Related Art

[0001] Historically, improvements in efficiency, emissions, and the like often add to production costs, and may reduce reliability of a product. In some applications, propulsion efficiency may be increased with the use of a contra-rotating propeller located immediately behind, and rotating opposite, the lead propeller.

[0002] In typical contra-rotating propellers, two propellers are mounted on concentric driveshafts. A first {e.g., outer) driveshaft rotates the first propeller, and a second {e.g., inner) driveshaft rotates the second propeller in the opposite direction. Such systems are typically complex, and the coupling of the driveshafts typically often reduces mechanical efficiency. Such systems do not inherently provide for navigation, and implementation of a rudder may be more difficult with a typical contra-rotating propeller system.

[0003] A ship may have a large propeller, for example as disclosed in PCT patent application publication no. WO 2009/126090, which is incorporated by reference herein. A large propeller may rotate more slowly than a smaller propeller (at equal thrust), which may improve efficiency. However, the added complexity and hardware may make implementation of a large propeller difficult. Managing the large flow field proximate to a vessel having a large area propeller may be challenging, and some implementations may be impacted by safety requirements.

[0004] A ship may have an azimuth thruster, which provides for directional control and forward thrust. Some azimuth thrusters incorporate a contra-rotating propeller. FR2927605 (Al) relates to a thruster having at least two water propulsion rotors. DE20315579 (Ul) describes a ship propulsion system that comprises two propellers rotating at different speeds mounted on concentric drive shaft arrangement inside tube with open ends. WO8601483 (Al) describes a propeller drive with contra rotating propellers, where the shafts of the outer propellers are supported by bearings inside the hollow shafts of the inner propellers. US5795200 (WO96/09954 ) describes a marine propulsion unit that has two contra-rotating propellers driven by a single shaft through gearing which includes a set of static planetary gears, an external gear, and an internal gear. US2011033296 describes a contra-rotating propeller marine propulsion device. US5441388 describes a ship drive with two counterturning propellers.

JP2004168222 (A), JP2010254293, EP 2873605, and FI 115210 describe counter-rotating propeller devices. WO9749604 (Al) describes a differential drive for a ship having two ship's screws.

[0005] It may be challenging to add a shroud to a propulsion system. For example, a shroud designed to enhance the flow around one propeller may degrade the flow around another propeller. A shroud moving in concert with one propeller could contact another propeller. For a propeller driven by a fixed driveshaft, the driveshaft typically locates the propeller with respect to the hull (e.g., the propeller may rotate around the driveshaft but the driveshaft, and thus the propeller, cannot move laterally with respect to the hull surface).

SUMMARY OF THE INVENTION

[0006] A first propeller may drive a ship. A navigation drive may comprise a second (e.g., contra-rotating) propeller, a propulsion system to drive (spin) the second propeller, and a transmission to controllably rotate (pivot) the second propeller's thrust direction with respect to that of the main propeller. The second propeller may be rotated around a steering axis (e.g., to impart a lateral thrust to the ship to navigate the ship). The second propeller may impart a forward or reverse thrust to the ship (e.g., instead of, or in addition to, the first propeller). The second propeller may be coaxial with the first propeller at a first position (e.g., in an aligned position), and not coaxial at other positions (e.g., that impart lateral thrust).

[0007] A ship may comprise a main propeller coupled to the ship and configured to impart a longitudinal thrust to the ship (e.g., to propel the ship forward or backward). A steer able contra-rotating propeller (steerable CRP) may comprise a CRP disposed longitudinally with respect to the main propeller (e.g., disposed immediately ahead of or aft of the main propeller). A CRP may be disposed such that the flow fields of the propellers interact (e.g., substantially overlap) during steaming. A CRP may be disposed to "harvest" rotational momentum imparted by the main propeller to the water.

[0008] A transmission may couple the CRP to the ship, and may drive the CRP (to provide thrust). The transmission may be configured to direct the thrust of the CRP over a range of angles (e.g., lateral angles from port to starboard and/or vertical angles). The transmission may align the CRP with the main propeller, such that they are coaxial and their thrust directions are parallel. The transmission may direct the thrust of the CRP independent of that of the main propeller by changing the thrust direction of the CRP (making it non-coaxial with that of the main propeller).

[0009] The transmission may direct the thrust of the CRP by rotating the CRP about a steering axis. For a CRP disposed behind the main propeller, the steering axis may be located ahead of the propeller plane of the CRP (e.g., by a distance that is at least as large as the radius, including at least as large as the diameter, including above the propeller planes of both propellers. For a CRP located in front of the main propeller, the steering axis may be located behind the propeller plane of the CRP (e.g., by a distance that is at least as large as the radius, including at least as large as the diameter, including above 150% of the diameter of the contra-rotating propeller) and may even be located behind the propeller planes of both propellers. Typically, the steering axis is located between the propeller planes of the main and CRP, including up to a distance from the main propeller plane, on the other side of the main propeller from the CRP, that does not exceed 10% of the diameter of the main propeller, including within 10% of the radius of the main propeller.

[0010] A transmission may direct the thrust of the CRP about a steering axis, which may be located (+/- 5% of propeller diameter) between the propeller planes of the CRP and the main propeller. For a CRP disposed aft of the main propeller, the steering axis may be located substantially ahead of the leading edge of the CRP. For a CRP disposed ahead of the main propeller, the steering axis may be located substantially behind the trailing edge of the CRP. Substantially may include a distance that is greater than the propeller radius, including greater than the propeller diameter, including greater than 150% of the propeller diameter.

[0011] The main propeller and/or steerable CRP may include a shroud, which may encompass at least one of, including both of, the CRP and the main propeller. The shroud and CRP may be directed as a single unit by the transmission. In an embodiment, a shroud axially encompasses the contra-rotating propeller, the transmission rotates the shroud and contra-rotating propeller about the steering axis, and the shroud extends from the contra-rotating propeller to axially encompass at least a portion of the main propeller. As shroud that encompasses the contra-rotating propeller may axially encompass the main propeller when the propeller planes of the main propeller and contra-rotating propeller are parallel (although the shroud may not completely encompass the main propeller when the propeller planes are not aligned). A shroud may encompass only one propeller (e.g., only the main propeller or only the CRP). In an embodiment, a shroud encompasses a CRP but not the main propeller. In an embodiment, the shroud encompasses the main propeller but not the CRP. A first shroud may encompass a first (e.g., main) propeller and a second shroud may encompass a second (e.g., CRP) propeller.

[0012] A ship may have a large diameter propeller (including one or both of the main propeller and the CRP) having a diameter that is greater than 50% of the draft of the ship, including greater than 80% of the draft. The ship may have a hull comprising a hull concavity. The hull concavity may have a radius of curvature that is larger (e.g., 5%, 10%, 50%, or even 100% larger) than the radius of the outer tip of the largest propeller (and/or a shroud around the propeller, if included). The radius of the hull concavity (e.g., in a plane orthogonal to the propeller axis) may be at least twice as large as the radius of the propeller and/or shroud. An exemplary hull may have a hull concavity with a radius between 2 and 90 meters, including between 4 and 70 meters, including between 6 and 50 meters. A hull concavity may have a radius that is, with respect to propeller radius, between 0.5 x and 40 x (the radius of the propeller), including between lx and 20x, including between 1.5x and lOx. A hull concavity may have a radius that, with respect to propeller radius, is between 0.5x and 100x(the radius of the propeller), including between lx and 70x, including between 1.5x and 50x, including between 2x and 20x.

BRIEF DESCRIPTION OF THE DRAWINGS

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

[0014] FIGS. 2A and 2B illustrate side elevations of a steerable CRP comprising a shroud, according to some embodiments.

[0015] FIGS. 2C and 2D illustrate an exemplary effect of steering axis location on shroud movement, according to some embodiments.

[0016] FIG. 3 illustrates a view of the stern of a ship, according to some embodiments.

[0017] FIG. 4 is a perspective illustration of an aft portion of a ship, according to some embodiments.

[0018] FIGS. 5A - E illustrate different steering configurations, according to some embodiments.

[0019] FIGS. 6A and 6B illustrate an exemplary embodiment.

DETAILED DESCRIPTION

[0020] A single propeller may create non-thrusting momentum (e.g., rotational and/or tangential momentum, such as swirl) in the water it passes through, resulting in reduced efficiency. A second, contra-rotating propeller disposed behind the first propeller may "harvest" at least a portion of this otherwise wasted momentum, converting it to forward thrust. A main propeller and contra-rotating propeller may be sized (e.g., with feedback from 3D CFD models and/or physical models of their flow fields at cruising speeds) such that a large portion of the non-thrust (e.g.,

rotational/tangential) momentum of the first propeller is substantially entirely harvested by the contra-rotating propeller. Preferably, the flow field of the fluid exiting the contra-rotating 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%, up to about 25%). A contra-rotating propeller may spread the thrust loading over a larger propeller area (e.g., over a larger number of blades), reducing the loading on each blade. Such a configuration may reduce wear (e.g., of bearings) and/or reduce fatigue.

[0021] Various aspects provide for a propulsion system (e.g., for propelling a marine vessel). The propulsion system may combine a main propeller (e.g., a fixed direction propeller providing only forward/reverse thrust) with a contra-rotating propeller whose thrust direction may be controlled independently of that of the main propeller (hereinafter: steerable CRP). The steerable CRP may be located immediately ahead of or behind the main propeller (such that the centerlines of the steerable CRP and the main propeller may be aligned coaxially on a "virtual" axis of rotation). This alignment may increase the hydrodynamic efficiency of the system. A transmission may direct the thrust direction of the CRP, independent of that of the main propeller, to impart a thrust in a direction different than the thrust direction of the main propeller. Thrust may be redirected by rotating the CRP about a steering axis. A transmission may be configured to vary the range of second directions over an angle (500) defined at least in part by a thrust direction of the CRP and a thrust direction of the main propeller that is at least 1 degree, particularly at least 5 degrees, particularly at least 10 degrees, particularly at least 20 degrees, and particularly not greater than 85 degrees, particularly not greater than 75 degrees, particularly not greater than 65 degrees, particularly between 5 and 45 degrees, particularly up to 40 degrees.

[0022] The propulsion system may comprise a main propeller and a steerable CRP having a contra-rotating propeller and a transmission coupling the contra- rotating propeller to the ship. The transmission may be configured to drive the CRP (i.e., "spin" or rotate the propeller blades per se). A transmission may include a central motor to spin the propeller blades via the center of the propeller. A transmission may include a rim-drive to spin the propeller blades via the outer portion of the propeller blades. A CRP and main propeller may rotate at substantially the same RPM

(revolutions per minute). A CRP may rotate at a different RPM than that of the main propeller.

[0023] The transmission may be configured to direct the thrust of the CRP (e.g., change the centerline of the CRP about a steering axis). The direction of thrust of the steerable CRP may be changed with respect to that of the main propeller, such that the rotation axis of the steerable CRP's blades is not aligned with that of the main propeller. The ship may be steered by controlling a thrust direction of the steerable CRP (e.g., to impart a lateral thrust to port or starboard). The transmission may comprise one or more hydraulic cylinders, electric actuators, geared (e.g., worm gear) actuators, and the like.

[0024] The centerlines of the propellers may be kept coplanar. In some embodiments, the steerable CRP is configured such that the transmission directs at least a portion of the thrust of the CRP in a vertical direction with respect to the main propeller. In some cases, the steerable CRP may be configured to impart a combination of vertical and horizontal thrust, such that the centerlines of the CRP and main propeller are not coplanar. At least one, including both, of the main propeller and CRP may include a shroud or nozzle. In some cases, neither propeller has a shroud or nozzle. [0025] FIG. 1 illustrates a view of the side 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, including at least 5 meters, including greater than 10 meters, including greater than 15 meters. An exemplary ship may have a draft between 4 and 25 meters, including between 6 and 18 meters.

[0026] A main propeller 110 may be coupled to an engine 103 (e.g., an LNG engine, a diesel engine, a turbine, an electric motor) and configured to impart a longitudinal thrust to the ship. In some ships, the main propeller may be a fixed propeller (e.g., coupled to the engine 103 via a rigid driveshaft) and only provide for forward or reverse thrust. A main propeller may provide for lateral thrust in some embodiments (e.g., as part of a pod or azimuth thruster).

[0027] The main 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 the main 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 have a diameter that is at least 1.5 meters, including at least 2 meters. A propeller may have a diameter be between 2 and 22 meters, including between 3 and 18 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).

[0028] The propulsion system may comprise a steerable contra-rotating propeller (steerable CRP) 120, which may be powered by engine 103 and/or another power plant. The steerable CRP includes a contra-rotating propeller (CRP) 122 configured to rotate opposite the direction of rotation of the main propeller. The CRP may have a diameter greater than 50%, greater than 75%, greater than 85%, greater than 90%, or even greater than draft 102. Typically, an aftward CRP may have a diameter slightly smaller than that of the main propeller. A CRP disposed ahead of the main propeller may have a slightly larger diameter. An exemplary (e.g., aftward) CRP may have a diameter between 40% and 90% of that of the main propeller, including between 45% and 80%, including between 50% and 70%, such as 55%-65%. The CRP may include a hub-driven propeller (as shown in FIG. 1), a rim-driven propeller (including an electromagnetically rim-driven propeller, including a permanent magnet rim drive), and other types of driven propellers.

[0029] The CRP may be powered by a transmission 130 that couples the CRP to the ship and drives the CRP (to provide thrust). Transmission 130 is typically configured to be controllable (e.g., via a navigation console) to direct thrust of the CRP (e.g., and the shroud) over a range of directions independent of the thrust direction of the main propeller. In an embodiment, a ship comprises a fixed-axle main propeller (that does not provide lateral thrust), and the steerable CRP may be directed to impart a range of lateral thrust that can be used to steer the ship from port to starboard. In a "straight ahead" alignment (of the CRP centerline with respect to that of the main propeller), the steerable CRP may propel the ship longitudinally (e.g., forward or backward).

[0030] CRP 122 may be aligned longitudinally with respect to the main propeller, such that their centerlines are coaxial and their flow fields substantially overlap. In some embodiments, the flow field of one propeller (e.g., the front propeller) substantially entirely encompasses the flow field of the other (e.g., following) propeller. In some cases, the CRP is disposed aft of the main propeller; in some cases (not shown) it is disposed ahead of the main propeller. A shroud may be substantially cylindrical (e.g., for propellers having the same diameter). A shroud may be conical (e.g., for one propeller having a larger diameter than the other).

[0031] The rotation axis of the steerable CRP may be directed independently of the rotation axis of the main propeller, providing for a controllable lateral thrust of the steerable CRP with respect to the thrust direction of the main propeller (regardless of the thrust direction of the main propeller). An azimuth thruster may combine a rim-driven propeller (e.g., a main propeller) and a center-driven propeller (e.g., a steerable CRP). In some embodiments, an azimuth thruster combines a main propeller (that itself can impart lateral thrust) and a steerable CRP (e.g., whose lateral thrust may be directed independently of that of the main propeller).

[0032] Various embodiments may comprise a lifeboat ramp configured to stow and deploy a lifeboat. The lifeboat ramp may be mounted along a centerline of the ship (e.g., aft of the transom and/or integrated with the stern). In FIG. 1, a fixed lifeboat ramp 140 is integrated with the hull and configured to stow and deploy a lifeboat 150. A lifeboat ramp may extend from the hull (e.g., aft of the transom or fore of the prow). A lifeboat ramp may be integrated with the hull. A lifeboat ramp and transmission 130 may be integrated into a single structural component, that may itself be integrated to the substructure of the rest of the hull. By keeping a lifeboat ramp in a continually "ready to deploy" position, the need to mechanically position the ramp to ready the lifeboat for deployment from the ship may be avoided, which may increase safety. In an

embodiment, a transmission is aft of the transom, and at least 50% of the lifeboat ramp is disposed aft of the transom.

[0033] A ship may comprise a main propeller and a steerable contra-rotating propeller (CRP). A steerable CRP may be coupled to the ship via a transmission configured to drive the CRP and direct the thrust of the CRP about a steering axis, which may be located ahead of the CRP when the CRP is aft of the main propeller, or behind the CRP when the CRP is ahead of the main propeller. A steering axis may be located proximate to the propeller plane of the main propeller. The steerable CRP may include a shroud axially encompassing the steerable CRP. A transmission may couple the CRP and shroud to the ship, and be configured to drive (spin) the CRP and direct the thrust of the CRP by rotating the CRP (and shroud, if included) about a steering axis to direct the thrust of the CRP over a range of directions (e.g., lateral directions) independent of the thrust direction of the main propeller. The shroud may axially encompass at least a portion of the main propeller. The shroud may axially encompass substantially all of the main propeller (e.g., when the propeller planes are parallel, for steering straight ahead). The shroud may encompass only a portion of the main propeller (e.g., when the propeller planes are not parallel, as for turning). In an embodiment, the shroud only encompasses a portion of the main propeller when the planes are not parallel (e.g., in a straight ahead configuration, such as turning the CRP by at least 5 degrees, including at least 10 degrees, about the steering axis) and when the propeller planes parallel, the shroud does not encompass the main propeller. Typically, the steering axis is located between the leading edge plane of one propeller and the trailing edge plane of the other propeller. The steering axis may be located between the leading edge planes of the main propeller and the CRP.

[0034] FIGS. 2A and 2B illustrate side elevations of a steerable CRP comprising a shroud, according to some embodiments. FIG. 2A illustrates a view of the starboard side; FIG 2B illustrates this view with the steerable CRP in section.

[0035] FIGS. 2A and 2B illustrate a ship 200 having a steerable CRP 220. In this example, steerable CRP 220 comprises a rim-driven (e.g., electromagnetically rim- driven) contra-rotating propeller 222, longitudinally disposed with respect to the main propeller, a shroud axially encompassing both the CRP and at least a portion of the main propeller, and a transmission coupling the CRP to the ship and configured to rotate the shroud and CRP about a steering axis. Rotation about the steering axis may direct a thrust of the CRP (independent of the thrust direction of the main propeller) over a range of directions (e.g., lateral directions). In this example, the steerable CRP comprises a shroud 221. A shroud (or nozzle) that axially "surrounds" a propeller may increase power and/or efficiency. A shroud and a rim drive may be integrated. A shroud may axially surround the main propeller (not shown), the CRP (e.g., FIG. 6A/B), and/or both the main propeller and the CRP (as in FIGS. 2A/B). In this example, the shroud axially encompasses both the contra-rotating propeller and the main propeller (at least when the propellers are oriented coaxially). The shroud may have a diameter slightly greater than that of the largest propeller it surrounds. A shroud may be substantially frustoconical (e.g., with a larger diameter at the front end, and a smaller diameter at the rear end). A frustoconical shroud may have an opening angle that is between 10 and 20 degrees, including between about 13 and 18 degrees. When integrated with a CPR, a shroud may also be steered by transmission 130, such that the CRP and shroud are directed as an integrated unit.

[0036] FIG. 2B illustrates a section of steerable CRP 220, and shows contra- rotating propeller 222 disposed aft of a main propeller 110. In this example, CRP 222 has a diameter that is slightly smaller than (e.g., 85-95% of) that of main propeller 110. A 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 main propeller as the transmission rotates the CRP). A steering axis may be located above (e.g., directly above) a point directly aft (or even aligned with) the main propeller aft terminus and/or a propeller plane of the main propeller (leading plane, midplane, maximum radius plane, terminal edge plane).

[0037] In some implementations, a "nose" 223 of the contra-rotating propeller is extended close to the aft terminus of the main propeller axis, (including into a corresponding recess into the axle body of the main 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 main propeller to the contra-rotating 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).

[0038] Transmission 130 may rotate CRP 222 (to provide thrust) and direct the thrust of CRP 222 (in this example, laterally). In this example, transmission 130 directs CRP 222 and shroud 221 about a steering axis 230 located just behind the main propeller aft terminus and directly above nose 223. A steering axis may be aligned with the nose of the contra-rotating propeller, particularly wherein a longitudinal distance from the nose (223) to a propeller plane of the contra-rotating propeller exceeds 50% of the radius, including exceeds the radius, of the contra-rotating propeller (e.g., at least 0.5 meters, including at least 1 meter from the CRP for a large propeller diameter). 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 main propeller. A steering axis may be located within a distance from the main propeller maximum diameter plane that does not exceed the diameter of the main propeller, including a distance that does not exceed 50% of the diameter, including within 20% of the diameter, including within 10% of the diameter, including within 7% of the diameter, including within 1% of the diameter.

[0039] FIGS. 2C and 2D illustrate an exemplary effect of steering axis location on shroud movement, according to some embodiments. To rotate the shroud in concert with the contra-rotating propeller, various geometrical factors (shroud dimensions, location of steering axis, tolerances, and the like) may be chosen to achieve a desired hydrodynamic response. FIGS. 2C and 2D are schematic illustrations of a longitudinal view (e.g., facing aft) of a planar cut of a shroud 220 at a location that is substantially coplanar with the main propeller. Orientation 220' may correspond to a "straight ahead" shroud (and CRP) position. Orientation 220" may correspond to a "steering" shroud position (e.g., to direct the contra-rotating propeller's thrust to port or starboard). The center points of the respective positions 220' and 220" are shown.

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

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

[0042] As shown by the coincident center points in FIG. 2D, the center point of the shroud plane (which is coplanar with the propeller in a "straight ahead" configuration) may be designed to remain coincident with the center point of the propeller plane as the shroud rotates around the steering axis. To an extent, the CRP/shroud rotate around the main propeller via appropriate choice of the location of the steering axis. Such a configuration may be enhanced with the combination of a contra-rotating propeller with a rim-drive or other mechanism that decouples the drive force for the CRP from a fixed location (e.g., as with a drive axle that prevents the CRP from moving laterally). In some embodiments, modest lateral motion of the contra- rotating propeller may be accommodated using a drive axle having an articulated (e.g., constant velocity) joint.

[0043] The 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] A steering axis 230 (FIG. 2B) may be located, with respect to forward direction of the ship, ahead of a propeller plane of the contra-rotating propeller, particularly up to a distance that is not ahead of a leading edge of the main propeller. In an embodiment, the steering axis is located ahead of the leading edge of the main propeller.

[0045] In some cases, steering axis (230) is located closer to a propeller plane of the main propeller than to a corresponding propeller plane of the contra-rotating 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 main propeller that does not exceed 50% of the radius the main 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 main propeller. In an embodiment (e.g., CRP aft of main propeller), a steering axis may be located between a longitudinal midpoint (between the leading and trailing edges of the shroud) and the leading edge of the shroud, particularly within the front 30% of the shroud. In an embodiment (e.g., CRP ahead of main propeller), a steering axis may be located between a longitudinal midpoint (between the leading and trailing edges of the shroud) and the trailing edge of the shroud, particularly within the aft 30% of the shroud.

[0046] FIG. 3 illustrates a view of the stern of a ship, according to some embodiments. In this example, steerable CRP 220 is illustrated in an "aligned" configuration, in which the centerlines of the CRP and main propeller are coaxial. FIG. 3 illustrates an exemplary outer radius 320 of a shroud 221 encompassing the main propeller and the CRP.

[0047] The centerline of the main propeller may be a vertical distance 330 from a baseline 106 of the ship. The centerline of the CRP may be a vertical distance 330' from the baseline. In some embodiments distances 330 and 330' are substantially the same (e.g., within a manufacturing tolerance or a tolerance within which hydrodynamic effects are negligible). The larger of distances 330 and 330' may be slightly larger than the radius of the larger propeller (or the outside of a shroud, if included) such that no portion of the steerable CRP extends below the baseline of the ship. In FIG. 3, both distances 330 and 330' are slightly larger than outer radius 320 of shroud 221. Vertical distances 330 and 330' may be within 10% of each other, including between 1% of each other. In some embodiments, distances 330 and 330' are different.

[0048] The propulsion system may be centered with respect to the port 104 and starboard 105 sides of the ship. In an aligned configuration, the centerline of the main propeller may be a horizontal distance 340 from the starboard side, the centerline of the steerable CRP may be a horizontal distance 340' from the starboard side, and these horizontal distances may be within 10% of each other, including within 1% of each other, including substantially the same. The centerlines may be substantially equidistant from the port and starboard sides, such that horizontal distance 340 equals horizontal distance 342 (centerline to port side) and horizontal distance 340' equals horizontal distance 342' (centerline to port side).

[0049] A ship may have a plurality of steerable CRP systems (e.g., with two or three main propellers). In an embodiment, a ship has a port and starboard main propellers, and each main propeller has a steerable CRP. A ship may have a center propeller and one or more propellers to each of the port and starboard sides. In some cases, all propellers include a steerable CRP. In some cases, not all of the propellers include a steerable CRP.

[0050] The hull 101 (FIG. 1) of a ship may have a hull concavity 300. Hull concavity 300 may improve the hydrodynamic flow properties of a ship having a large diameter propeller. Hull concavity 300 may have a radius (e.g., orthogonal to the centerline of the main and/or contra-rotating propellers) that is larger than the largest propeller radius (and shroud radius, if a shroud is included). The hull concavity may be finite (i.e., not have an infinite radius). In an embodiment, a propeller has a diameter that is greater than 50% of the draft of the ship (including above 75%, including above 90%, or even above 100%). The hull may comprise a hull cavity 300 having a radius of curvature 310 that is larger than the outer radius of the propeller, particularly at least 40% larger, including at least 60% larger, including at least twice as large, including at least four times as large. A hull concavity 300 may have a radius of curvature 310 that is larger than the draft of the ship, including 2x, larger, 3x larger, 5x larger, lOx larger, or even 20x larger. A hull concavity typically has a finite curvature (i.e., is not completely flat).

[0051] In some cases, radius 310 is large enough that a portion (e.g., the center top portion) of hull concavity 300 extends above the waterline of the ship (when the ship is stationary). Such a concavity may fill with water when the ship is moving. In some embodiments, a shroud 221 axially encompasses the main and steerable propellers (of which at least the main propeller is a large diameter propeller). The shroud may have an outer radius 320 (FIG. 3), and the hull may have a hull concavity 300 with a radius of curvature 310 that is larger than, including at least twice as large, including at least three times, or even five times as large, including lOx as large as outer radius 320 of the shroud. Typically, radius 310 of the hull concavity will not be so large that efficiency is reduced (e.g., as determined in a computer simulation of steaming conditions). Radius 310 may be smaller than 50x, including smaller than 20x, including smaller than lOx, the largest of the radii of the main propeller, CRP, and (if included) shroud. In an implementation, radius 310 may be greater than 3 meters, including greater than 8 meters, such as above 15 meters, and may be less than 80 meters, less than 40 meters, including less than 30 meters.

[0052] FIG. 4 is a perspective illustration of an aft portion of a ship, according to some embodiments. A steerable CRP may be implemented at the aft portion of a ship, as shown. A steerable CRP may be implemented at the bow of the ship (not shown). Hull concavity 300 may improve the flow of water past the steerable CRP, improving efficiency.

[0053] FIGS. 5A - E illustrate different steering configurations, according to some embodiments. An angle 500 between the thrust directions of the main propeller and steerable CRP 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.

[0054] 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 CRP not significantly degrade the performance of the main propeller and/or interact with the hull. Angle 500 may be limited by mechanical interaction between the propellers and/or a shroud.

[0055] FIG. 5A illustrates an "aligned" configuration, in which the schematic thrust directions of the main propeller and steerable CRP 220 are the same (in this case, thrusting aftward to move the ship ahead). FIG. 5B illustrates a configuration in which steerable CRP 220 is directing its thrust partially to starboard. FIG. 5C illustrates a configuration in which steerable CRP 220 is directing its thrust partially to port. In these illustrations, point 502 represents the intersection between the propeller plane of the contra-rotating propeller and its axis of rotation. In FIG. 5A, point 502 is aligned with the main propeller (in this case, on the centerline of the ship). As the shroud and contra- rotating 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. Such displacement may be facilitated with a transmission (to drive the CRP) having a with a drive axle with an articulated joint and/or (for larger displacements) a rim drive.

[0056] 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 CRP 220. In an embodiment, a hull comprises at least one hull concavity and at least one, including at least two, pockets. A hull may comprise a hull concavity without pockets; a hull may comprise a pocket without a hull concavity.

[0057] FIGS. 5D and 5E provide additional interior detail on the embodiment shown in FIGS. 5A-C, showing the shroud in sectional view. FIG. 5D illustrates the steerable CRP in a "straight ahead" configuration; FIG. 5E illustrates the steerable CRP in a "turning" configuration. In this example, steering axis 230 is located at a nose 223 (FIG. 2B) of the contra-rotating propeller that is disposed within a corresponding recess in the axial structure of the main propeller. Location of a steering axis with respect to the main propeller may be defined with respect to a rearmost plane 113 (of the rearmost edge), a maximum radius plane 111 (of the tip edge with largest radius), a leading edge plane 114, a central plane, a plane of maximum blade load and/or another plane.

[0058] 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 main propeller to about 200% of the diameter of the main 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 contra-rotating propeller, including 1.5 times larger, including more than twice as large.

[0059] 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.

[0060] 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 main 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 main propeller without contacting it. A propeller tip may be correspondingly concave with a convex shroud portion.

[0061] FIGS. 6A and 6B illustrate an exemplary embodiment. A ship 600 may comprise a CRP 620 having a shroud 621 associated with the CRP but not the main propeller. FIG. 6A illustrates a side elevation of ship 600. FIG. 6B illustrates the shroud in section. In this example, transmission 130 directs both shroud 621 and CRP 222 about steering axis 230 to direct at least a portion of the thrust of CRP 222 laterally. In an embodiment, the shroud is designed such that in a "straight ahead" configuration with the propeller planes parallel (as shown in FIGS. 6A and 6B), the shroud does not encompass the main propeller. The shroud may be designed to encompass a portion of the main propeller only when the CRP/shroud is in a "turning" configuration (e.g., as in FIG. 2D). The shroud may be designed to not encompass the main propeller in any configuration. In some cases, a shroud encompasses the main propeller but not the CRP.

[0062] Various features described herein may be implemented

independently and/or in combination with each other. An explicit combination of features in an embodiment 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.