A marine propulsor for submersible vessels or surface vessels powered by underwater propulsion units. A shaftless motor with disk-shaped rotor and stator(s) is mounted in the vessel structure with a blade hub mounted on the rotor, the hub including propeller blades extending beyond the circumference of the vessel housing. The motor is substantially iron-free with much of the rotor/stator volume being occupied by windings, thus providing sufficient power without taking up substantial space or adding burdensome weight. The rotor is journal mounted in the vessel and circumferential thrust bearing assemblies are provided around the rotor/blade hub assembly. A water cooling/lubricating system is provided for the bearings and rotor/stator(s). Power is supplied individually to two stators mounted on either side of the rotor to control electromagnetic forces on the rotor to offset thrust forces and to reduce the magnitude of propulsor induced structural vibration. Rotor excitation current may be inductively supplied, or permanent magnets or low reluctance magnetic material may be mounted in the rotor. Sound insulation is interposed between stators and the vessel structure to dampen ac noise/vibrations. In a dual counter-rotating propulsor embodiment, a thrust transference member is interposed between propulsor assemblies.

FIELD OF INVENTION 
This invention relates generally to propulsion systems and more 
particularly to propulsors for submersible or semisubmersible vessels or 
surface ships. 
BACKGROUND OF THE INVENTION 
Various systems have been proposed for water-going vessels in which one or 
more rotating propellers are disposed beneath the water line of the vessel 
for semi-submersible vessels or disposed within a portion of the hull of 
submersible vessels. 
Typically, the propellers in submersible systems have been driven by diesel 
power, steam turbines or electric motors mounted within the hull of a 
vessel. A propeller shaft extends through the hull to the propeller 
mounted on the shaft outside the hull. Such systems have the disadvantages 
of shaft vibration and noise radiating from the shaft. Further, leaking 
around the shaft occurs when the seal becomes loose or worn. Alternative 
systems have been suggested using shaftless electric motors mounted 
outside of the hull with only electric power cables passing through the 
hull. U.S. Pat. No. 3,182,623 provides one such example of shaftless 
motors used to drive impellers mounted within the tail section of a 
submarine. A disadvantage of such system is that propulsors (electric 
motors and impellers) occupy almost the entire interior of the tail 
section. Further, traditional shaftless electric motors are either too 
small to effectively move a vessel or, if large enough, add significant 
weight to the vessel. U.S. Pat. No. 3,101,066 suggests another shaftless 
electric motor for propelling a submersible vessel. Again, however, the 
traditional motor disclosed in the U.S. Pat. No. 3,101,066 has 
insufficient power to drive the vessel in which it is mounted and, if the 
size of the conventional motor with cylindrical iron rotor and stator is 
increased, the size and weight of the motor become a major disadvantage. 
These prior systems have failed to address the additional problems of 
handling the thrust imparted by the propellers in large vessels, or the 
problem of handling cooling and lubrication of bearings associated with a 
shaftless motor mounted in a submersible vessel. Further, prior systems 
have failed to adequately address the problems of cooling/lubricating 
bearings in contaminated or muddy waters. In addition, prior systems have 
not solved the problem of electrical noise imparted to the vessel hull by 
the stator of an electric motor mounted to the hull or mechanical 
vibration imparted by the rotor. These and other disadvantages of the 
prior art systems are overcome by the unique features of the propulsion 
system of the present invention. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved propulsor 
for submersible vessels, or surface vessels or semi-submersible vessels 
with underwater propulsion arrangements. 
It is another object of the invention to provide a propulsor with a 
shaft-free motor of sufficient power to drive a marine vessel but with a 
motor of minimum weight and size. 
A further object of the invention is to provide an improved mounting 
arrangement for the motor rotor including a journal bearing and a fluid 
supply for cooling/lubricating the journal bearing. 
Another object of the invention is to provide an arrangement for handling 
and distributing thrust imparted by the propulsor including 
circumferential thrust bearings, and a fluid supply for 
cooling/lubricating the thrust bearing(s). 
It is another object of the invention to control power to the propulsor so 
that a certain amount of propulsor thrust can be countered with 
electromagnetic forces and the position of the rotor can also be 
controlled by varying electromagnetic forces to dampen propulsor induced 
structural vibrations. 
It is yet a further object of the invention to provide a shaft-free 
submersible motor in which rotor excitation current is inductively 
supplied. 
It is another object of the invention to provide a marine propulsor with 
sound insulation between the vessel hull and the propulsor. 
A further object of the invention is to provide a thrust transmitting 
arrangement in a dual rotor, counter-rotating marine propulsor. 
Another object of the invention is to provide an improved marine propulsor 
with a shroud extending around the propulsor blades. 
Yet a further object of the invention is to provide a marine propulsor that 
is modular to facilitate assembly and removal. 
The marine propulsor of the present invention includes one motor in a first 
embodiment or two motors with counter-rotating propellers in a second 
embodiment. The motors include a disk-shaped rotor and two disk-shaped 
stators mounted in the body of vessel and being axially aligned with a 
longitudinal axis of the vessel. If the vessel is not itself completely 
submersible, the motors can be mounted in pods or cylindrically shaped 
casings attached below the bottom surface of the vessel. Unlike many 
conventional marine motors, the motor of the present invention is 
shaft-free. The stators are mounted in fixed positions to the vessel or 
propulsion pod housing and the rotor is journal mounted to the vessel or 
pod housing. Hereinafter, although reference may be made only to a vessel, 
it is to be understood that the invention is equally applicable to 
submersible vessels and to pods or propulsor housings mounted below a 
surface vessel. Electrical power is supplied to the stator windings 
through waterproof cables and connectors. 
The propulsor blades are mounted on a hub assembly that is removably 
attached to the rotor. A shroud, which can be mounted to the hub assembly 
or the vessel housing covers the propulsor blades. In the dual 
counter-rotating embodiment, a single shroud can cover both blade 
assemblies or individual shrouds can be provided for each. 
In contrast to the traditional heavy, bulky electric motors used in marine 
propulsors, the rotor and stators utilized in the present invention are 
substantially iron free. Much of the volume of the motor is occupied by 
electrical conductors. As a result, the disk-shaped motor is more compact 
and lighter than traditional motors and yet power is not sacrificed 
because the space utilized for conductors is greatly increased. 
The thrust created by the propeller blades, attached to the rotor by the 
blade hub assembly, is transferred to the vessel through a circumferential 
thrust bearing. The thrust bearing can be a single annular bearing or a 
plurality of spaced bearing assemblies mounted to the vessel structure 
around the circumference of the vessel and positioned to cooperate with a 
bearing surface on the rotor or propeller hub. The rotor is journal 
mounted on a vessel inner housing. A journal bearing mounted on a 
circumferential surface of the inner housing cooperates with a 
circumferential bearing surface of the rotor. 
The propulsor assembly with rotor, stators, thrust and journal bearings are 
outside the vessel housing (which may be a pressure hull in a submergent 
vessel) and are exposed to water. The rotor, stator and bearings are water 
cooled and water lubricated. To ensure a supply of clean water to these 
elements when operating in contaminated waters, a forced seawater supply 
is provided. This water supply may be either filtered water or clean water 
from an internal tank. This forced clean water is pumped to the journal 
bearings and then drawn by centrifugal force up through fluid channels in 
the rotor toward the thrust bearings located near the periphery of the 
rotor. Additionally, water can be pumped through channels in the stator 
for cooling. 
Electrical power is generated by traditional means, including fossil fuel 
systems or nuclear generators. The electrical power is delivered to the 
two stators through a variable frequency converter or controller, for 
example a cycloconverter. Thus, the speed of the motor can be controlled 
by varying the frequency of input power. The magnitude of propulsive 
thrust produced by the rotating propeller blades through the water will be 
proportional to magnitude of power delivered to the motor. The motor speed 
can be brought up slowly from zero cycles/second to synchronize the speed 
of the rotor with the speed of the rotating magnetic field of the stator, 
thus preventing any slip and producing maximum propulsive thrust from the 
input power. 
The two stators are mounted on opposite sides of the rotor. Each stator may 
be provided with an excitation winding (primary) and the rotor may be 
provided with a secondary excitation winding and rectifiers to provide the 
rotor excitation current. Alternately, a rotor magnetic field may be 
provided by permanent magnets mounted in the rotor. The rotor magnetic 
field reacts with a rotating magnetic field of the stator to produce 
rotational torque upon the rotor. Power is supplied separately to each 
stator from the variable frequency controller so that the magnitude of 
power supplied to one stator can be different than the magnitude of power 
supplied to the other stator. In addition to the rotational force applied 
to the rotor caused by the rotating magnetic field in the stators, the 
stators, placed on opposite sides of the rotor, will exert lateral force 
on the rotor due to the stator's magnetic field. If equal power is 
supplied to both stators, the attractive force on the rotor will be 
balanced between the two. However, if one stator is overenergized and the 
other stator is underenergized, the balance of attractive force can be 
shifted to the overenergized stator. This capability can be used to absorb 
some of the thrust forces applied against the thrust bearings. By shifting 
the balance of attractive magnetic forces on the rotor, its position while 
rotating between stators can be controlled to a certain degree to reduce 
the magnitude of rotor vibration that would normally be transmitted to the 
neighboring vessel structure. 
The major components of the propulsion assembly including rotor, stators, 
propeller blade hub, and shroud can be individually installed or removed 
from the vessel. Further, each of these components may be of split 
construction. 
This modular assembly greatly facilitates initial construction and later 
maintenance. The stators can be bolted to an aft member of a vessel hull 
section and the rotor journal mounted on an inner hull section. The 
propeller blade hub assembly is removably attached to the rotor and the 
propeller shroud can be removably attached to the blade hub or to the 
vessel hull. The second stator can then be mounted to a forward member of 
the vessel hull to complete the modular assembly of the major propulsor 
components. Interposed between the stators and the vessel hull sections to 
which they are attached will be a sound insulation layer, e.g. rubber, to 
absorb or reflect alternating current vibrations produced by the stator 
windings. 
In one embodiment of the present invention, dual, counter-rotating 
propulsors are used with a thrust transference member interposed between 
the two propulsors. In this arrangement, two rotors (one with reverse 
blades) are journal mounted in the vessel structure on either side of the 
thrust transference member. Stators (four total) are mounted on either 
side of each rotor and are powered such that the rotors rotate in opposite 
directions. Thrust bearing assemblies are mounted on either side of the 
transference member and on the fore and aft sections of the vessel hull. 
In this arrangement, during forward movement, propulsion force of the 
forward rotor/blade assembly is transferred directly to the vessel 
structure through the forward thrust bearings and propulsion force of the 
rearward rotor/blade assembly is transferred through the thrust 
transference member, its thrust bearings and the forward thrust bearings 
to the vessel structure. During rearward movement, the opposite sequence 
of propulsion force transference occurs. 
These and other features, objects and advantages of the present invention 
will be apparent from the foregoing drawings and detailed description.

DETAILED DESCRIPTION 
FIG. 1 illustrates a fragmentary view of the marine vehicle in which the 
present invention would be incorporated. This vehicle could be a 
submersible vessel, as shown for example in U.S. Pat. No. 3,101,066, in 
which instance body 10 may be a pressure hull. Alternately, the vehicle 
could be a surface vessel, in which instance the propulsor of the present 
invention may be mounted in a pod or cylindrical or cigar-shaped propulsor 
housing attached below the bottom surface of the vessel and below water, 
as shown for example in U.S. Pat. No. 4,389,197. In either situation, the 
marine propulsor of the present invention will be installed in a section 
12 of body 10, having generally a cylinder or frustum shape or 
configuration. Also shown in FIG. 1 is a shroud 14 covering the blades of 
the propulsor assembly. Fins 16 may be mounted at the end of a submersible 
vessel or on an attached pod or propulsor housing. 
As best shown in FIG. 2, the propulsor assembly is of modular construction, 
facilitating installation and removal. FIG. 2 illustrates the general 
relationship of the major components of the assembly, with details 
omitted. Shroud 14 is shown with four rib supports 18 that include 
openings 20 to accommodate rotating blades 22. Openings 20 divide each rib 
support 18 into preswirl struts 18a and postswirl struts 18b (FIGS. 2 and 
3.). Shroud 14 may of course be much larger than shown in relation to the 
blades 22 and blade hub 24. In installing the propulsor assembly, forward 
stator 26 may be mounted to a forward section of vessel housing 10 and 
rearward stator 28 may be mounted to a rearward section of vessel housing 
10. Rotor 30 is journal mounted to an inner housing section. Blade hub 24 
is fit over rotor 30 with rotor keys 32 being received within hub key 
slots 34 thus fixing hub 24 relative to rotor 30. As rotor 30 rotates, hub 
24 and blades 22 are rotated through the water causing a propulsion force 
that is transmitted to the vessel through thrust bearings in contact with 
thrust bearing surface 36. Bearing surface 36 may be on rotor 30 rather 
than hub 24 as shown. Each of the stators 26 and 28, rotor 30, blade hub 
24, and shroud 14 may be of split construction to further ease 
installation and removal. 
FIG. 3 is a cross sectional view of the vessel body section 12 with 
propulsor assembly installed. The forward stator 26 and rearward stator 28 
are mounted to front body portion 38 and rear body portion 40 of the 
vessel. Interposed between stators 26,28 and body sections 38 and 40 is 
sound attenuating material shown generally as 42. This material 42 may be 
any suitable acoustic insulation, e.g. rubber, wood, or fiberglass. This 
material will absorb or deflect alternating current (ac) waves generated 
by the stator windings and will dampen ac vibrations of the vessel body. 
This insulation prevents the body of the vessel from amplifying and 
transferring noise from the motor to the water surrounding the vessel. 
The stators 26 and 28 and rotor 30 are disk shaped, as best shown in FIG. 
2, without a drive shaft or other mechanism occupying their center. As a 
result, the central section 44 of the vessel body 10, which is a 
substantially cylindrical housing, can be open as shown generally at 120 
in FIG. 3. Central housing 44 can provide space for assemblies connecting 
the fore and aft of the vessel, thus permitting the aft of the vessel to 
be used, for example, for launching smaller vehicles or torpedoes (e.g. 
where the vessel is a submarine). The rear body section 40 of the vessel 
can be detachably connected to front section 38 by a bolting flange 46 
extending from central housing 44. Rear section 40 can be connected to 
flange 46 by bolts 48, or other suitable means. 
The rotor 30 is journal mounted on central housing 44. A journal bearing, 
shown generally at 48, carries the rotor 30. The journal bearing 48, 
described in greater detail in connection with FIG. 4, is water cooled and 
lubricated. Forced seawater is supplied from either a clean water tank 
(not shown) or through a filtered seawater inlet (not shown) through main 
fluid conduits 50 and secondary fluid conduits 52 which lead to journal 
bearing 48 and also to thrust bearing assemblies 54. The thrust bearing 
assemblies 54 are mounted on a peripheral area of the front and rear body 
sections 38 and 40 around their circumference to transfer (through bearing 
surface 36 of blade hub 24) the propulsive force created by the rotating 
blades 22 of the assembly to the vessel body 10. Bearing surface 36 may 
also be on rotor 30. The thrust bearing assemblies may comprise solid 
annular bearings or a plurality of spaced thrust bearing assemblies (e.g. 
conventional tilting pad type) mounted on the circumference of each of the 
front and rear body sections. The water forced through conduits 52 cools 
and lubricates thrust bearing assemblies 54. 
The rotor 30 and stators 26 and 28 are located outside of body 10 and are 
cooled by the surrounding water. They operate in water and at submergent 
water pressure, thus requiring no complex seals between the motor and the 
body of the vessel. In muddy or contaminated waters, clean or filtered 
water can be passed through the rotor and stators through tertiary 
passages 56. For the sake of simplicity, tertiary passages 56 are shown 
passing through rotor 30 only in the upper half of FIG. 3 while passages 
56 are shown passing through stators 26 and 28 only in the lower half of 
FIG. 3. Cooling water can be pumped through the rotor and/or stators 
through fluid conduits 52. Alternatively, cooling water can be brought in 
through a filtered inlet 58 in shroud 14. Again, for simplicity in the 
drawing, this alternative is shown only in the upper half of FIG. 3. Inlet 
58 feeds a cooling water passage 60 that extends through shroud 14, 
pre-swirl strut 18a and post swirl strut 18b. Struts 18a and 18b can be 
movably mounted to shroud 14 and/or body 10 to improve backing 
capabilities and maneuverability. Cooling passage 60 extends through 
limbers or passages 66 that supply water to tertiary passages 56. 
FIG. 4 illustrates the relationship of tertiary passages 56 in the rotor 30 
to the journal bearing. Filtered seawater or a fluid is pumped (by a 
conventional pump, not shown) through main fluid conduit 50 and secondary 
conduit 52 until it reaches the journal bearing 48. Alternatively, 
seawater may be directed through shroud 14 and limbers or passages 66 to 
journal bearing 48, as best shown in FIG. 3. Journal bearing 48 may take 
many forms, such as a plain cylindrical bearing, or as shown in FIG. 4 may 
include a plurality of spaced rubber staves or plates 68 mounted on an 
outer circumferential surface of vessel central housing 44. Water passes 
out of conduit 52 into the interstices 70 between staves 68. As rotor 30 
turns, water is drawn between staves 68 and rotor bearing surface 72 to 
lubricate and cool journal bearing 48. Also, due to centrifugal force, 
water is drawn up through passages 56 in rotor 30 to cool the rotor and 
also to supply a cooling and lubricating fluid to thrust bearing 
assemblies 54. As best seen in FIG. 3, the outlets of passages 56 are in 
the vicinity of thrust bearing assemblies 54 and bearing surface 5 of 
blade hub 24. 
FIG. 5 illustrates in general block form the electrical power system. The 
vessel propulsion power 74 may be any conventional power generator, e.g. 
fossil fuel, nuclear, etc. Optionally, the system may include an ac-to-dc 
converter system 76 to provide dc power on dc bus 78 to variable frequency 
(v.f.) inverter controllers 80 and 82. The v.f. inverter controllers 
invert power supplied on the dc bus 78 to ac power at a desired frequency. 
Two v.f. inverter controllers are provided to individually supply power to 
stators 26 and 28. Alternatively, as shown in FIG. 5A, ac-to-dc converter 
system 76 may be eliminated by utilizing v.f. cycloconverters 80A and 82A 
that directly convert the vessel propulsion ac power to the desired 
frequency. Power is supplied to the stator windings through waterproof 
cables and connectors, shown generally as 106 and 108. A conventional 
controller 88 (e.g. programmable computer) is utilized to operate v.f. 
controllers 80 and 82 to vary allocation of power between stators 26 and 
28. The speed of rotor 30 will be proportional to the frequency of power 
supplied to stators 26 and 28. An advantage of the v.f. controller is that 
the motor speed can be increased slowly from zero cycles/second to a 
desired operating speed while maintaining synchronization between the 
rotor speed and the speed of the rotating magnetic field of the stator, 
thus preventing slip and maximizing torque. 
Utilization of separate power inputs for the two stators 26 and 28 provides 
a unique advantage in controlling the electromagnetic forces on the rotor 
30. Stators 26 and 28 are mounted on opposite sides of the rotor as 
illustrated in FIGS. 2 and 3. Alternating current in the stator windings 
will set up a rotating magnetic field that will react with a rotor 
magnetic field to produce torque and a resultant propulsive force via 
blades 22. In addition, the stator magnetic fields will exert lateral 
force on the rotor. If controller 88 is operated to cause v.f. controllers 
80 and 82 to supply equal magnitudes of power to stators 26 and 28, the 
attractive forces on the rotor will be balanced between the two stators. 
However, controller 88 can be operated to cause v.f. controllers 80 and 82 
to overenergize one stator and underenergize the other, thereby shifting 
the balance of attractive forces to the overenergized stator. This 
capability can be used to electromagnetically counter the thrust forces by 
increasing the balance of attractive magnetic forces in a lateral 
direction opposite to the direction of thrust. This can relieve the forces 
applied against thrust bearings 54 and eliminate contact of the bearing 
surfaces. As a result, transmission of vibration forces of the rotor 
assembly to the supporting structure is minimized. The electromagnetic 
imbalance effectively absorbs propulsive thrust forces on the rotor 
assembly and reduces propulsor induced structural vibration. 
As mentioned previously, the motor of the present invention is shaftless, 
has a disk-shaped rotor and stators, and is substantially iron-free. 
Iron-free, disk-shaped machines are disclosed generally in U.S. Pat. No. 
4,691,133. In contrast to conventional iron motors generally used in 
marine vessels, the rotor/stator windings of the motor of the present 
invention contribute a larger proportion of the weight of the rotor and 
stators. Utilizing the space of the rotor/stator for windings provides 
sufficient power to drive the vessel without increasing the size or weight 
of the motor as would be required in a conventional iron motor. Referring 
to FIG. 3, the radial dimension D of the rotor 30 and stators 26 and 28 is 
at least 10% of the nominal diameter ND of the vessel body section 12 at 
the point where the propulsor assembly is mounted. This radial dimension 
provides sufficient space for a large number of windings to be mounted in 
the rotor and stators, yet leaves the inner portion of the vessel open for 
utilization of the aft section. 
FIG. 6 shows an example of the rotor assembly including a salient pole 
construction of the rotor, the blade hub, and blades. A large number of 
salient pole windings 84 is mounted in the disk-shaped rotor with frame 
122. Rotor excitation windings (secondary), shown generally at 86, are 
mounted in a slot in the interior circumference of rotor 30, and the 
journal bearing surface is shown at 72. Blade hub 24 is mounted on rotor 
30 as previously discussed. Thrust bearing surface 36 may either be a 
surface on hub 24 or rotor 30. Blades 22 (individual blades not shown) are 
fixed on the outer circumference of blade hub 24, as best shown in FIG. 2. 
FIG. 7 is an illustration of rotor frame 122 showing a plurality of 
mounting slots 88 for the rotor salient pole windings. Rotor excitation 
winding (secondary) slots are shown at 90. As shown in FIG. 8, first slots 
92 and second slots 94 are provided for dual salient pole rotor windings 
where the motor would be mounted in a vessel with backup, redundant 
systems. FIG. 8 further illustrates the rotor excitation winding slots 90 
and a wiring channel 96. Rectifiers may also be mounted in wiring channel 
96. FIG. 9 is an illustration of a typical salient pole winding 84 mounted 
via mounting bolts 98 in slots 88. FIG. 10 is a partial view of the stator 
26 or 28 with a plurality of salient pole stator windings 100 mounted 
inside, and primary excitation windings shown generally at 102. FIG. 11 is 
an alternate embodiment of stators 26 or 28 with the stator windings 104 
in a lap wound configuration. 
FIG. 12 is a representative view of the apparatus for providing rotor 
excitation current. A rotor excitation controller 114 supplies an 
alternating current source to primary excitation windings 102 through 
cables 116 and 118 respectively. Controller 114 permits the rotor 
excitation to be variable. Secondary excitation windings 86 in rotor 30 
are inductively coupled to primary windings 102 to provide rotor 
excitation current. This ac current is rectified by dc rectifier circuits 
110 to provide a dc current to rotor windings 84, thus creating magnetic 
poles as shown. Rectifier circuits 110 are connected to provide, with 
duplicate rotor windings 84, a redundant circuit. As an alternative to the 
induction circuit shown in FIG. 12, permanent magnets (111) may be 
installed in rotor 30. As a further alternative, low reluctance magnetic 
material may be mounted in the rotor, or the induction circuit shown in 
FIG. 12, permanent magnets, and low reluctance magnetic material may be 
combined in the rotor/stator assembly to provide the rotor magnetic field. 
FIG. 13 is a representation illustrating the dual, counter-rotating 
propulsor embodiment of the present invention. In this embodiment, two 
rotors 30 (one with reverse blades 22 mounted on hub 24) are journalled in 
the vessel body 10 on either side of a thrust transference member 112. 
Stators 26,28 (four total) are mounted in section 12 of vessel body 10 on 
either side of each rotor 30, and are powered in a conventional manner 
such that rotors 30 rotate in opposite directions. Thrust bearing 
assemblies 54 are mounted on either side of thrust transference member 112 
and on fore and aft sections of the vessel body 10. In the embodiment 
illustrated in FIG. 13, during forward movement, propulsive forces 
produced by the forward rotor assembly (rotor 30, hub 24, and blades 22) 
are transferred directly to the vessel structure through forward thrust 
bearing assemblies 54. Propulsive forces of the rearward rotor assembly 
are transferred through the thrust transference member 112, to the thrust 
bearing assemblies 54 mounted on the thrust transference member 112, to 
the forward thrust bearing assemblies 54, and finally to the vessel 
structure. Similarly during rearward movement, the propulsive forces 
produced by the rearward rotor assembly are transferred directly to the 
vessel structure while propulsive forces produced by the forward rotor 
assembly are transferred through thrust transference member 112 and the 
intermediate thrust bearing assemblies. As an alternative, and shown only 
in the lower half of FIG. 13, the thrust transference member may be 
secured to the vessel structure as shown generally at 122 or be a portion 
of the vessel structure. In this alternative embodiment, propulsive forces 
of both rotor assemblies are transferred directly to the vessel structure 
through the thrust bearing assemblies. 
The present invention has been described and shown in relation to various 
preferred embodiments. Detailed descriptions and illustrations of certain 
known components and operations have been omitted for the sake of clarity 
and understanding of the present invention. Conventional components and 
principles will be readily appreciated by those having ordinary skill in 
the art as will various modifications and changes that can be made to the 
embodiments disclosed, such embodiments, modifications, and changes being 
intended to fall within the scope of the invention a defined by the 
following claims.