Electric marine propulsion system

An AC marine propulsion system which provides a constant continuous rated horsepower availability at the standard rated engine RPM, over the full useful operating range of the propeller. A prime mover diesel engine is operated at its continuous rated speed to provide for maximum fuel efficiency. An alternator connected to the prime mover is driven at a speed which is higher than the standard rated alternator speed, thus producing higher than standard rated voltage and frequency. The combination of prime mover and alternator provides for an optimum system mass to power ratio. An AC motor is coupled through a gearbox to a fixed pitch propeller system. A frequency converter is dispsoed between the alternator and AC motor. Propeller RPM is controlled by changing the frequency of the voltage supplied to the AC motor. Variable shaft RPM at constant horsepower output is supplied to a fixed pitch propeller by selectively varying the frequency of the voltage supplied by the converter from 100% of maximum RPM to 40% of maximum RPM at 100% shaft horsepower.

BACKGROUND OF THE INVENTION 
The present invention relates to electric marine propulsion systems. 
Specifically, an AC electric motor drive is provided for a fixed pitch 
propeller wherein full power is available at variable shaft RPM and 
torque. 
Marine propulsion systems are of two basic types with regard to the 
structural design of the propeller. The first basic type is a propeller of 
"fixed pitch", usually solid cast, wherein the propeller RPM is controlled 
to comply with the demands of the system. Electric motor drives for fixed 
pitch propellers are described in various references, including U.S. Pat. 
No. 4,114,555, and in a paper entitled "SCR Controlled Electric Propulsion 
System" by Harry W. O'Brien, Jr., delivered at the spring meeting of the 
Society of Naval Architects and Marine Engineers in New Orleans, La. on 
Apr. 1, 1977. These referenced systems generate alternating current, and 
subsequently convert the AC current to DC for driving a DC motor. The 
prime mover driving the alternator is usually a diesel engine operated at 
a governed speed, which can produce maximum horsepower upon demand. As 
such, the prime mover is operated at its continuous rated speed to 
produce, upon demand, its rated horsepower output. These systems typically 
control motor RPM by varying the applied DC voltage through the use of an 
SCR controller. As such, the variable DC voltage supplied to the motor 
determines the propeller shaft RPM and shaft horsepower, as required. The 
DC motor is usually coupled to the propeller shaft through a reduction 
gear box. 
The foregoing referenced electric propulsion system provides for control of 
propeller shaft RPM over a first and second operating speed range, thus 
producing constant torque up to 100% of base power and shaft RPM, and a 
variable reducing torque, at 100% of base power, while operating at a 
shaft RPM above base RPM. The horsepower output of the propulsion system 
also varies proportionately with changes in shaft RPM up to 100% of base 
power, thus providing a variable horsepower, constant torque system, up to 
100% of base power and shaft RPM. A constant horsepower reducing torque 
system is provided once full horsepower output at 100% of base horsepower 
is attained at the top of the first range of operational RPM, wherein 
horsepower remains constant during operation of the vessel, from the 
bottom to the top of the second range of operational RPM. The foregoing 
electric propulsion system is extremely expensive and is inherently 
burdened with a large mass to power ratio. 
Al alternative to the fixed pitch propeller, DC electric drive is the 
non-electric controllable pitch propeller drive, usually of precision 
machined fabricated design, which provides the availability of full engine 
horsepower at a constant propeller shaft RPM. During operation of a 
controllable pitch propulsion system, the engine can maintain its full 
horsepower and RPM while complying with the system's various load demands 
by increasing or decreasing the pitch of the propeller. The advantage of 
the controllable pitch propeller system is a fully mechanical power train, 
with the ability to maintain 100% full horsepower while operating under 
the various load conditions experienced during operation of the vessel. 
The controllable pitch propulsion system, however, incurs proportionately 
excessive frictional drag losses when operating under low pitch, high slip 
conditions, which diminish the overall propulsive efficiency of the 
propeller system. As the pitch of the propeller is reduced while shaft RPM 
and power remain constant, the horsepower which is sacrificed to overcome 
frictional drag losses becomes very large, when compared to the greatly 
reduced frictional drag loss of a fixed pitch propeller capable of 
operating at much lower RPM, while still maintaining full engine RPM and 
horsepower output. The present invention combines the advantages realized 
in controllable pitch propulsion systems with those realized in DC or 
AC-DC electric propulsion systems, while eliminating their respective 
disadvantages. The system of the invention can provide a constant 
horsepower output to a fixed pitch propeller over a wide range of shaft 
RPM to affect a variable torque system which substantially reduces 
propeller frictional drag losses while operating at low shaft RPM, full 
power and high propeller slip. The electric propulsion system of this 
invention provides substantially 100% horsepower over the full useful RPM 
range of the propeller, while maintaining optimum utilization of the 
available horsepower. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a marine propulsion system 
which produces a variable torque propeller drive, utilizing constant 
horsepower over a wide range of shaft RPM. 
It is a more specific object of this invention to provide a totally 
alternating current marine propulsion system which drives a propeller, 
such system having a minimum mass to power ratio compared to other 
electric propulsion systems. 
It is a principal object of this invention to provide a variable torque 
propulsion drive system, which is capable of producing a more ideal 
balance between frictional and induced propeller drag losses at high slip 
ratios, which minimizes total drag losses and enhances the propulsive 
efficiency of the system by at least 5% over other types of propulsion 
systems. 
These and other objects are provided by the marine propulsion system in 
accordance with the invention. 
The propulsion system in accordance with the present invention, converts 
prime mover internal combustion engine power output into an alternating 
current. 
Typically, the prime mover is a diesel engine operated at its continuous 
rated RPM to provide for continuous rated horsepower upon demand. This 
operating condition for the prime mover produces its maximum efficiency 
for fuel consumption. An alternator is driven by the diesel engine at a 
rotational speed which produces a voltage and frequency greater than the 
rated voltage and frequency for the alternator. Thus, the prime mover 
operating at its continuous rated speed, in conjunction with an alternator 
driven so as to produce a voltage and frequency higher than its rated 
voltage and frequency, becomes a major system operative parameter. The 
generated voltage from the alternator at higher than standard voltage and 
frequency, provides the operative voltage for running an AC main 
propulsion motor. 
The higher than standard frequency is converted to any selected frequency 
using frequency conversion apparatus, and applied to an AC motor. The 
motor is subsequently coupled by reduction gear means to the propeller 
shaft having a fixed pitch propeller attached to its outboard end. 
The propulsion system, in accordance with the present invention, makes it 
possible to achieve a constant engine horsepower and RPM at maximum fuel 
efficiency, while changing the propeller RPM to attain an improved 
propulsion efficiency. Motor RPM is easily changed by changing the output 
voltage frequency of the conversion apparatus. When it is desirable to 
maintain a constant horsepower output at increased propeller loads, 
propeller shaft torque is inversely proportional to the shaft RPM. 
Optimum energy conversion is provided with the present invention by 
maintaining the prime mover at its continuous rated speed, while 
permitting the alternator to assume an operating RPM, higher than its 
standard rated RPM. As such, the prime mover need not be oversized so as 
to compensate for the diminished horsepower output often realized, when 
driving an alternator at the alternator's standard rated rotational speed. 
Thus, a minimum system mass to power ratio is achieved. 
During operation of the propulsion system of this invention, the fixed 
pitch propeller will be rotated at variable RPM as selected by the 
frequency converter. The fixed pitch propeller will, however, absorb less 
total drag losses at high slip than a conventional controllable pitch 
propulsion system, and the increased torque at reduced RPM will more 
efficiently handle the increases experienced in propeller loads.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a block diagram of a propulsion system 
in accordance with one embodiment of the invention. The embodiment 
depicted in FIG. 1 is a twin screw marine vessel, having separate power 
controls 47, 48 for each propeller 44 and 45. The propulsion system 
provides for a constant horsepower variable torque drive to each 
propeller. 
The twin screw propulsion system of FIG. 1 includes first and second diesel 
engine prime movers 11 and 12. The diesel engines 11 and 12 are preferably 
Model 16 V149T1, General Motor diesel engines, each having a continuous 
rated SHP of 1500 at 1800 RPM. The engines 11 and 12 are operated at a 
constant speed of 1800 RPM as set by a governor included with the engine. 
A third smaller size engine 13 is shown which is employed for emergency 
shipboard power. The diesel engine 13 will drive alternator 17 to provide 
converted shipboard utility power at 60 cycles. 
Alternators 15 and 16 may be standard rated at 530 kw., 60 Hz.- 8 pole, 240 
volts having a continuous rated RPM of 900. Alternator 17 may be standard 
rated at 100 kw., but is otherwise the same as alternators 15 and 16. 
Alternators 15, 16 and 17 are driven by the diesel engines 11, 12 and 13 
at the controlled speed of 1800 RPM, thus producing a higher than standard 
frequency of approximately 120 Hz. Additionally the voltage output of 
alternators 15, 16 and 17 will double from 240 to 480 volts under these 
input drive conditions. The kw of the alternators 15, 16 and 17 will also 
double from 530 to 1060 kw., and from 100 kw. to 200 kw., respectively, 
but, of course, do not produce an increased current output under these 
drive conditions. 
The foregoing combination of engines 11, 12 and 13 and alternators 15, 16 
and 17 provides for a maximum energy conversion from diesel power to 
electric power at a minimum system mass to power ratio. As such, the 
diesel engine of this invention may be selected to have a rated horsepower 
which is optimum, thus permitting the engine to be run at its optimum 
speed of 1800 RPM. Alternators 15, 16 and 17 are driven at this optimum 
engine RPM at higher than standard frequency. In the prior art electric 
power propulsion systems, the alternators 15, 16 and 17 would be driven at 
their standard rated speed to produce their standard rated voltage and 
frequency. As such, the prior art devices often impose an inefficiency on 
the diesel engine prime movers 11, 12 and 13 not incurred by the present 
invention. Since these prior art systems generate voltage at a standard 
frequency, the prime mover must always be sized so as to make up for the 
25-30% loss in horsepower output normally experienced by running the prime 
mover at the alternator's standard rated RPM. When running the alternator 
at higher than its rated RPM so as to match the standard rated RPM of the 
prime mover, the prime mover of this invention need not suffer any 
operating loss, while the propulsion system is further enhanced by a 
proportional increase in alternator kw. output. This relationship between 
prime mover and alternator which achieves a minimum mass to power ratio is 
described more fully in our previous patent application, Ser. No. 191,856, 
filed Feb. 29, 1980, hereby incorporated by reference. 
A common bus 18 combines the power output of alternators 15 and 16. The 
combined alternator outputs of 15 and 16 are, of course, phased together 
as is known to those skilled in the art, such that the 480 volts at 120 
Hz. is maintained. This three-phase power is to be converted through 
frequency converters 20, 21, 22 and 23 to a range of variable frequency 
suitable for driving propulsion motors 28, 29, 31 and 32. 
This three-phase power also supplies the main shipboard electrical service 
through the interlock system wherein it is first transformed by 
transformer 26 to a suitable voltage, and then converted through frequency 
converter 24 to standard 60 Hz. frequency. 
Each of the propellers 44 and 45 is connected by shafting to a reduction 
gear box 41, 42. The propellers are a conventional solid cast, 4-blade 
design of approximately 120 inches diameter, with a 92-inch pitch and 
having a pitch ratio (P/D) of 0.76. The propellers are designed to operate 
at the full 1,340 SHP, over a range from 171 to 68 RPM. The foregoing 
propeller specifications are of the type which will satisfy the 
requirements of a typical riverboat towing vessel. The reduction gear box 
41 and 42 is a double pinion reduction gear, designed with a 10.5 to 1 
reduction ratio. The gear box design will handle 100% SHP through the full 
range of 171 down to 68 shaft RPM, with the SHP becoming proportional to 
the shaft RPM (decreasing) at shaft speeds lower than 68 RPM. 
AC motors 28, 29, 31 and 32 are coupled to the input of reduction gear 
boxes 41 and 42. Coupling members 35, 36, 39 and 40 connect the 
appropriate motors to the reduction gear boxes 41 and 42. The AC motors 
may be a four-pole design, sized and rated to produce their full power of 
670 horsepower each through an operating range of 1800 RPM down to 720 RPM 
without excessive heating. The foregoing motors, when operated below 720 
RPM or 40% of their maximum RPM, will produce an output horsepower 
proportional to the reduced operating speed. 
Each of the AC motors 28 and 29 or 31 and 32 is supplied a voltage from a 
respective frequency converter 20, 21, 22 and 23. Frequency converters 20 
through 23 receive as an input the higher than standard voltage and 
frequency of the alternators 15 and 16. Frequency converters 20, 21, 22 
and 23 will supply constant voltage at a variable frequency under the 
control of power controls 47 and 48. Each of the power control panels 47 
and 48 is linked to the frequency converters 20 through 23 such that the 
pilot can independently control the RPM of propellers 44 and 45. Control 
links 49 and 50 may be electrical cables which will vary the resistance of 
frequency controlling elements of each frequency converter. Alternatively, 
mechanical linkage may be utilized to vary those potentiometers shown in 
U.S. Pat. No. 3,579,086. 
The frequency converters 20 through 23 are of a type known as a 
cyclo-converter, similar to those described in U.S. Pat. No. 3,579,086. 
The frequency converter employs an oscillator which will reduce the input 
frequency in proportion to the oscillator frequency. The frequency 
converter 20 is shown more descriptively in the Patent, hereby 
incorporated by reference. As such, the control of the propeller is 
accomplished by changing the frequency of a voltage applied to the AC 
motor. 
Frequency converter 24 is a fixed frequency converter, similar to the 
aforementioned cyclo-converter described in U.S. Pat. No. 3,579,086. A 
transformer 26 supplies the frequency converter 24. As such, alternator 17 
and diesel engine 13 may be driven at a higher than standard frequency 
RPM, and the correct 60 cycle voltage will be realized from the frequency 
converter 24. This resulting, transformed 60-cycle voltage may be 
distributed throughout the vessel to be utilized as ships service power. 
Referring to the aforementioned Patent, there is shown the general layout 
of a frequency converter for reducing a 120-cycle input voltage to a 
60-cycle voltage frequency, and then to a variable frequency voltage from 
60 Hz. to 0 Hz. Each of the propellers 44 and 45 are controllable by the 
aforesaid pilot house controls 47 and 48 which control the frequency of 
the voltage produced from a respective converter. 
The performance of the system of FIG. 1 is shown more completely in FIG. 2. 
FIG. 2 demonstrates the system efficiency for motor input voltage 
frequencies from 60 to 15 Hz. As is shown to the marine design engineer, a 
variable torque propulsion system is excellent, if it can provide full 
power at a maneuvering RPM equal to 50% of its maximum RPM. As such, it is 
seen that in the 60 Hz. to 24 Hz. operating range of this invention this 
design criteria is easily met. The entire system efficiency is shown to 
vary between 90 and 65% over the operating motor frequency of 60 to 24 Hz. 
As the motor frequency is varied between 60 Hz. and 24 Hz., the propeller 
shaft torque is shown to increase while the motor RPM decreases. Although 
shown as a straight linear change, the propeller shaft torque will, in 
reality, show a very slight curve between the 60 to 24 Hz. frequency 
range. However, for all practical purposes, propeller shaft torque may be 
considered linearly proportional to the RPM and motor frequency. 
Over this wide operating range of motor frequency, a constant horsepower 
output may be applied to the propellers 44, 45. The result of this type of 
operation can be more clearly shown in the following comparison of the 
theoretical performance of the present invention over that of a 
conventional controllable pitch propeller system. As shown in the Table 
below, the solid cast fixed pitch system produces frictional drag losses 
which rapidly decrease from maximum RPM to 40% of maximum RPM. Although 
the induced drag losses increase substantially over the same operating 
range, in the embodiment of the present invention, the induced drag losses 
are more than offset by the reduction in frictional drag losses, plus the 
accompanying increase in torque. As such, the large increase in propeller 
shaft torque at 100% power constitutes a system efficiency advantage of 
the present invention over the controllable pitch propeller system. 
Thus, one can see that the benefits of a controllable pitch propulsion 
system are more efficiently realized with the present invention. Full 
horsepower at increased torque availability is provided over the most 
important segment of the operating range of the propulsion system. 
Therefore, optimum vessel speed can be more nearly maintained when an 
increased load is realized by the vessel. The availability of the full AC 
system horsepower for varying propeller RPM provides a performance 
preferable to the controllable pitch propeller propulsion system, and also 
preferable to the referenced AC/DC constant torque electric propulsion 
system. 
Thus, there has been described with respect to one embodiment of the 
invention, an AC electric marine propulsion system providing constant 
horsepower with variable torque to a solid cast fixed pitch propeller. 
Those skilled in the art will recognize yet other embodiments described by 
the claims which follow. 
__________________________________________________________________________ 
COMISON BETWEEN TWO PROPULSION SYSTEMS OF EQUAL SHAFT 
HORSEPOWER AND PROPELLER SIZE 
FIXED PITCH PROPELLER "AC" ELECTRIC SYSTEM, VARIABLE TORQUE 
VERSUS 
CONTROLLABLE PITCH PROPELLER MECHANICAL SYSTEM, CONSTANT TORQUE 
B C D E H 
A Propeller 
Propeller 
Propeller 
Propeller F G Induced 
Propeller 
Shaft HP 
RPM at Pitch (Ratio) 
Torque at Propeller 
Fric. Drag 
Drag (H1) 
System at 100% 
100% Power 
at 100% Power 
100% Power Slip % 
Losses % 
Losses % 
__________________________________________________________________________ 
SHP 
Fixed Pitch Propeller `AC` Electric System 
1,340 SHP 
Changeable from Max. to 40% of Max. RPM 
Fixed Pitch Ratio (P/D) Solid Cast 
##STR1## 40% 80% 
25% 10% 10% 25% (H2) 
Controllable Pitch Prop. Mechanical System 
1,340 Constant at Max. RPM Only 
Changeable Pitch Ratio (P/D) Con- trollable Pitch 
##STR2## 40% 80% 
25% 25% 10% 15% 
__________________________________________________________________________ 
(H3) 
In the foregoing comparison between a `Variable` Torque and a `Constant` 
Torque system, the "relative" drag losses shown would normally occur, onl 
during conditions such as "maneuvering" operations, or whenever the 
propeller is subjected to rapid "slip acceleration", or reversal, as in 
switching from full ahead to full astern, and vice versa. Under these 
severe (zero to max.), thrust conditions, the Variable Torque system is 
capable of faster (thrust), recovery, and hence, greater utilization of 
the available shaft horsepower. Consequently, in a towing operation, it 
can provide improved "overall" vessel performance. 
(H1) Induced drag, is the "rotational" component imparted to the water by 
the action of the propeller. Propellers of comparatively heavy PitchRatio 
(P/D), generally produce relatively large `induced` drags when operating 
at high slips. On the other hand, propellers of fine PitchRatio (P/D), 
generally produce relatively small `induced` drags when operating at high 
slips. 
(H2) The relatively large increase in `induced` drag at high propeller 
slip and (comparatively), heavy PitchRatio (P/D), is accompanyed by a 
similar increase in torque. The increase in `induced` drag is however, 
offset by a similar decrease in frictional drag. This even exchange of 
drag losses, leaves all of the increased torque available to handle the 
increased load (increased slip and thrust), without any further sacrifice 
of power, or speed of advance. 
(H3) The relatively small increase in `induced` drag at high propeller 
slip and (comparatively), fine PitchRatio (P/D), is accompanyed by 
constant torque. The small increase in `induced` drag is, however, in 
addition to the maximum frictional drag. The additional drag losses, 
require that a further reduction be made in propeller pitch (controlable) 
to satisfy the additional sacrifice of useful power and subsequent lower 
speed of advance. Although the `slip` of the Controlable Pitch propeller 
may be the same as the Fixed Pitch propeller, the resultant thrust and 
speed of advance will be less, due to the fact, that the "speed of the 
propeller" (RPM .times. Pitch), for the CP propeller, has become less tha 
that of the Fixed Pitch propeller.