Marine propeller with transversal converging ribs

A marine propeller which includes arcuate ribs extending from each blade surface. Each rib is widely spaced at the blade's leading edge and curves inwardly towards the propeller hub to substantially converge at the blade's trailing edge. Each rib is further configured so as its height above the blade surface is highest at the blade's trailing edge and is tapered to a lower height at the leading edge. This rib configuration provides a greater blade surface area to produce greater thrust and overall higher propeller efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a propeller used to produce thrust when 
driven by a marine propulsion system, and more particularly, to a marine 
propeller which is capable of replacing a conventional marine propeller 
and includes a series of arcuate ribs on each blade used to increase 
hydrodynamic efficiency. 
2. Description of the Prior Art 
A propeller often is defined as a bladed device that rotates on a shaft to 
produce thrust in the direction of the shaft axis. Propellers are used in 
a wide variety of applications, including mechanical and electrical air 
fans, aircraft, and boating. In a marine environment, the propeller 
attached to a motor is the means by which the vessel or boat is driven 
through the water. Marine propellers may operate either clockwise or 
counterclockwise in relation to the boat's stern and may be shaft or 
stern-driven. Also, marine propellers are manufactured in a variety of 
shapes and sizes, depending on application. These types include standard 
outboard, cleavers, modified cleavers, choppers, and "elephant ears." As 
the propeller is driven or rotated by the motor, a negative pressure is 
created on one side of each blade surface while a positive pressure is 
created on the blade's opposite side. This pressure differential causes 
water to be drawn into the submerged propeller from an area forward of the 
rotating blade where it is then pushed or accelerated at a high velocity 
from its rear. 
In the past a number of methods have been used to modify the marine 
propeller in order to increase propeller efficiency. Increased efficiency 
allows the propeller to produce more thrust per revolution. One commonly 
used method is where two or more of the blades of the propeller are offset 
in a forward direction from a perpendicular line extending through the 
center of the blade hub. This offset is called the blade rake and 
increases a boat's ability to operate in both cavitational and ventilation 
situations. Propeller cavitation is generally the formation of both 
combustion exhaust vapor and air filled bubbles or cavities in water near 
or on the surface of the rotating propeller. These cavities occur when 
their pressure falls below the vapor pressure of water. Ventilation occurs 
when the blade is either fully or partially exposed from the water while 
in operation. 
A second and more common modification to the propeller includes blade pitch 
adjustment which is the amount of twist or turn of each blade in relation 
to the propeller hub. This twist allows the blade to form a helical 
surface. The propeller pitch is commonly set to determine the amount of 
water which will be pulled through the propeller per revolution, which in 
turn produces a corresponding amount of thrust. 
A third and less often used method to increase propeller efficiency has 
been the addition of a supplementary vane or rib blade which extends 
perpendicular to the propeller surface. This rib helps to draw water from 
the outer periphery of the blade towards the blade root or hub. This has 
the overall effect of increasing propeller efficiency since the ratio of 
thrust horsepower produced by the propeller is increased in relation to 
the shaft horsepower as delivered by the motor to the propeller. British 
patent 9930 discloses the use of a propeller which utilizes additional rib 
blades positioned on a main blade. The ribs protrude from the blade 
surface at a uniform height and extend linearly from leading edge to 
trailing edge. 
Additionally, U.S. Pat. Nos. 4,757,587, 4,128,363, and 3,294,175 further 
teach the use of rib configurations which extend across the blade surfaces 
in order to enhance propeller blade effectiveness. Although these ribs may 
be present on the blade surface, each rib does not include a cup area, nor 
are the ribs staggered in height from the center of the hub to the outer 
tip, nor are they tapered from a leading blade edge to trailing blade 
edge. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a propeller construction used 
primarily with marine engines comprises several rotatable blades, each 
extending from a center support section or hub. Each blade includes a 
plurality of arcuate ribs which extend transversely on the forward blade 
surface from the leading edge of the blade to the trailing edge. The ribs 
are widely spaced at the leading edge and are positioned so as to arc 
inwardly towards the propeller boss or hub, substantially converging at 
the blade's trailing edge. Each rib is further characterized by its unique 
height in that the rib protrudes upwardly from the blade surface and forms 
a gradient so that the rib height at the blade's trailing edge is greater 
than the rib height at the blade's leading edge. Moreover, the innermost 
rib, closest to the blade hub, has the lowest mean height as compared with 
the subsequent ribs extending outwardly from the blade hub. This provides 
a staggered rib configuration which traps water passing over each rib 
during rotation from the innermost rib to outwardly extending ribs. The 
height of each outwardly extending rib may be increased proportionally in 
size to the rib directly adjacent at its hub side. Further, each rib is 
slanted or angled inward towards the forward rotation of the blade and 
includes a curved upper rib cup, which allows each rib to draw and hold a 
greater amount of water. 
It is therefore the principal object of the invention to produce a 
propeller with arcuate ribs with a unique tilt angle, taper and cup, which 
will increase each blade's overall efficiency. 
It is a further object of the invention to produce a marine propeller that 
is compatible with an existing marine drive system and will increase both 
speed and hydrodynamic efficiency while reducing vibration. 
It is still further an object of the invention to produce a marine 
propeller that will decrease the amount of slip produced by a boat while 
increasing thrust, so as to allow the boat to be in proper trim, 
increasing boat speed. 
It is still further an object of the invention to produce a marine 
propeller that allows an outboard or stern drive, shaft drive, surface 
drive, or unit to be mounted higher on the transom, thereby reducing the 
drag created by a motor mounted lower on the boat stern. 
In accordance with these and other objects which will be apparent 
hereinafter, the instant invention will now become described with 
particular reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Although the following detailed description is drawn to a propeller with 
three rib configuration, it should be recognized by those skilled in the 
art that any number of ribs may be positioned on each blade surface and 
the optimal number will depend upon propeller size, number of blades, 
blade configuration, engine horsepower and torque, varying with each 
individual application. 
With reference to FIGS. 1 and 2, the outboard propeller generally shown at 
1 includes an outer hub 3 and inner hub 5. An exhaust passage 7 extends 
through the inner hub 5 and acts as an aperture to allow outboard engine 
exhaust gases to discharge into the water while in operation. A series of 
blades 9 are fixedly attached to the outer hub 3. Each blade 9 includes a 
blade back 11, blade face 12, and blade tip 13. 
A cup 15 is located on the trailing edge of the blade and is generally 
defined as a small curve or lip which permits the propeller blade to hold 
water momentarily within the lip during rotation. 
The blade tip 13 separates the leading edge 17 of the blade from the 
trailing edge 19. The blade tip 13 may be formally defined as the maximum 
reach of each blade from the center of the inner hub 5. The leading edge 
17 is that portion of the blade, which in view of the blade pitch, is 
nearest to the boat hull and first contacts the blade surface during 
rotation. The trailing edge 19 is that part of the blade furthest from the 
boat hull and is the edge from which water leaves the blade. Trailing edge 
19 extends from the blade tip 13 to the inner hub 5. 
The propeller 1 further includes a plurality of arcuate ribs 21 which are 
placed on each blade back 11. In the embodiment shown in FIGS. 1-5, a 
series of three ribs 21a, 21b, and 21c extend from the leading edge 17 to 
the trailing edge 19 on each blade back 11. Each of the ribs 21a, 21b, and 
21c are widely spaced at the leading edge 17, and each bends or arches 
toward the inner hub 5 at a point 23 and is angled so as to substantially 
converge as each of the ribs 21a, 21b, and 21c reaches the trailing edge 
19 of blade 9. 
By way of example, one way to position each of the ribs 21a, 21b, and 21c 
onto blade back 11 is through the use of concentric circles. In FIG. 2A, 
three concentric circles CC1, CC2, and CC3 are drawn about a center point 
CP of inner hub 5. The radius of these circles are drawn by using the 
distance x between center point CP and the blade tip 13. The radius of 
each of circles CC1, CC2, and CC3 is then calculated by taking a 
percentage of distance x. In this example, 0.9x, 0.7x, and 0.5x have been 
used for circles CC3, CC2, and CC1, respectively. 
After calculation of each concentric circle radius, circles CC1 and CC3 are 
next positioned so as to tangentially intersect circle CC2 at point T. 
Point T is defined as a point substantially 270.degree. , measured from 
the line at 0.degree. created by points CP and blade tip 13. Movement of 
CC1 and CC3 creates a series of arcs 21a.sub.arc, 21b.sub.arc, and 
21c.sub.arc, which pass across blade back 11. The portion of these arcs 
which pass across the blade back define a relative position where each of 
ribs 21a, 21b, and 21c may be initially placed. It should be pointed out 
that this is only one method which may be used to position ribs and other 
mathematical methods are possible. 
An additional aspect of ribs 21a-21c is that each is raised and extends 
substantially perpendicular to the surface of blade back 11. As best seen 
in FIGS. 5A and 5B, each rib 21a, 21b and 21c has a negligible height 
h.sub.1 at the leading edge 19, which may be substantially flush with the 
blade back 11 of blade 9. Each of the ribs 21a 21b and 21c increases in 
height in a gradient, where each ultimately reaches its respective maximum 
height h.sub.2 at the trailing edge of the blade 9. Each respective height 
varies according to specific application and may also vary according to 
hub size, motor torque, and horsepower. These engineering parameters are 
matters of design choice which will be obvious to those skilled in the 
art. 
In order to more fully show this relationship FIG. 4 illustrates an 
enlarged top view of ribs 21a, 21b, and 21c extending transversely from 
the leading edge 17 and substantially converging at the trailing edge 19. 
FIG. 5A illustrates a side sectional view of the blade 9 drawn through 
sectional lines Va--Va shown in FIG. 4, showing ribs 21a, 21b, and 21c 
extending from the leading edge 17 to trailing edge 19, forming a gradient 
therebetween. Each respective rib is at its lowest height h.sub.1 at the 
leading edge, and slowly tapers to a greater and greater height, reaching 
its maximum h.sub.2 at the blade's trailing edge. FIG. 5A further 
illustrates that each respective rib increases to its maximum height 
h.sub.2 which is higher than its inwardly adjacent rib. For example, rib 
21b reaches a greater height than rib 21a, and rib 21c reaches a greater 
height than rib 21b. This staggered increase in blade height would 
continue outwardly to each successive rib, no matter how many ribs were 
used on the blade surface. FIG. 5A further shows a curved upper area or 
rib cups 30a, 30b, and 30c on each respective rib surface. Each rib cup 
30a, 30b, and 30c is a curved area extending along the entire top edge of 
each respective rib and acts to trap and retain additional water during 
propeller rotation. 
FIG. 5B illustrates a side sectional view drawn from sectional lines Vb--Vb 
shown also in FIG. 4. Each rib 21a, 21b, and 21c is at a respective height 
d.sub.1, d.sub.2, d.sub.3 above the blade back 11. This Figure further 
illustrates that although the ribs may protrude substantially 
perpendicular from the blade back 11, each is tilted or angled forward at 
a respective angle .alpha., .beta., and .theta.. This forward angle is in 
the direction of blade rotation and may be set anywhere between 89.degree. 
and 1.degree.. These angles will depend on specific application since the 
lesser the forward angle, the greater amount of water will be trapped 
under each respective rib during rotation. 
FIG. 6 further illustrates an alternative embodiment showing a two rib 
configuration where ribs 21d and 21e extend across blade surface 12. This 
embodiment is different from that described above in that only two ribs 
extend across each blade back 11. This embodiment offers the advantage of 
less hydrodynamic drag, yet has the disadvantage of producing less thrust 
per revolution since only two ribs are present, having an overall lesser 
surface area. 
It should be recognized that the propeller design as described is not 
limited to any specific number of blades and any number of ribs with 
various minimum or maximum heights h.sub.1 or h.sub.2, tilt angles 
.alpha., .beta., and .theta., or rib cups are possible on any type or 
number of propeller blades. 
OPERATION 
As seen in FIGS. 1 through 3, as blade 9 spins in the direction shown by 
the arrow, the propeller draws or pulls water in from its front end 
through an imaginary cylinder approximately the size of the propeller 
diameter. The front end of propeller 1 is that end that faces the boat 
rear surface or hull. As the propeller rotates, water accelerates through 
each blade 9, creating a slightly smaller water stream or jet of higher 
pressure water behind each of the propeller blades. The water jet exiting 
the propeller blade is smaller in diameter than the actual diameter of the 
propeller. This type of water jet action of pulling water in through the 
propeller blade 9 and pushing it out the rear of the propeller produces a 
resultant force generally referred to as "thrust." It is this type of 
pulling and pushing force that drives a boat in a forward direction. 
FIG. 2 specifically depicts the propeller 1 moving in a direction shown by 
the accompanying arrow with the differential pressure areas graphically 
illustrated. Water is pulled into the propeller and as the propeller 9 
rotates downward, water is pushed down and back, away from blade face 12. 
The thrust produced by the propeller of the instant invention comes to a 
focal point rearward of the blade, before dispersing. This is distinct 
from propellers of the prior art, which push water directly from the blade 
without bringing the water from each blade to a focal area. This results 
in a greater positive pressure area shown in FIG. 3 by the positive "+" 
sign. Similarly, water is being drawn in on the top side of the blade as 
the blade rotates. This results in a negative pressure, illustrated in the 
drawing by the "-" or negative sign. This negative pressure pulls the 
propeller along through the water while the resulting opposite positive 
force acts to simultaneously provide a pushing action. 
Due to the increase in overall surface area because of the addition of the 
ribs, each rib 21a, 21b, and 21c on the blade back 11 acts to more rapidly 
channel a greater amount of water to the center or inner hub 5 of the 
propeller 1 than that of a conventional marine propeller. This additional 
water channeled to the center creates a greater thrust. 
Further, the wide gap between ribs at each blade's leading edge 17 acts to 
gather a greater volume of water towards the blade face 12. These 
converging ribs have an effect of reducing the cross sectional area normal 
to the water flow across the blade surface. This decrease in area produces 
a resulting increase in flow pressure as the water, which is 
non-compressible, is continually forced or squeezed into a narrower and 
narrower passage. 
The tilt angles .alpha., .beta., and .theta. of ribs 21a, 21b, and 21c, and 
rib cups 30a, 30b, and 30c, as seen in FIGS. 5A and 5B, trap and retain 
even a greater amount of water moving across the rib surface. Any water 
which may escape from under these rib cups moves up and over the top of 
the respective rib. Since the next outwardly adjacent rib protrudes at a 
greater height than that which the water escapes, the water moving in that 
direction is caught in this rib. The water is then pulled underneath the 
rib cup and the same sequence of events occurs. Hence, this allows even a 
greater amount of water to be trapped by each rib, producing a greater 
pressure differential than normally could be achieved between blade back 
11 and blade face 12 if a conventional marine propeller design were used. 
Overall, the propeller configuration of the instant invention has the 
advantage of producing less slip and greater thrust, while ultimately 
allowing a boat using this propeller to attain a higher velocity. The use 
of arcuate ribs allows the propeller blades to be more hydrodynamically 
efficient, producing an overall greater thrust per propeller revolution. 
In contrast to propellers which are selected for either their off-the-start 
or high velocity capability, the propeller of the instant invention allows 
a boat to take off and accelerate from the start at a quicker rate. The 
propeller allows for both a very quick start while still attaining a high 
top speed. The propeller produces less wake while in operation, further 
demonstrating its hydrodynamic efficiency. 
The instant invention has been shown and described herein in what is 
considered to be the most practical and preferred embodiment. It is 
recognized, however, that departures may be made therefrom within the 
scope of the invention and that obvious modifications will occur to a 
person skilled in the art.