High performance boat prop guard with high strength attachment bracket

A propeller guard including a cylindrically shaped ring which includes a plurality of evenly spaced openings therethrough. The openings in the ring are formed so as to allow water to pass therethrough during operation so as to improve the performance characteristics, specifically acceleration, plaining, speed and steering, of the motor. The ring is attached to the motor by an attachment bracket which is designed to inhibit fatigue and cyclical loading failure caused by vibrations of the ring during use. The ring is formed in a generally tapered recangular cross section in order to minimize the drag and vibration characteristics thereof during use. The ring is formed so as to be attachable to an outboard motor to surround the propeller thereof to protect the propeller from underwater objects.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates generally to guard members for shielding the 
propeller of an outboard motor. More specifically, the present invention 
relates to a propeller guard which is designed for maximizing the 
performance of an outboard motor having a propeller guard attached 
thereto, and maximizing the strength and performance characteristics of 
the bracket which attaches the guard to the motor. 
2) Prior Art 
The propeller of an outboard motor typically rests below the bottom surface 
of the boat when in use, and propels the boat through the water. Due to 
its position during operation, the propeller of as outboard motor tends to 
be very susceptible to damage from under water objects such an rocks, 
sandbars, marine life and the like. If the propeller of the outboard motor 
becomes damaged due to its impingement on underwater objects, it may 
become unable to perform as designed. The need for repair and/or 
replacement of a propeller damaged thus, generally occurs at very 
inconvenient times and is always a very expensive repair. Therefore, a 
need exists to develope a device which can protect the propeller of an 
outboard motor to prevent its being damaged by underwater objects. 
Also, the propeller of an outboard motor spins at an extremely high RPM 
during use. Should a passenger, skier, swimmer or other person be 
accidentally hit by the propeller during operation of the motor, serious 
injury will inevitably result. Therefore, a guard which will inhibit 
accidental contact of a person with the propeller to prevent accidental 
bodily injury is also needed. 
Many prior art attempts have been made to solve the above problems. Several 
prior art devices which are representative of the many previous attempts 
to develope a prop guard responding to the above identified needs are 
shown in U.S. Pat. No. 2,551,371 to Grieg; U.S. Pat. No. 2,963,000 to 
Fester; U.S. Pat. No. 2,983,246 to Manley; and, U.S. Pat. No. 4,078,516 to 
Balius. In each of these devices an enclosure, generally including a 
hollow cylindrical member, is attached to the outboard motor so as to 
surround the propeller. The device is designed to allow water to have 
fluid flow access to the propeller in order to allow the propeller to 
function as designed. Although these devices are somewhat successful in 
preventing damage to the propeller by preventing contact of the propeller 
with underwater objects, several severe drawbacks nevertheless remain. 
Most importantly, each of these devices tend to severely reduce the 
performance characteristics of the outboard motor. 
As is well understood, an outboard motor pushes a boat forward in reaction 
to the propellers of the motor forcing water backwards. However, an 
outboard motor which also includes a propeller guard is inhibited in its 
performance due to the fact that water flowing past the propeller tends to 
be dispersed and/or disrupted by the guard. Also, water impinging on the 
guard during operation of the motor increases the drag characteristic 
thereof, thus decreasing performance. Further, the presence of the guard, 
since not necessarily designed as an integral part of the motor, can cause 
instability, vibrations, control degradation, and unpredictability of 
motor response during use. Finally, prior art propeller guards are 
attached to the motor in such a manner as to be incapable of preventing 
damage or failure of the attachment members during high speed use. 
Accordingly, it is needful that a propeller guard be developed and 
designed which affords protection against contact between the propeller 
and underwater objects, and which at the same time is designed so as to 
maintain or improve the performance characteristics, such as steering, top 
end speed, planing, acceleration, attachment bracket durability, etc. of 
the motor. 
U.S. Pat. No. 4,680,017 to Eller, attempts to address the problem of 
maintaining and/or improving performance characteristics of the motor 
through the design a propeller guard. The propeller guard of the Eller 
invention functions to prevent radial dissipation of water passing through 
the propeller, to thereby cause all water to be directed in a linearly 
rearward direction as it passes through the propeller, so as to be useful 
in generating forward motion of the boat. However, in operation, the drag 
characteristics of Eller's propeller guard tend to off-set any advantages 
of its use. Further, control characteristics of Eller's motor are 
significantly degraded due to the presence and design of Eller's propeller 
guard. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a principle object of the present invention to provide a propeller 
guard for an outboard motor which can prevent inadvertent contact of 
underwater objects with the propeller thereof during use. 
It is also a principle object of the present invention to provide a 
propeller guard which is designed to maintain or improve the performance 
characteristics of the motor. It is another object of the present 
invention to design a propeller guard attachment bracket which is not 
subject to failure due to forces thereon such as vibration or cyclical 
loading in the manner of prior art attachment brackets. 
It is another object of the present invention to design a propeller guard 
which, improves the control characteristics such as planing, top speed, 
acceleration, and steering, etc. of the outboard motor. 
It is further an object of the present invention to design a propeller 
guard which is simple to manufacture and therefore relatively inexpensive, 
yet durable and reliable in design. 
These and other objects of the present invention are realized in a specific 
embodiment thereof which includes a generally cylindrical ring having 
diameter which is greater than the diameter of the propeller over which it 
is to be attached. The ring includes a small concave attachment plate at 
one position thereabout and an attachment bracket diametrically opposed 
thereto. The attachment plate and bracket allow the ring to be securely 
attached to the outboard motor at the bottom of the anti ventilation fin 
and the lower fin of the motor respectively. The ring includes a series of 
openings therethrough which allow the water passing over the exterior of 
the ring to be diverted through the openings by vacuum force, from the 
exterior of the ring to the interior thereof, as the guard passes through 
the water. The openings are oriented around the ring in a generally 
uniform manner and are formed through the ring in a manner which causes 
the water passing therethrough to be given a slight radial component of 
flow directed directly toward the propeller in order to give increased 
control (specifically steering), to the motor. The size, number and 
general distribution of the holes about the ring are calculated to allow 
sufficient radial flow of water to allow precise control and steerability 
of the motor, while avoiding problems due to flow blockage during a sharp 
turn which moves the ring into blocking relationship with water moving 
past the propeller. The ring is formed with a tapper in the interior side 
of the entrance opening and a similar tapper on the exterior side of the 
exit opening. The shape of the ring functions to diminish the drag and 
vibration characteristics of the guard while at the same time improving 
the controllability thereof by generating a vacuum through the openings in 
the sides of the ring which pulls water in toward the propeller. The 
attachment bracket of the invention secures the ring to the motor in a 
rigid and stable fashion and is designed to minimize vibrations and other 
cyclical loading thereon in order to prevent premature metal fatigue, 
cracking, or failure.

DETAILED DESCRIPTION OF THE INVENTION 
The prop guard 10 of the present invention is adapted for use on motor 
boats having either an outboard motor 13 as shown in dashed lines in FIG. 
or may be used on an inboard/outboard motor (not shown). The motor 
includes a propeller 14 having blades 15 which is operatively connected 
inside motor 13 to a conventional type drive. The prop guard 10 of the 
present invention is attached to the motor 13 at a location on the 
underside of the cavitation plate 16 by an attachment plate 18 and is also 
attached to a lower fin 17 of the motor 13 by an attachment bracket 12. 
As best seen in FIGS. 1 and 2, the prop guard 10 of the present invention 
is shown for purposes of the present disclosure as being attached to an 
88-90 Merc and Mariner 100-115 hp outboard motor. It is well within the 
scope of the present invention however to attach the prop guard 10 to any 
sized or type of outboard or inboard-outboard motor. For the purposes of 
the following description however is should be understood that the 
particular dimensions identified relate to the above mentioned motor and 
therefore would likely be modified should the prop guard 10 be adapted for 
placement on other types or sizes of motors. 
The propeller guard 10 is formed from a flat elongated rectangular section 
of metal which has been rolled into the shape of a circular ring shape 11. 
It is preferred that the metal be aluminum, and in the preferred 
embodiment of the invention the aluminum has a length of four feet, a 
width of four inches and a thickness of one quarter inch. When rolled into 
a circle, the diameter thereof is preferably seventeen and one quarter 
inches. 
The attachment plate 18 is preferably formed of a similar aluminum having a 
length of approximately seven inches, a width of approximately four inches 
and a thickness of one quarter inch. The attachment plate is preferably 
formed with a slight (one quarter inch) inward bow formed centrally 
therein which corresponds to the curvature of the bottom of the cavitation 
plate 16 of the motor 13 to which it is to be attached. 
The attachment plate 16 is drilled with four holes 19, each approximately 
5/16ths inches in diameter and each located at a corner of the attachment 
plate 16 approximately three quarters of an inch in from each side forming 
each corner. The attachment plate 18 is then placed into the ring 11 and 
welded to each end thereof to form a complete continuous circumference. 
The plate 16 is formed in the ring so as to cause the bowed shape thereof 
to be directed inwardly into the interior of the ring 11. It should be 
noted that the entire ring may be cast if desired instead of cut and 
welded as described without departing from the spirit and scope of the 
invention. 
As best shown in FIGS. 2 and 3, the attachment bracket 12 is preferably 
formed of stainless steel, and in the embodiment of the present invention, 
is formed of T304 stainless steel. The attachment bracket 12 includes a 
generally flat, rectangular metal plate 20 preferably approximately seven 
inches long by one inch wide by one-half inch thick, and a pair of side 
plates 22. The side plates 22 are cut to form front edges 21 which are at 
an angle (a) of approximately 40 degrees with the top edges 23 thereof. In 
the preferred embodiment the angle (a) is approximately 38 degrees. The 
rear edge 24 of side plates 22 are cut to form an angle (b) of 
approximately sixty degrees with the bottom edge thereof and in the 
preferred embodiment the angle (b) is approximately sixty degrees. 
The metal plate 20 is attached to the ring at a position diametrically 
opposed to the center line 25 of the attachment plate 18. 
The plate 20 is formed with three holes 26 therein which are one quarter 
inch in diameter and which are evenly spaced along the length of plate 20 
which contacts the ring 11. The plate 20 includes an extension 27 which is 
of a reduced thickness and which includes holes 28 of quarter inch 
diameter which correspond with holes 29 of the side plates 22 when the 
side plates are properly attached to the metal plate 20. Side plates 22 
are attached to the metal plate 20 by mean of bolts 30 and metal plate 20 
is attached to ream 11 by bolts 31. 
As best shown in FIG. 2, ring 11 includes an entrance opening 32 and an 
exit opening 33 and a central longitudinal axis 34. 
From center line 25 of attachment plate 18, proceeding in both directions 
around the interior surface 35 of the ring 11, ports 36 have been drilled 
through the ring 11 at four inch intervals around the entire circumference 
of the ring 11. The total preferred number of ports around the ring 11 of 
the particular embodiment shown is 10. Each port is centered approximately 
one and on quarter inches back from the inlet opening 32 of the ring 11. 
Each port is drilled through the ring 11 at an angle (c) of approximately 
34 degrees from the central axis of the ring 11. It is preferred that the 
two ports 36 closest to attachment plate 18 be of a diameter of 
approximately seven tenths of an inch and formed at an angle (c) of 
approximately thirty four degrees, with the remaining ports 36 being of a 
diameter of approximately one-half inch and at an angle (c) of 
approximately twenty two degrees. 
As shown in FIG. 4(a), the interior surface 35 of the ring 11, at the inlet 
opening thereof is formed with a somewhat rounded tapered end surface 37. 
Similarly, the exterior surface 44 of the ring 11 at the outlet end 
thereof is shaped with a taper 38 therein. 
The prop guard 10 is attached to a motor 13 by locating attachment plate 18 
against the bottom surface of cavitation plate 16 and bolting the 
attachment plate 18 thereto by bolts 39 passing through openings 19. The 
side plates 22 are then bolted by bolts 40 to the lower fin 17 of the 
motor 13. 
As best shown in FIG. 4(b), the prop guard 10 when in operation, is 
generally oriented to allow water to pass along the longitudinal axis 34 
thereof (as shown by lines 41). End surface 37 of ring 11 is shaped so as 
to cause water flowing there against, such as represented by line 42, to 
be diverted to flow along interior surface 35. Because line 42 has been 
diverted. As is well known in fluid dynamics, the diversion of water flow 
as represented by line 42 causes the speed of fluid represented by line 42 
to be increased. This in turn causes a vacuum effect through port 36. The 
vacuum effect generated in port 36 causes water to be drawn from the 
exterior surface 44 of ring 11, through port 36 and into the interior 
thereof (as shown by flow line 43). 
The net effect of surface 37 and ports 36 on the water flowing therepast is 
to draw a portion of the water from the exterior 44 of the ring 11 into 
the interior area of the ring 11 where it can impinge upon the propeller 
14, thus increasing the thrust capability of the motor 13. 
As shown in FIG. 4(c), when the motor 13 is moving at a velocity through 
the water, and the propeller is rotated relative to the flow of water in 
order to effect a turn, it can be seen that water flow lines 41 will 
impinge upon exterior surface 44 of the ring 11, and be diverted over end 
surface 38. 
In prior art prop guards, when the propeller is rotated to effect a turn, 
water is deflected away the propeller by the exterior surface of the ring. 
However, in the present invention, rotation of the propeller 14 to effect 
a turn causes ring 11 to orient a plurality of the ports 36 (those ports 
36 lying directly in line with the water flow lines 45) to be oriented 
such that water can flow directly through ports 36 and into the blades 16 
of the propeller 14, without interference from the ring 11. 
Prior art prop guards completely inhibit flow of water directly into the 
propeller during a turn such as shown by FIG. 4(c), and therefore cause 
cavitation in the area of the propeller. The cavitation causes a loss of 
thrust of the motor which in turn causes a loss of steering control. 
The design of the present invention, including the ports 36, insures that a 
flow of water is always directed into the propeller 14, even while 
effecting sharp turns at high speeds. Thus, the problem of prior art prop 
guards creating cavitation at the propeller thereof during turns has been 
solved in the design of the present invention. 
The prop guard of the present invention has been tested to verify the 
performance characteristics thereof during actual use. In a first test, a 
115 hp outboard engine was fitted with a 19 pitch stainless steel 
propeller. The motor was attached to a boat and run at top speed (with 
throttle wide open) and reached a speed of 37 mph at 5500 rpms. The prop 
guard of the present invention was then attached to the motor and the 
motor was again run to top speed. The top speed of the motor including the 
prop guard attached thereto was 36 mph at 5250 rpms. The prop guard of the 
present invention was then removed and replaced with a prior art prop 
guard. Top speed of the motor with the prior art prop guard attached 
thereto was 30 mph at 4800 rpm. 
A second series of tests were performed using the prop guard of the present 
invention. In this test, a 115 hp outboard motor with a 15 pitch aluminum 
prop was fitted with a ring similar to ring 11 of the present invention, 
however without any ports 36 extending therethrough and without any 
tapering of the front or rear end surfaces 37 and 38 thereof. The motor 
was run at full throttle and reached 29 mph at 5000 rpms. 
Next, the ring 11 was tapered at the rear end surface 38 thereof, in the 
manner as described in the disclosure, and the motor was again run at full 
throttle. The motor reached 30 mph at 5300 rpms. 
Next, the ring 11 was tapered at the front end 37 thereof, and the motor 
was run at full throttle. The motor reached 32 mph at 5000 rpms. 
Next, the ports 36 were drilled in the ring 11 at the angle as described 
above in the disclosed, and the motor was again run at full throttle. The 
motor reached 34 mph at 5750 rpms. The prop guard was then completely 
removed from the motor and the motor was run at full throttle and reached 
a speed of 35 mph at 5750 rpms. 
A third test was conducted to determine the period of time necessary to 
cause the boat to come to a level position from a starting position dead 
in the water, to an ending point at top speed. Using a 19 pitch stainless 
prop with the 115 hp outboard motor, and the prop guard 10 of the present 
invention, the average time to level the boat was 15 seconds. Without the 
prop guard 10, the average time to level the boat was 17.6 seconds. The 
test therefore showed that the prop guard 10 of the present invention 
helped the boat come to a level position more quickly than without its 
use. This is important because the boat operators visibility is impaired 
until the boat reaches a level position. Further, a quicker leveling of 
the boat allows the boat to be used to pull water skiers with greater 
ease. 
As has also been shown there is very little power loss with the pro guard 
10 of the present invention attached to the motor. Further, the control 
and handling of the boat with the prop guard of the present invention is 
easier since cavitation is greatly reduced or eliminated, especially 
during hard turns. 
In actual use, there have been other noted advantages of the present 
invention. For example in very choppy water, the prop guard 10 eliminates 
keel waking, thereby aiding in keeping the boat under control. Also, as 
shown by the tests above, with the use of the present invention it is 
possible to achieve similar top speeds from either a 15 pitch propeller or 
a 19 pitch propeller. This allows the use of a lower pitch prop (which has 
the advantages of higher power when accelerating from a dead stop) yet 
allows the prop nevertheless to retain a good top end speed (such as is 
generally the purpose of a higher pitched prop). In other words, use of 
the prop guard of the present invention allows a lower pitched prop to 
take on the characteristic of a more elaborate (and expensive) two speed 
prop. 
It is to be understood that the above described embodiment is only 
illustrative of the application of the principles of the present 
invention. Numerous modifications or alternative arrangements or 
embodiments may be devised by those skilled in the art without departing 
from the spirit and scope of the present invention.