Marine propulsion device including propeller protection means

Disclosed herein is a marine propulsion device including a lower unit having a gear case, a laterally extending anti-cavitation plate extending above a propeller carried on a propeller shaft rotatably mounted in the gear case, and a pair of generally flat side fins, each of which has a laterally extending trailing edge located wholly forwardly of the propeller and which either extend laterally in coplanar relationship from the opposite sides of the gear case, extend laterally in coplanar relationship from the opposite sides of the lower unit at a location between the anti-cavitation plate and the gear case, extend outwardly and downwardly in an angular relationship from the opposite sides of the gear case, or extend laterally in coplanar relationship from the opposite sides of the lower end portion of a vertical fin extending downwardly from the gear case.

BACKGROUND OF THE INVENTION 
The invention relates to marine propulsion devices such as outboard motors 
and stern drive units and, more particularly, to such marine propulsion 
devices including means for protecting the propeller against engagement 
with underwater objects. 
Various guard arrangements have been proposed to protect the propeller of 
marine propulsion devices, such as outboard motors and stern drive units, 
against damage by floating or submerged objects such as driftwood and 
other debris. Prior art propeller guard arrangements typically are quite 
expensive, are limited somewhat to a particular propeller size or shape 
and/or tend to affect propeller performance. Examples of prior propeller 
guard constructions are disclosed in the U.S. Modin Pat. No. 1,869,977 
issued Aug. 2, 1932, the U.S. Brown Pat. No. 2,894,447 issued July 14, 
1959 and the Canadian Liaaen Pat. No. 509,171 issued Jan. 18, 1965. 
SUMMARY OF THE INVENTION 
The invention provides a marine propulsion device including a lower unit 
having a gear case normally submerged in water, a propeller shaft mounted 
in the gear case for rotation about an axis and carrying a propeller, and 
a pair of generally flat side fins affixed on the lower unit, each having 
a laterally extending trailing edge located wholly forwardly of the 
propeller. Each of the side fins preferably include a generally straight 
outermost edge extending fowardly of the trailing edge and generally 
parallel to the propeller shaft axis and a leading edge extending at an 
incline outwardly and rearwardly toward the outermost edge. 
In one embodiment, the side fins are affixed on and extend laterally in 
substantially coplanar relationship from the opposite sides of the gear 
case with the leading edges of the side fins preferably extending from the 
forward end of the gear case. 
In another embodiment, the side fins are affixed on the lower unit at a 
location between a laterally extending anti-cavitation plate extending 
above the propeller and the gear case and extend laterally in 
substantially coplanar relationship from the opposite sides of the lower 
unit. 
In a further embodiment, the side fins are affixed on and extend downwardly 
and outwardly from the opposite sides of the gear case at an angular 
relationship to each other. 
In a still further embodiment, the marine propulsion device includes a 
vertical extending fin extending downwardly from the gear case and the 
side fins are affixed on the vertical fin, preferably on the lower edge 
thereof, and extend laterally in substantially coplanar relationship from 
the opposite sides of the vertical fin. 
One of the principal features of the invention is the provision of a marine 
propulsion device including simple means for protecting the propeller 
against contact with objects submerged in the water. 
Another of the principal features of the invention is the provision of a 
marine propulsion device including such propeller protection means which 
does not significantly affect propeller performance. 
Another of the principal features of the invention is the provision of a 
marine propulsion device including such propeller protection means which 
permits the use of propellers having a variety of sizes and blade 
configurations. 
Other features and advantages of the embodiments of the invention will 
become apparent upon reviewing the following detailed description, the 
drawings and the appended claims.

Before explaining the embodiments of the invention in detail, it is to be 
understood that the invention is not limited in its application to the 
details of construction and the arrangements of the components set forth 
in the following description or illustrated in the drawing. The invention 
is capable of other embodiments and of being practiced and carried out in 
various ways. Also, it is to be understood that the phraseology and 
terminology employed herein is for the purposes of description and should 
not be regarded as limiting. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Illustrated in the drawing is an outboard motor 10 including a propulsion 
unit 12 which is dirigible about a generally upright swivel post 14 
carried by a transom bracket 16 which is mountable on the transom of a 
boat. The propulsion unit 12 includes a power head 20 which houses an 
engine (not shown) and is carried on a lower unit 22 having a drive shaft 
housing 24 rigidly supporting the power head 20. The lower unit 22 
includes an enlarged, streamlined or torpedo-shaped gear case 26 which is 
rigidly attached to the lower end of the drive shaft housing 24 via a 
streamlined strut 27. The strut 27 has a curved leading edge or surface 28 
and a more pointed trailing edge or surface 29 (FIG. 2). 
Rotatably mounted in the gear case 26 for rotation about an axially 
extending axis 30 is a propeller shaft 32 which carries a propeller 34, 
for common rotation therewith, rearwardly of the aft end 36 of the gear 
case 26. The propeller shaft 32 is drivingly connected to a drive shaft 33 
which is rotatably supported in the drive shaft housing 24 and is 
drivingly connected to the engine in the usual manner. 
Extending laterally from the strut 27 above the gear case 26 and above the 
propeller 34 is a horizontal anti-cavitation plate 38 which extends 
rearwardly over the propeller 34. The anti-cavitation plate 38 is arranged 
to restrain the downward flow of air along the sides of the lower unit 22, 
particularly along the sides of the strut 27, and thereby retain water 
beneath the anti-cavitation plate 38 in a "solid" condition. 
The lower portion of the lower unit 22, including the anti-cavitation plate 
38, the gear case 26, and other components located below the 
anti-cavitation plate, normally is submerged in water when the boat is 
being propelled. The engine exhaust gases can be discharged under water 
through a discharge snout 40 extending rearwardly at an angle from the 
strut 27 beneath the anti-cavitation plate 38 and connected in 
communication with the interior of the drive shaft housing 24 as shown or 
discharged through a passage (not shown) within the propeller hub 42. 
Extending downwardly from the gear case 26 and forwardly of the propeller 
34 is a vertical fin or skeg 44 which is arranged to protect the propeller 
34 against damage by objects in the water passing beneath the boat. The 
trailing surface or edge 46 of the skeg 44 is located forwardly of the 
propeller 34 and the leading surface or edge 48 of the skeg 44 is inclined 
downwardly and rearwardly from the forward end 49 of the gear case 26. 
Additional means is provided for protecting the propeller 34 against 
engagement with underwater objects. More specifically, in the construction 
illustrated in FIGS. 1-3, such propeller protection means comprises a pair 
of generally flat, horizontally extending side fins 50 and 52 which are 
affixed on and extend laterally in substantially coplanar relationship 
from the opposite sides of the gear case 26 at or about the centerline or 
rotational axis 30 of the propeller shaft 32. In the specific construction 
illustrated, the gear case 26 has an upper portion 54 and a lower portion 
56 which are suitably fastened together at a joint 57 located at the 
centerline or rotational axis 30 of the propeller shaft 32. The side fins 
50 and 52 are disposed just below the joint 57 and are cast as an integral 
part of the lower gear case portion 56. 
As best shown in FIG. 2, each of the side fins 50 and 52 have a laterally 
extending trailing edge 58 located wholly forwardly of the propeller 34, a 
generally straight outermost edge 60 extending forwardly from the trailing 
edge 58 in generally parallel relation to the centerline or rotational 
axis 30 of the propeller shaft 32, and a leading edge 62 extending at an 
incline rearwardly and outwardly from a location adjacent the forward end 
49 of the gear case 26 toward the outermost edge 60. The trailing edges 
can extend forwardly at a small angle (e.g., 1.degree.) with respect to a 
plane intersecting and extending perpendicularly to the centerline or 
rotational axis 30 of the propeller shaft 32. Each leading edge 62 
preferably includes a convexly curved portion 64 which merges smoothly 
with the forward end portion of the outermost edge 60. The forward ends of 
the leading edges 62 can merge smoothly with the curved outer surface of 
the nose or forward end 49 of the gear case 26 as shown or can be spaced 
rearwardly from the forward end of the gear case. 
With this arrangement, the side fins 50 and 52 not only serve as guards for 
deflecting objects submerged in the water away from the propeller as the 
boat is propelled forwardly, but for some reason not fully understood at 
this time, have been found to reduce the tendency for the propeller 34 to 
cavitate or ventilate. In addition, termination of the trailing edges of 
the fins wholly forwardly of the propeller avoids interference with the 
propeller wash. The overall width of the side fins 50 and 52 preferably 
approximate, but can be slightly less than, the outside diameter of the 
propeller 34. That is, the outermost edges 60 extend to a lateral position 
generally corresponding to the peripheral path of the propeller 34. 
Further, since the trailing edges 58 of the side fins 50 and 52 terminate 
forwardly of the propeller 34, a variety of propellers having different 
blade sizes and shapes can be used for any given propulsion unit. 
FIGS. 4 through 6, illustrate alternate constructions for the side fins. 
Components constructed in the same general manner as the embodiment 
illustrated in FIGS. 1-3 have been assigned common reference numerals. 
In the alternate construction illustrated in FIG. 4 the side fins 66 and 68 
extend laterally in substantially coplanar relationship from the opposite 
sides of the strut 27 at a location between the anti-cavitation plate 38 
and the gear case 26, preferably immediately above the enlarged portion of 
the gear case 26. 
The laterally extending trailing edges 58a of the side fins 66 and 68 are 
located forwardly of the propeller 34. The outermost edges 60a, the 
leading edges 62a, and the curved portions 64a are arranged in the same 
general manner as in the embodiment illustrated in FIGS. 1-3. The forward 
ends of the leading edges 62a can be spaced rearwardly from the leading 
surface or edge 28 of the strut 27 as shown or can merge smoothly with the 
curved leading surface or edge 28 of the strut 27. 
In the alternate construction illustrated in FIG. 5, a vertical fin or skeg 
is omitted and the side fins 70 and 72 extend outwardly and downwardly 
from the opposite sides of the gear case 26 at an angular relationship to 
each other, and preferably from a location on the lower portion 56 of the 
gear case 26 below the centerline or rotational axis 30 of the propeller 
shaft 32. The laterally extending trailing edges 58b of the side fins 70 
and 72 are located forwardly of the propeller 34. The outermost edges 60b, 
the leading edges 62b, and the curved portions 64b are arranged in the 
same general manner as the embodiment illustrated in FIGS. 1-3, except the 
side fins 70 and 72 are slightly longer so that the outermost edges 60b 
extend laterally to a location generally corresponding to the peripheral 
path of the propeller 34. 
As with the embodiment illustrated in FIG. 4, the forward ends of the 
leading edges 62b can be spaced rearwardly from the forward end of the 
gear case 26 as shown or can merge smoothly with the curved outer surface 
of the nose or forward end of the gear case 26. 
In the alternate construction illustrated in FIG. 6, generally flat side 
fins 74 and 76 are affixed on the lower portion of the vertical fin or 
skeg 44 and extend laterally from the opposite sides of the skeg in 
substantially coplanar relationship. The laterally extending trailing 
edges 58c of the side fins 74 and 76 are located forwardly of the 
propeller and can be generally co-terminus with the trailing surface or 
edge 46 of the skeg 44. The outermost edges 60c, the leading edges 62c, 
and the curved portions 64c are arranged in the same general manner as in 
the embodiment illustrated in FIGS. 1-3. The lowermost surfaces of the 
side fins 74 and 76 can be generally coplanar with the lowermost edge or 
surface of the skeg 44 with the forward ends of the leading edges 62c 
being generally co-terminus with the leading surface or edge 48 of the 
skeg 44. If desired, the side fins 74 and 76 can be affixed on the skeg 44 
at a location intermediate the lowermost edge or surface thereof and the 
gear case 26 and the forward ends of the leading edges 62 can be spaced 
rearwardly from the leading surface or edge 48 of the skeg 44. 
Various features of the invention are set forth in the following claims: