Marine engine exhaust muffler

An exhaust muffler for mounting on the transom of an inboard or inboard/outboard motorboat around the exhaust pipes which extend from the transom. The muffler comprises a housing which has a pair of sidewalls each adjacent to one of the sides of the boat, so that the housing is approximately equal in width with the width of the boat at the stern. Inside the housing is a baffle extending downwardly from the top of the housing which directs the exhaust downwardly around the bottom of the baffle to reduce the noise emanating from the exhaust. The top of the housing preferably forms a swim platform which provides a functional and attractive appearance to a muffler located outboard of the boat. The large size of the muffler results in enhanced noise reduction without adversely affecting engine performance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring more particularly to the drawings, and initially to FIG. 1, there 
is shown the muffler 20 of the present invention mounted at the stern of a 
motorboat 21. The motorboat 21 is a conventional inboard or 
inboard/outboard motorboat having a marine engine located within the boat 
and having an exhaust pipe 22 (FIG. 5) extending aft. The stern of the 
motorboat 21, as shown in FIG. 1, includes a pair of sides 23 and a 
transom 24. The exhaust pipe 22 extends through the transom 24 of the 
motorboat slightly above the water line. In the motorboat 21 shown in FIG. 
1, there is a single inboard engine with either one or two exhaust pipes 
22 located near the center of the transom 24. As will be described later, 
the muffler of the present invention is capable of modification to 
accommodate motorboats having dual engines with multiple engine exhausts. 
In addition, because the muffler of the present invention is symmetrical 
in design, the muffler can be used with motorboats in which the exhaust 
pipe extends through either the port or the starboard side of the transom. 
The muffler 20 of the present invention mounts on the transom 24 of the 
motorboat 21, and extends rearwardly from the transom. As shown in FIGS. 
1, 2 and 3, the muffler 20 comprises a housing formed by a top portion 26 
and a bottom portion 27. The top portion 26 forms the upper half of the 
muffler 20 and is comprised of a pair of upper exterior sidewalls 28 on 
each side of a top surface 29. The bottom portion 27 forms the lower half 
of the muffler 20 and is comprised of a pair of lower exterior sidewall 
30, a pair of rear walls 31, one adjacent to each of the lower exterior 
sidewalls 30, a pair of contoured interior sidewalls 33 each extending 
from one of the rear walls 31 toward the transom 24 of the motorboat, two 
lower bottoms 35 extending between each of the interior sidewalls 33 and 
the exterior sidewalls 30, and a recessed upper bottom 37 extending 
between the two interior sidewalls 33 directly beneath the top surface 29. 
The bottom portion 27 also includes a forward wall 38 (FIGS. 4 and 5) 
adjacent to the transom 24 and having openings 39 through which the 
exhaust pipes 22 extend. 
The top portion 26 and the bottom portion 27 may each be formed of the same 
material used in the hull of the motorboat 21, such as Fiberglas, or they 
may be made of aluminum or other suitable laminating material. Preferably, 
fiber-reinforced plastic, such as Fiberglas, is used, so that the muffler 
20 of the present invention is completely non-corrosive and long-lasting. 
The top portion 26 and the bottom portion 27 are preferably bonded 
together with epoxy or other suitable material, and they may also be 
fastened together with pop rivets at the gunwale. 
As shown in FIG. 1, the exterior sidewalls 28 and 30 each extend aft from 
the transom 24, and are generally adjacent to the sides 23 of the 
motorboat, so that the muffler 20 extends from side to side across 
substantially the entire stern of the motorboat 21, and the muffler is 
approximately equal in width to the width of the motorboat at the stern. 
As shown in FIGS. 1 and 2, each of the lower exterior sidewalls 30 
comprises lower sidewall portions 43 and 44 formed in the bottom portion 
27 of the muffler 20. The upper sidewalls 28 extend from near the top of 
the transom 24 to the rearward edge of the top surface 29, and provide 
support for the top surface 29 as it functions as a swimming platform. 
The top surface 29, which extends rearwardly from the transom 24, provides 
a swimming and diving platform at the stern of the motorboat which may be 
conveniently used for entering and exiting the boat and as a staging area 
for water skiing. As with other swimming and diving platforms which are 
mounted on the sterns of motorboats, the platform formed by the top 29 
should be located above the water line but sufficiently close to the water 
line to function as a platform. 
The top 29 comprises a horizontally extending aft portion 46 and a smaller, 
sloped forward portion 47 adjacent to the transom 24. As shown in FIG. 5, 
the sloped portion 47 is provided for clearance for the exhaust pipe 22, 
which extends through the transom 24 of the motorboat. The top 29 also 
includes an upwardly extending portion 48 which extends upwardly from the 
sloped portion 47 adjacent to the transom 24 and terminates at the front, 
upper portion of the exterior sidewall portion 28, providing a downwardly 
stepped configuration from the stern. 
The recessed upper bottom 37 supports the rearward edge of the top surface 
29 along the length between the interior sidewalls 33. The upper bottom 37 
extends between the interior sidewalls 33 directly beneath the aft end of 
the top surface 29. 
A pair of hollow muffler chambers 51 are thus formed in each side of the 
muffler 20. Each of the chambers 51 extends rearwardly from the transom 24 
to the rearward edge of the top surface 29. Each chamber 51 is formed by 
one of the exterior sidewalls 30, one of the rear walls 31, one of the 
contoured interior sidewalls 33, a portion of the forward wall 38, the 
lower bottom 35, and a portion of the top surface 29. The forward wall 38 
is provided with a pair of openings 39 through which the exhaust from the 
exhaust pipe 22 enters each of the chambers 51. Each of the chambers 51 
extends from the middle of the motorboat toward the outside to permit the 
flow of exhaust from the exhaust pipe 22 to enter the chamber. 
As shown in FIG. 4, the exhaust is channeled outwardly toward the sides of 
the muffler 20 by the contoured interior sidewalls 33. The exhaust leaves 
each of the chambers 51 through ports 53 formed in the rear walls 31. For 
the muffler 20 shown in FIGS. 1-5, a pair of ports 53 are provided in the 
rear walls 31, with one port provided on each of the rear walls. The area 
of each of the exhaust ports 53 must be larger than the cross section of 
the exhaust pipe 22 in order to prevent excessive back pressure within the 
chamber 51, which back pressure could have an adverse effect on engine 
performance. The bottom 35 of the chamber 51 is below the water line, so 
that water fills the chamber up to the level of the bottom of the exhaust 
port 53, since the only way in which water can exit through the chamber is 
through the exhaust port. The exhaust ports 53 should each be 
approximately just above the water level. If the exhaust ports 53 are 
located too high, a large amount of water accumulates in the chamber 51, 
which may create excessive back pressures. 
Within each of the exhaust chambers 51, suitable muffler or baffle means 
are provided. In the embodiment of the invention shown in FIGS. 1-5, 
baffle means are employed in each of the chambers 51 comprising a 
downwardly extending baffle 61 mounted from the top surface 29 in the 
region in which the chamber narrows as the interior sidewall 33 approaches 
the exterior sidewall 30. Each of the baffles 61 includes a protrusion 53 
extending across the top of the baffle which is inserted into a 
corresponding slot in the bottom of the top surface 29. Alternative 
muffler means will be described hereinafter. 
The bottom 35 and the interior sidewall 33 of each of the chambers 51 are 
contoured in such a manner as to direct the flow of exhaust water from the 
exhaust pipe 22 around toward the outside of the chamber and through the 
exhaust port 53. Each of the baffles 61 extends downwardly from the top 
surface 29 to require the exhaust flow to turn downwardly below the water 
line and then back upwardly to the exhaust port 53. If the baffle 61 does 
not extend a sufficient distance down from the top surface 29, the exhaust 
flow can flow through the chamber 51 without any baffling effect 
whatsoever. However, the baffle 61 should not extend down within the 
chamber 51 to a point in which the area beneath the baffle is such that 
excessive back pressure may be induced within the chamber. 
Since the bottom 35 of the chamber 51 is beneath the water line, water 
normally fills the chamber as previously noted. As shown in FIG. 5, the 
baffle 61 may extend downwardly beneath the water line. The exhaust from 
the exhaust pipe 22 usually includes water, and the water/exhaust mixture 
is sent through the chamber 51 and must pass beneath the baffle 61 before 
exiting through the port 53. This convoluted path of the water with the 
exhaust gas produces optimum muffling effect. 
An alternative muffler design for motorboats having dual engines and 
multiple exhausts is disclosed in FIGS. 6-8. In this design, two inner 
muffler chambers 52 are provided in addition to the two outer chambers 51. 
The two inner chambers 52 are located between the two outer chambers 51 
and extend rearwardly from the transom 24. The inner chambers 52 do not 
extend to the rearward edge of the top surface 29 as the outer chambers 51 
do, so that clearance is provided for the engine propellers. The two inner 
chambers 52 are formed by a recessed rear wall 32, a pair of interior 
sidewalls 34, the forward wall 38, and a lower bottom 36. The interior 
sidewalls 34 are located between the interior sidewalls 33. The two inner 
chambers 52 are separated from each other by a divider 56. The forward 
wall 38 is provided with additional openings 40 through which two 
additional exhaust pipes 22 may extend. Two additional ports 54 are 
provided in the rear wall 32. Exhaust enters the inner chambers 52 through 
the additional openings 40 in the forward wall 38 and leaves through the 
ports 54. 
As with the outer exhaust chambers 51 already described, similar suitable 
muffler or baffle means may be provided in the inner chamber. In the 
embodiment of the invention shown in FIGS. 6-8, a downwardly extending 
baffle 62 is mounted from the top surface 29 adjacent to the exhaust pipe 
22 to direct the exhaust downwardly. The considerations in the placement 
and dimensions of the baffle 62 are similar to those of the baffles 61 
already described. 
Other alternative muffler means are shown in FIGS. 9-20. 
In the first alternative embodiment shown in FIGS. 9 and 10, downwardly 
extending baffles 71 and 72 are mounted from the top portion 26 adjacent 
to the outer end of the exhaust pipes 22 to direct the exhaust downwardly, 
and upwardly extending baffles 73 and 74 are mounted from the bottom 
portion 27 rearwardly of the baffles 71 and 72 to reverse the flow of the 
exhaust. 
In the second alternative embodiment shown in FIGS. 11 and 12, downwardly 
projecting muffler pipes 81 and 82 extend from each of the exhaust ports 
53 and 54. The pipes 81 and 82 each have a downwardly directed elbow so 
that the exhaust from the exhaust pipe 22 is mixed with water already in 
the chamber 51, which then enters through the bottom of the pipes 81 and 
82 and exits through the ports 53 and 54 when the water level in the 
chamber becomes higher than the level of the port. 
In the third alternative embodiment shown in FIGS. 13 and 14, conventional 
muffler chambers 91 and 92 are employed within the chambers 51 and 52. 
Each muffler chamber has an entry pipe 93 or 94 connected directly to the 
outer end of one of the exhaust pipes 22, and has an exit pipe 95 or 96 
connected to one of the ports 53 and 54. 
In the fourth alternative embodiment shown in FIGS. 15 and 16, pairs of 
downwardly extending baffles 101-104 are mounted from the top portion 26. 
The exhaust from the exhaust pipes 22 strikes the first baffles 101 and 
102 and is directed into the intermediate space 105 or 106 between the 
first baffles 101 and 102 and the second baffles 103 and 104. The exhaust 
then travels downwardly around the second baffles 103 and 104 before 
exiting through the ports 53 and 54. 
A fifth alternative muffler design is shown in FIGS. 17 and 18. This 
muffler design employs pairs of downwardly extending baffles comprising 
first baffles 111 and 112, and second baffles 113 and 114. In comparison 
to the baffles 101-104 shown in FIGS. 15 and 16, the baffles 111-114 are 
sloped rearwardly and thus produce a different flow through the chambers 
51 and 52, and may result in different pressures within the chambers. 
A sixth alternative muffler design is shown in FIGS. 19 and 20. This 
muffler design comprises single downwardly extending baffles 121 and 122. 
Unlike downwardly extending baffles 61 and 62, shown in FIGS. 5 and 8, the 
baffles 121 and 122 are sloped rearwardly, thus producing an exhaust flow 
having different characteristics. 
The particular design of the muffler means used within the chambers 51 and 
52 depends upon the exhaust pressures provided through the exhaust pipe 
22, the desired reduction in noise level, the back pressures experienced 
within the chambers, and other factors which are known in the art. It is 
contemplated that one of the alternative muffler designs herein shown, or 
other suitable design, may be substituted for the designs shown in the 
preferred embodiments of FIGS. 1-8 to produce a suitable muffler which has 
the desired reduction in noise level without producing excessive back 
pressures or other effects which may adversely affect the performance of 
the marine engine. 
By combining the muffler of the present invention with a swimming platform, 
it is possible to make a much larger muffler chamber and still maintain 
the aesthetic appearance of the boat. Thus, with a larger muffler chamber 
than is typical in the prior art, it is possible to have a muffler which 
results in a larger amount of noise reduction than with the mufflers of 
the prior art. 
Since the exhaust of marine engines typically includes cooling water, the 
design of the muffler should be such that it accommodates the flow of 
cooling water mixed with exhaust gases which would pass through the 
muffler. Thus, it is important that the exhaust ports 53 and 54 provided 
in the rear wall of the muffler be positioned to permit the flow of water 
out of the muffler and to avoid the accumulation of excessive amounts of 
water in the muffler chamber. 
While a preferred design of the muffler of the present invention is mounted 
on the transom of an inboard or inboard/outboard motorboat, alternative 
designs are possible. For example, the muffler of the present invention 
may be integrally formed with the motorboat rather than attached to the 
exterior of the boat. Thus, the rear of the motorboat may include a 
housing integrally formed with the boat providing a top which is below the 
normal rear transom of the boat, forming a swimming platform, with one or 
more chambers being formed beneath this top, each chamber having a muffler 
means within to provide the advantages of the present invention. 
Other modifications are also possible. For example, a muffler may be 
designed with a single large chamber for use on a motorboat having only a 
single exhaust pipe, in which case, the rear wall would be provided with a 
single exhaust port. 
The muffler of the present invention may also be employed with outboard 
engines in which the engine exhaust is provided through an exhaust pipe at 
the stern of the motorboat. With the exhaust pipe provided at the stern of 
the boat, the muffler of the present invention may be mounted on the 
transom to provide the benefits of this invention. 
The present invention also provides a swim platform which has many 
advantages over prior art swim platforms. In contrast to the prior art 
swim platform made of wood, the platform of the present invention is 
formed of fiber-reinforced plastic, such as Fiberglas, and thus is less 
likely to deteriorate as wood would, and is less vulnerable to damage 
during docking and through other use of the boat. Furthermore, the swim 
platform provides natural protection to the swimmers from the outboard 
drive which is normally located beneath the swim platform. 
While the invention has been shown and described with respect to particular 
embodiments thereof, this is for the purpose of illustration rather than 
limitation, and other variations and modifications of the specific 
embodiments herein shown and described will be apparent to those skilled 
in the art, all within the intended spirit and scope of the invention. 
Accordingly, the patent is not to be limited in scope and effect to the 
specific embodiments herein shown and described, nor in any other way that 
is inconsistent with the extent to which the progress in the art has been 
advanced by this invention.