Wet marine exhaust muffler

A wet marine exhaust muffler includes a metallic housing and a polymeric jacket substantially encapsulating the metallic housing. The metallic housing defines one or more chambers in the interior of the housing. An inlet pipe and an outlet pipe each extend through openings in the polymeric jacket and communicate with at least one chamber in the interior of the housing. The metallic housing is preferably formed from stainless steel and the polymeric jacket is preferably formed from a thermoplastic material having a thickness of approximately 0.001 inch to 0.1 inch, and more preferably of approximately 0.005 inch to 0.060 inch. According to one preferred method for inhibiting the explosion of a wet marine exhaust muffler, the muffler housing (but not the inlet and outlet pipes) is at least partially surrounded by a heat shrinkable thermoplastic tubing and the heat shrinkable thermoplastic tubing is heated by conventional means to shrink the heat shrinkable thermoplastic tubing over the muffler housing to form the thermoplastic jacket. It is one object of the invention to provide a wet marine exhaust muffler, and a method for manufacturing such a muffler, effective to prevent or minimize explosion damage without substantially increasing the manufacturing cost or compromising other desirable muffler characteristics.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
The present invention pertains to exhaust mufflers for watercraft, and more 
particularly to a method and apparatus for preventing or minimizing 
explosion damage by wet marine exhaust mufflers. A preferred embodiment of 
the invention comprises a muffler having a metallic housing encapsulated 
in a polymeric jacket to promote the integrity of the muffler when 
subjected to high internal pressure surges. 
2. Description Of The Related Art 
Motorized watercraft typically have included exhaust mufflers for muffling 
or attenuating exhaust noise generated by the operation of marine engines. 
Such mufflers have had to be capable of handling not only exhaust gas 
itself, but also droplets of water injected into the exhaust flow from the 
cooling system of the engine. The water injected from the engine cooling 
system typically performed two functions, namely, absorbing engine exhaust 
noise and cooling the exhaust gas so that the gas might be safely 
discharged through the hull of the craft without presenting a fire hazard. 
Mufflers of various designs have been placed in the exhaust conduits 
running between the engines and the discharges. Typically, marine mufflers 
include housings which enclose one or more chambers for permitting 
expansion of the exhaust gas to attenuate noise. One example of a wet 
marine exhaust muffler is disclosed in U.S. Pat. No. 5,588,888 to 
Magharious, the disclosure of which is incorporated herein by reference. 
Often, marine muffler designs have been closely akin to the mufflers used 
on automobiles but have been constructed of fiber reinforced polymer 
materials such as FIBERGLASS materials which can better tolerate the 
marine environment. 
Drawbacks to the use of fiber reinforced polymer materials as opposed to 
metals in the fabrication of exhaust mufflers have included lower strength 
and greater elasticity in comparison to metals. For example, acoustical 
energy in the exhaust gas passing through the muffler could induce 
sympathetic elastic vibrations in the walls of the housing. Such 
vibrations reduce the ability of the muffler to attenuate acoustical noise 
and may also contribute to possible failure of the housing walls. 
Under certain circumstances, wet marine exhaust mufflers have been exposed 
to abnormally high internal pressure surges (that is, "backfires") 
sufficient to cause the mufflers to explode. Such explosions are capable 
of causing damage to the craft in which the mufflers were installed as 
well as injury to persons positioned in or near the craft. 
Past efforts to prevent or reduce explosion damage have included changes in 
the configurations (e.g., geometries, chamber sizes, pipe positions and 
the like) of the mufflers and in the materials from which the mufflers 
were made. Such changes significantly increase the costs of manufacturing 
the mufflers. The changes also require compromises in other desirable 
characteristics of the mufflers, such as corrosion resistance and 
accoustical characteristics. 
Consequently, there remains a need for a wet marine exhaust muffler, and 
for a method for manufacturing such a muffler, effective to prevent or 
minimize explosion damage without substantially increasing the 
manufacturing cost or compromising other desirable muffler 
characteristics. 
SUMMARY OF THE INVENTION 
This need is addressed by means of the wet marine exhaust muffler of the 
present invention. The preferred wet marine exhaust muffler of the present 
invention includes a metallic housing and a continuous, unbroken polymeric 
jacket or surface layer substantially encapsulating the metallic housing. 
The metallic housing defines one or more chambers in the interior of the 
housing. Though the structure by which exhaust gas is admitted and 
discharged from the one or more chambers in the interior of the housing is 
not critical to the invention, the muffler preferably includes an inlet 
pipe and an outlet pipe. The inlet and outlet pipes each extend through 
openings in the polymeric jacket aligned with the inlet and outlet, 
respectively, and communicate with at least one chamber in the interior of 
the housing. Preferably, the exterior surfaces of end portions of the 
inlet and outlet pipes are left bare of polymer material to facilitate 
coupling with other components in the marine wet exhaust system. 
The preferred muffler structure substantially improves the ability of a 
muffler having a conventional configuration and formed from conventional 
materials to withstand high internal pressure surges (that is, 
"backfires") without rupturing. Even though the wall thickness of the 
polymeric jacket is preferably small compared to the wall thickness of the 
muffler housing which it encapsulates, the jacket will withstand internal 
pressure surges up to 5 kpsi or more prior to bursting. Furthermore, due 
to the elasticity of the polymer material, the jacket will stretch 
slightly in response to an internal pressure surge. This capacity to 
stretch enables the jacket to retain its integrity and to contain the 
remains of the muffler structure even if the muffler structure fractures 
due to the pressure surge. 
The configuration of the muffler is not critical to the present invention. 
Conventional muffler configurations useful in connection with the present 
invention include that disclosed in U.S. Pat. No. 5,588,888 to Magharious. 
In an especially preferred embodiment, the muffler housing is formed from 
stainless steel to provide added strength along the exterior of the 
muffler. 
The polymeric jacket is preferably made from a conventional thermoplastic 
material. Preferred thermoplastic materials include polyester, polyamide, 
polyolefin and polycarbonate materials. The criteria for selecting the 
thermoplastic material include sufficient resistance to combustion and to 
softening due to heat, petroleum-based lubricants, solvents and the like 
to remain durable in a marine propulsion system environment. In an 
especially preferred form, the polymeric jacket is formed from a heat 
shrinkable material, such as heat shrinkable polymer materials available 
from Advanced Polymers, Inc. of Salem, Mass.; Electro-Insulation Corp. of 
Arlington Heights, Ill. and Cary Industries of Maryland Heights, Mo. The 
thickness of the polymer jacket is preferably on the order of 0.001 inch 
to 0.1 inch, and even more preferably on the order of 0.005 inch to 0.060 
inch. 
The process by which the polymeric jacket is formed on the metallic housing 
is not critical to the invention in its broadest sense, but certain 
methods are preferred due to their low manufacturing costs. According to 
one preferred method, the muffler housing is at least partially surrounded 
by a heat shrinkable thermoplastic tubing and the heat shrinkable 
thermoplastic tubing is heated by conventional means, such as by a heat 
gun. The heating shrinks the heat shrinkable thermoplastic tubing over the 
muffler housing to form a thermoplastic jacket encapsulating the muffler 
housing. In an especially preferred form in which the muffler includes 
inlet and outlet pipes, the heat shrinkable thermoplastic tubing is 
positioned so as to substantially surround the metallic housing but not 
the inlet and outlet pipes so as to leave end portions of the inlet and 
outlet pipes bare of polymer material. 
Alternative preferred methods for forming the polymeric jacket include 
exposing the muffler housing to a liquid polymer precursor and solidifying 
the polymer precursor to form the polymeric jacket. The polymer precursor 
may be prepared by any conventional means and its precise morphology is 
not critical to the invention. Likewise, the technique by which the 
muffler housing is exposed to the polymer precursor is not critical, 
though preferred techniques include painting the polymer precursor on the 
muffler housing and dipping the muffler housing in the precursor. 
Therefore, it is one object of the present invention to provide a wet 
marine exhaust muffler, and a method for manufacturing such a muffler, 
designed so as to prevent or minimize explosion damage without 
substantially increasing the manufacturing cost or compromising other 
desirable muffler characteristics. The invention will be further described 
in conjunction with the appended drawings and following detailed 
description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring initially to FIG. 1 of the drawings, a first embodiment 10 of a 
wet marine muffler comprises a substantially cylindrical metallic housing 
12, preferably formed of stainless steel or other durable metal and 
defining one or more chambers (not shown) in the interior of the housing 
12; an inlet pipe 14 and an outlet pipe 16, each communicating with a 
chamber of the one or more chambers in the interior of the housing 12; and 
a polymeric jacket 18 (shown in section) substantially encapsulating the 
housing 12. The polymeric jacket 18 is preferably made from a conventional 
thermoplastic material. Preferred polymer materials include polyester, 
polyolefin and polycarbonate materials. In an especially preferred form, 
the polymeric jacket 18 is formed from a heat shrinkable material. 
One preferred method for inhibiting the explosion of the muffler 10 
comprises the steps of at positioning a heat shrinkable thermoplastic 
tubing (not shown) over the muffler housing 12 and heating the heat 
shrinkable thermoplastic tubing by conventional means, such as by a heat 
gun. The heating shrinks the heat shrinkable thermoplastic tubing over the 
muffler housing 12 to form the polymeric jacket encapsulating the muffler 
housing. The heat shrinkable thermoplastic tubing may be, for example, in 
the form either of a continuous sleeve or of a sheet or film rolled over 
to form a tube. Preferably the heat shrinkable plastic tubing has a 
diameter sufficiently large for the tubing to fit easily over the housing 
12 and a thickness sufficient such that the finished polymeric jacket is 
approximately 0.001 inch to 0.1 inch, and even more preferably 
approximately 0.005 inch to 0.060 inch, thick. Optionally, the finished 
polymeric jacket 18 is cured to improve its mechanical and chemical 
properties. 
It is preferable that the heat shrinkable thermoplastic tubing have an 
axial length substantially corresponding to the length of the finished 
polymeric jacket. In this manner, the heat shrinkable thermoplastic tubing 
surrounds the housing 12 but not end portions 20 and 22 of the inlet and 
outlet pipes 14, 16 so that the inlet and outlet pipes 14, 16 extend 
through openings 26 and 28 in the finished polymeric jacket 18. 
Other preferred methods for inhibiting the explosion of the muffler 10 
include exposing the housing 12 to a liquid polymer precursor (not shown), 
either by painting the polymer precursor onto the exterior surface of the 
housing 12 or dipping the housing 12 into the polymer precursor, and 
solidifying the polymer precursor to form the polymeric jacket 18. The end 
portions 20, 22 of the inlet and outlet pipes 14, 16 may be masked by 
conventional means during the exposure to the polymer precursor. 
Optionally, the finished polymeric jacket 18 is cured by conventional 
means to improve its mechanical and chemical properties. Conversion of the 
polymer precursor to desired polymer may be achieved by conventional 
techniques including irradiation of an ethylenially unsaturated monomer, 
for example, to form the desired polymer through free radical (chain 
addition) techniques. Additionally, thermoplastic polymers can be formed 
as the encapsulating medium from appropriate monomers by step-reaction 
(condensation) or ionic and coordination chain polymerization techniques. 
FIG. 2 shows a second embodiment 50 of a wet marine exhaust muffler 
comprising a housing 52, preferably formed of stainless steel or other 
durable metal and defining a chamber (not shown) in an interior (not 
shown) of the housing 52; an inlet pipe 54 (shown partially in phantom) 
and an outlet pipe 56 (shown partially in phantom), each communicating 
with the chamber in the interior of the housing 52; and a polymeric jacket 
58 (shown in section) substantially encapsulating the housing 52. As was 
true of the polymeric jacket 18 of FIG. 1, the polymeric jacket 58 is 
preferably made from a conventional thermoplastic material. In an 
especially preferred form, the polymeric jacket 58 is formed from a heat 
shrinkable material. The thickness of the polymeric jacket 58 is 
preferably on the order of 0.001 inch to 0.1 inch, and even more 
preferably on the order of 0.005 inch to 0.060 inch. 
The polymeric jacket 58 is preferably formed by heat shrinking a heat 
shrinkable thermoplastic tubing (not shown) over the housing 52 or by 
exposing the housing 52 to a liquid polymer precursor by techniques 
similar to the preferred techniques used in forming the polymeric jacket 
18 of FIG. 1. End portions 60 and 62 of the inlet and outlet pipes 54, 56 
extend through openings 64 and 66 in the polymeric jacket 58. In forming 
the polymeric jacket 58 by heat shrinking a heat shrinkable thermoplastic 
tubing over the housing 52, a tubing having an axial length greater than 
the height 68 of the housing 52 should be used so as to facilitate 
coverage of opposite sides 70 and 72 of the housing 52. 
FIGS. 3 and 4 show a third embodiment 100 of a wet marine exhaust muffler. 
As best shown in FIG. 4, the muffler 100 comprises a housing 102, 
preferably formed of stainless steel or other durable metal and defining 
an interior 104; an internal baffle 106 cooperating with the housing 102 
to define a first chamber 108 and a second chamber 110 in the interior 
104; a pair of inlet pipes 112 and 114 communicating with the first 
chamber 108; a baffle pipe 116 communicating between the first and second 
chambers 108, 110; an outlet pipe 118 communicating with the second 
chamber 110; and a polymeric jacket 120 substantially encapsulating the 
housing 102. The configuration and operation of the muffler 100 is 
discussed in more detail in U.S. Pat. No. 5,588,888 to Magharious, the 
disclosure of which is incorporated herein by reference. 
The polymeric jacket 120 is preferably made from a conventional 
thermoplastic material. In an especially preferred form, the polymeric 
jacket 120 is formed from a heat shrinkable material. The thickness of the 
polymeric jacket 120 is preferably on the order of 0.001 inch to 0.1 inch, 
and even more preferably on the order of 0.005 inch to 0.060 inch. 
The polymeric jacket 120 is preferably formed by heat shrinking a heat 
shrinkable thermoplastic tubing (not shown) over the housing 102 or by 
exposing the housing 102 to a liquid polymer precursor by techniques 
similar to the preferred techniques used in forming the polymeric jacket 
18 in the embodiment 10 of FIG. 1 and the polymeric jacket 58 of the 
embodiment 50 of FIG. 2. End portions 122, 124 and 126 of the inlet and 
outlet pipes 112, 114, 118 extend through openings 126, 130 and 132 in the 
polymeric jacket 120. In forming the polymeric jacket 120 by heat 
shrinking a heat shrinkable thermoplastic tubing over the housing 102, a 
tubing having an axial length greater than the length 134 of the housing 
102 should be used so as to facilitate coverage of opposite sides 136 and 
138 of the housing 102. 
The present invention substantially improves the ability of the muffler 10, 
50, 100 to withstand high internal pressure surges (that is, "backfires"). 
Even though the wall thicknesses of the polymeric jackets 18, 58, 120 are 
preferably small compared to wall thicknesses of the housings 12, 52, 102, 
the polymeric jackets 18, 58, 120 will withstand internal pressure surges 
up to 5 kpsi or more prior to bursting. Furthermore, due to the elasticity 
of the polymer material, the jackets 18, 58, 120 will stretch slightly at 
the moment of explosion. This capacity to stretch enables the jackets 18, 
58, 120 to retain their integrity and to contain the remains of the 
housings 12, 52, 102 even if the housings 12, 52, 102 fracture due to the 
explosions. 
At the same time, it is noted that the configurations of the mufflers 10, 
50, 100 and, in particular, the geometries of the housings 12, 52, 102, 
are all conventional. No modifications to the configurations are required 
in order to carry out the present invention and properties such as 
acoustical properties (e.g., "tuning") are not compromised. Likewise, 
conventional materials such as stainless steel can be, and preferably are, 
used to construct the housings 12, 52, 112 and the internal structures 
(e.g., the internal baffle 106 and the pipes 102, 114, 118 of the muffler 
100). The use of conventional materials and configurations, as well as of 
low cost techniques for forming the polymeric jackets 18, 58, 120, helps 
to minimize manufacturing costs. 
Having described the invention in detail and by reference to preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the appended claims.