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| Tony's Tips |
ARTICLE DATE: 10/01/2006 |
| Marine Exhaust Systems |
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| Designing a marine exhaust
system |
| Designing a marine exhaust system for
a boat is something that apparently takes the back seat during the planning
stages when doing a repower. In new boat construction, the design of many systems
seems to center around the "cookie cutter" philosophy, as builders always seem
to want to or work around a factory designed wet elbow system that is supposed
to "fit all". Besides that, many new builders are more concerned about getting
the engine below a low deck than worrying about the exhaust and exhaust outlet
of the engine being close, at, or BELOW the water line. |
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| QSC Starboard Cross-over
riser - 4" dry to 6" wet mixer to 6" surge tube. |
Cummins 6CTA 8.3 with
"twisted" dry riser -4 " dry to 6" wet - This design eliminated 3 wet 90's
that the original system had. |
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Also, and another all too
common error in exhaust design, is the use of an anti-siphon valve as a "fix-all"
for an otherwise poor and sure to fail design. It's not that an anti-siphon
valve is not needed in many applications, but "Average Joe" has no
clue as to what they can do and more important, what they CANNOT do, and how
small changes in the basic design of an anti-siphon valve can greatly enhance
its effectiveness. More on "Anti-Siphon" valves and their shortcomings below.
The most BASIC of all supposedly understood but not followed diagrams of a
marine exhaust layout is below. Why is it we continually see the LWL of a vessel
within just a few inches of the exhaust spill-over point? Notice the word MINIMUM?? |
CARDINAL RULE #1 - Wet Exhaust Height Above LWL
REMEMBER this as the "SPILL-OVER" point |
The diagram above seems to elude new boat
designers and "repower experts" in so many designs that have cost vessel owners
millions of dollars over the years in ruined engines. Simply unbelievable to
me that something as simple as building an exhaust with a minimum of 12" of
safety margin height is not followed.
Moving on to the total design of the system is where most of the emphasis needs
to be when repowering or building a boat. With "design" we mean the entire
system from the engine exit point (the turbo outlet in most cases) all the
way to where the exhaust leaves the boat. This includes the "size(s)" (just
figured that one out), from the engine to the final exit, type of muffler,
if any, type of material for both the dry and wet side of the system, and the
general routing and actual installation of the system. It seems that many installers
and builders plan this very important part of the vessel as an "afterthought"
because many great engine rooms are built with no room remaining to build a
safe and well planned exhaust system. |
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| Starboard QSM riser -
4" dry to 6" wet. |
Cummins 330B / V-drive
- Simple Riser points directly to stern exhaust exit. |
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Material choices when building exhaust
systems can vary, but 25+ years of experience and 100's of exhaust systems
under our belt in high hour commercial applications have shown us that 304L
or 316L is good for all of the dry sections and shows no difference in lasting
ability regardless of what "street talk" says. Using 316L for all wet sections
is always best. Use 316L rod for all welding (304 or 316) and if the design
uses any mild steel for the dry section, then use 309L for the mixed joints
/ weld area. TIG is the method of choice for all welding. For all custom FRP
exhaust work, the of use resins that are Class 1 fire retardant isophthalic
polyester resins are our choice and easy to source. Common industry names are
Reichhold (DION) and McNichols and are used extensively thru the FRP structural
pultrusion process and filament winding manufacturing industry. You may also
find many of the more common iso-tooling resins meet these specs and are also
an excellent choice to use, although some do not have the Class 1 rating. IMO,
99% of the time, Class 1 resins are not needed unless the spec calls for it.
Resin rich lay-ups are best with plenty of thickness ( ¼" to ½") in all joints
building up with glass-strand mat with a layer or two stitched fabric in-between.
We finish with iso-gel coat and surfacing agent when applicable / paint as
to what looks right for the job - in all cases our work is always vastly more
stout than "factory supplied" FRP parts, and we always put a 2" - 4" long tube
doubler inside all tubes where the hose clamps go. This eliminates the tube
crushing that factory pipes/ muffler spigots seem to be very susceptible to.
With sizing and basic construction covered, let's get into exhaust design,
as this is the most abused and least understood phase of the overall system.
"Gravity," I'll say it again, "G-R-A-V-I-T-Y," is the most important aspect
of the design that needs to be addressed and how you can use that force to
help you design a safe and effective exhaust system. Next is understanding
and knowing where your waterline is in both static and all running conditions.
With those ideas fresh in your mind these are some pointers and concepts that
you need to consider in the initial planning stages of the design layout: |
| 1) Understand the difference
between a "requirement" and a "recommendation" from the engine
manufacturer regarding exhaust design. They may "require" a minimum specified
exhaust height for safety, but "recommend" a particular size for the
system based on past experience. Many times smaller exhaust sizes can
be used to everyone's advantage and employing an experienced company
w/ hands-on experience for this part of the vessel construction or repower
is time and $$ well spent. |
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| In the shop
exhaust "mock-up". |
Measuring for
a custom exhaust riser from drawings / dimensions supplied from
owner 2000 miles away. |
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| 2) Always
use gravity to your advantage. Water flows downhill so, if you
have a system that holds water (water jacketed risers for instance) and
this system fails internally (it's not IF it is going to fail, it's WHEN
it will fail), where will the water go?? Into the turbo/exhaust manifold/cylinders??
Think about YOUR riser or elbow should it fail internally where you can
not see it and what might happen. Remember, WET risers are an absolute no-no
for any long term application unless they are "coolant cooled." Internal
failure of "wet elbows" and custom water jacketed risers is an old and ongoing
problem, regardless of material choice and/or other claimed construction
features. (See Tip #7). |
| 3) Always
use all of the available height in the engine room for the riser
(where needed and is practical) BEFORE turning over the top and injecting
water; i.e. always inject the water on the downhill side, or down stream
of the top of the riser. A wet exhaust system with a steep downward slope
is always better and safer. The cheap way out, by using a factory supplied
"cookie cutter" designed wet elbow, is not always a good or safe option.
When thinking marine exhaust, remember "one size" DOES NOT FIT ALL. |
| 4) Be sure
that IF the option presents itself in the design of a wet exhaust system,
allow for all of the water to drain itself from the exhaust when not running. Although this can't always be done, you can still build a safe system by
utilizing other simple design ideas, custom mufflers, surge tubes, etc.
An important point to remember, IF your muffler, etc., holds water in the
static position, then the system is also holding water when lifting/pulling
the boat at the yard. ALWAYS lift the bow first (noticeably bow high) and
hold it there for a minute or so to let the water drain from the system.
I have seen quite a few destroyed engines that had water slosh up into the
cylinders from this exact scenario of water rolling back and forth or lifting
the stern first when pulling a boat - Usually this is not discovered until
launch time and by then the engine is toast. |
| 5) When using a "lift muffler"
design, remember that in most cases you can make the system "inherently
safe" by being sure that the engine "spill-over" height is higher
than the lift muffler spill over point. When the water injection is below
the water line, you can also design an "active" anti-siphon valve that
is much safer than the typical "auto-type valve" shown or used in most
applications-(IMO, they are a poor design and should be avoided unless
fully understood as to their shortcomings and checked for proper operation
on a continuous basis ). |
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| Some of the fallacies that still
persist today are shown in the "copied" lift-muffler design below that
is shown in current high dollar color catalogs touting their exhaust
muffler products. Thinking that this is an applicable base type design
for a lift muffler system is 100% hogwash and in many cases leads to
ruining a perfectly good engine because of water rising within the system
before the engine starts and after it is shut down. What they should
be saying is to design a riser BEFORE the water injection to use all
of the height available within the engine room, and to design an anti-siphon
system that allows an active and "open" siphon break to be on the upward
rise of the water injection system allowing water to flow over the side
BEFORE the water flows into the lift muffler filling the system. |
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| Again, another instance of 100%
"couch engineering" from an engineer that has probably never done and/or
seen an install of a marine engine at or below the waterline and only
gives or sells his "expertise" based on theoretical circumstances on
paper calculations. This kind of literature angers me as I know it's
total garbage. However, I do admit that when this type of engineering
is taken for "gospel," I make lots of $$ because it assures me of continuous
new engines sales down the road. |
To exhaust elbow |
From heat exchanger
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| Anti-siphon outlet / dump |
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| Active "Anti-siphon"
bypass/valve installed on the uphill side to overboard discharge. |
Active "Anti-siphon" bypass / valve installed
on the uphill side to overboard discharge.
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The general layout of the "anti-siphon"
valve. In reality, it's not a valve at all, but an active bypass that
should be installed on the uphill water flow before it goes over the
top, and discharged over the side above the waterline in a VISIBLE location.
This allows cranking without flooding the lift muffler and adds an enormous
layer of safety to the system.
6) Overall Exhaust design
can usually be made better in many ways if you DO NOT use "cookie cutter"
type exhaust components. Typically, many use factory 90 degree
wet elbows, and an array of 45 and 90 degree bend hoses and clamps routing
an exhaust. I guess most installers think of a marine exhaust system
like copper plumbing in a house.
A simple change from a 90 degree angle bend to a 75 degree bend, an angled
input to a muffler, or an added twist in a riser can make a world of
difference in overall exhaust layout. In other words, don't just think
"straight, 45 & 90" when designing an exhaust system. In the 100's of
exhaust systems we have designed over the last 25+ years, I am sure that
at least 50% of them had to have something custom done to a "factory
muffler" in order to make the exhaust layout "fit" design criteria. |
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| Yanmar 4LHA 230
- Modified 4" Muffler inlet - Eliminated (1) wet 45 and (1) wet
90. |
"Modifing a muffler
like this will lower exhaust pressure, save valuable space and
cleans up the total system." |
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| An Interesting Note: I have tested
the pressure drop of a typical 75 degree custom elbow from both 6" to
8" and 5" to 6". In both cases, at 430 HP on the 5-6" elbow, and at 660Hp
on a 6" to 8" elbow, I never saw more than ¼" Hg delta. Going from smaller
to larger allows a quick expansion and lowers pressure restriction overall
even in relatively sharp wet bends. Less bend would even be better. |
| 7) Never, never, never do
you want a boat that has saltwater cooled wet risers or pipes unless
they are installed in such a manner that when they leak they are downhill
of the engine "spill-over" point. It is not IF they are going
to leak, it's when they are going to leak as it is a 100% given that
they will. If this is the only viable option, then be sure that you realize
that they need to be inspected annually (or more often), or changed out
after every few years to be safe. A few examples below of perfectly good
low hour engines that had a "wet riser" or something similar and when
they failed internally, the owner was into the BIG bucks as to repair. |
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| Destroyed Turbo
are the results from the failed "wet riser" on the right. |
This "wet" riser
failed in less that 700 total hours hitting the owners wallet big
time. As mentioned, you NEVER want a wet riser cooled w/ salt water,
even as "cool" as you might think they are. |
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8) When the vessel design
is such you are very limited as to the physical dimensions of the exhaust
size ( like installing a muffler in a confined space) and you
need to reduce back pressure but you cannot install larger pipes, tubes,
etc, another option would be to bypass some of the water that is normally
mixed into the wet exhaust.
In most cases, the engine seawater system pumps more than enough water
to cool the engine and sometimes as much as 2+ times water than needed
to cool the exhaust.
This can vary as to engine design AND exhaust design, but bypassing approx
1/3 of the water that leaves the heat-exchanger or cooling system on
the engine and sending it over the side of the boat can easily reduce
back pressure by 1" Hg or more in some cases.
An added benefit of this is that it can add a "visual" as to water flow
and, in many boats, this would be a plus as the seawater water flow is
sometimes impossible to determine. |
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| Exhaust Water
By-Pass - just before the mixing elbow. |
Port & Starboard
Exhaust Bypass. |
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| Knowing that you are pumping seawater
always adds comfort to your experience. If this is something that you
feel would be needed, use the services of a company that has a track
record in this type of work as it would be time and effort well spent. |
| 9) Reference
sketches and ideas: |
| Sketch
A - "Typical Underwater Exit Design" with
Muffler AFT of Exit |
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| Sketch
B - Custom "twin outlet"
lift muffler can be used when the existing main exhaust run is too small and
cannot be upgraded easily - With a 5-6" inlet and two smaller outlets ( 3.5"
- 4.5") this base design can safely be used in applications up to 400 HP. This
system was recently incorporated into a 46 ft Chris Craft Romer 'gas to QSB'
twin-engined repower of a when no other viable option was available. |
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| Sketch
C - Custom twin outlet lift muffler to underwater
exit |
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| Sketch
D - Safe "Dry Exhaust Thru-Hull"
Design |
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| Sketch
E - Typical Fiberglass Surge Tube Connection |
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