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| Tony's Tips |
ARTICLE DATE: 10/01/2006 |
| Marine Exhaust Systems |
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| Let's start our main discussions with
exhaust sizes (NOT overall design, as we must first figure out what is required
to meet engine requirements), dissect them a touch, and then decide what is
needed and what will work. You must also understand that when we talk "marine
exhaust" we are usually talking a "wet" system, but also realize that inside
this "wet system" we have two distinct parts. In most exhaust systems that
are in the type of boats discussed in these forums (150-800 HP diesel engines),
there are TWO distinct parts of the exhaust system/piping. The DRY part and
the WET part. Even on the factory supplied 90 degree "wet elbow," these two
sections exist, though many people don't realize it. |
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| Cummins QSB 380 in a 27
ft Farallon - 4" dry to 5" mixer. Under 25" H2O
at WOT. |
Port and starboard Cummins
risers - 4" dry to 5" mixers - Very simple and basic in construction. |
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| The inner pipe of this "wet elbow" is
actually/usually a 90-degree dry bend, or section, surrounded by raw water
to keep the surface cool. At the end of this "wet" elbow, where the exhaust
hose attaches, is where the water is introduced (hence the term "mixing elbow")
and the exhaust NOW becomes wet. Inside this elbow is a smaller diameter (typically
around 2 ½ - 6" ID) which is the dry side, and where the hose attaches (it
expands to (or is surrounded by) 4-10" tubing/OD piping, depending on the engine
size, etc.) |
| EXHAUST SIZE can be determined by a few rules,
but all really come down to two things: |
| #1 - Meeting the engine requirements as to total restriction
(back pressure). |
| #2 - Designing a system that will FIT inside the boat's
constraints. |
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Most Important Point to Understand - Very simple, if you can't meet Rule #1 and Rule #2, then
nothing else matters and the exhaust is the WRONG SIZE. One,
it won't meet engine specifications / requirements, and two, it won't fit within
the constraints of the boat.
As to meeting engine/manufacturers' requirements
regarding "back pressure," this is where it is so vital to select the correct
sizes of piping & mufflers, design the actual layout or flow path, and then
put it all together so exhaust flow is not allowed to build up over this maximum
pressure limit assuring safety in all aspects related to the installation.
Exhaust back pressure measurements are a very low number, something like what
it takes to blow up a balloon. The pressure is usually measured in units of
Water Column Height or Mercury (Hg). Typical maximum limits are from 1.5" Hg
to 3" Hg or 20" to 40" of water (right around 1 PSI), depending upon the manufacturer.
John Deere, Lugger and Detroit seem to want back pressure lower (about 30"
water max) than Cummins or Yanmar (40" to 50" water), and this makes it
even tougher to meet the requirements and build a system that will fit. A new
specification for exhaust back pressure limit from Cummins for the QSB was
just released that will easily allow a well designed 5" WET system to meet
back pressure requirements on the QSB 380 and possibly even the newest 480
version if a well thought out design is used. Cummins now allows 5" Hg (about
2.45 PSI-68" of water column) on all the QSB's. This is a very friendly spec
and can make for a much easier and less expensive exhaust system installation. |
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| 370 Cummins, with V-Drive
with high mount turbo 3" dry to 5" mixer - One 90 degree wet bend
- Under 10" H2O
at WOT - Very simple and safe arrangement!! |
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| Yanmar 6LYA custom riser
- 4" dry to 5" wet mixer to 6" transom exhaust - Under 35"
H2O
at WOT. Notice 160 degree F temperature alarm on "cold part" of
mixer. |
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But, back to sizing. Exhaust flow is
determined from the amount of HP the engine makes and the more HP, the more
exhaust flow; therefore it will take larger piping and/or less bends, shorter
lengths etc., to meet back pressure requirements. Also, a very simple to understand
concept (even though most installers seem to forget) is the "bend" equation
part of planning an exhaust. Figure a smooth radius 90 degree bend is equal
to about 6-10 FT of DRY piping and 15 FT of wet piping. Another good rule of
thumb to remember is for every 100 PRODUCED horsepower, the engine makes about
200 CFM of exhaust gases, but this does not include the water and / or steam
that becomes part of the mix when water is introduced. That's why dry piping
can be smaller than wet piping, WATER and steam add to the total flow in a
substantial manner; therefore once water is introduced, the piping MUST BE
LARGER.
Past experience as to what size will work and meet requirements is always a
plus when in the initial planning. But not all are so lucky, so they turn to
the engine specs or installation guide and see what is "recommended," and/or
they use the factory supplied wet elbow size to go by. In some cases this is
fine and gets you by with the least amount of cost and effort. In most cases,
the hose size that fits the factory elbows will work in 90% or more of the
applications out there.
Below are some notes on 3 most common WET exhaust sizes that have shown to
meet all restriction requirements for Cummins engines over the last 20 years
and are based upon well over 300 Installation Reviews and exhaust tests. Keep
in mind that the dry section and wet section are different and must be treated
so. Also take into consideration the type of muffler, the overall length of
the system, the amount of both dry and wet bends, the amount of water that
you inject into the system (some of it, if not all of it), the angle of the
discharge AFTER you inject water and how the system exits the vessel. All of
these affect the total system back pressure. I am using Cummins engines as
that is what most of my experience is with and I am using 3" Hg or 41" of water
column as the maximum back pressure limit: |
| NOTE: |
Learn to understand the nomenclature
difference between "pipe," "tube" and "hose" dimensions.
For this discussion, pipe size is always shown as a nominal size in SCH
10 wall ( .120"-ish) - I.E.
2.5" = 2 7/8" OD,
3" = 3.5" OD,
4" = 4.5" OD,
6" = 6 5/8" OD, etc.
"Wet" size is always the actual ID of the exhaust hose. Tube is always
the actual OD of the tube (pipe) and is usually about 1/8" wall meaning
a 5" WET tube or FRP (fiberglass) exhaust tube or pipe has approx a 4
¾" ID.
Exhaust hose (any hose for that matter) is always sized as an ID measurement
and always is sized to FIT OVER a tube OD dimension or pipe outer diameter
measurement. |
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4" Tube, 4" pipe size ( 4.5" OD) & 5"
WET SYSTEMS - 6BT 210, early 6BTA's - ( 200-250 HP) Factory recommendation
is 5" WET (Yanmar 6YLA too), or most any diesel in between 180 and 250 HP.
Experience has shown that 2.5" pipe (2.875 OD) for a short length or one "90,"
OR 3" pipe size ( 2-3 90's) for the dry section, and 4" tube to 4" pipe for
the wet section and wet muffler is usually fine. This would also apply to a
4LHA 240 Yanmar and 6LPA 315.
6" WET SYSTEMS - 6BTA's - 300 ~ 370 Diamonds and QSB's thru 480's, QSC's , QSL's ( 300-500
HP) - Factory recommendation is 6" wet. |
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| 5" to 6" transition |
Typical 6" 8" wet FRP "low restriction"
transition wet elbows |
Heavy Wall Construction to prevent factory
type "crush" failures |
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| Well, this is where we have some "fudge factor" if you want to design it right
and make it "fit" the boat. Going from a 5" wet to a 6" wet ( typically in
300-450+ HP engines ) is HUGE and opens many install capabilities. We like
6" mufflers and piping from the typical engine room bulkhead aft to the exit
near or at the stern. But we like to do our engine risers/custom elbows and
wet mixers in 4" pipe for the dry section and 5" WET outlets where space is
an issue. Using a properly designed dry riser w/ a 5" mixing elbow, and then
transitioning to 6" at the engine room bulkhead, saves valuable space in smaller
engine rooms, saves money for the customer, and is just about always easier
and a better overall and "politically cleaner" install. The transition from
5" to 6" wet is always easy as 5" to 6" FRP tube is an easy make for a custom
elbow of any degree at 45 or less. If a highly restrictive muffler is used,
or the available space allows the use of 6" close to the engine, then a 6"
mixer might be the best choice. |
Transition from 5" to 6" wet
6" tube to mufflers to stern
5" wet mixer
4" pipe size (4.5" OD) dry riser
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| 4" Dry riser, 5" WET mixer to 6" wet FRP
pipe transition |
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8" WET SYSTEMS - Some "C" & QSC installs,
QSM's-400++ to 800-ish HP-Cummins has "recommended" both 6" and 8" and seems
wishy-washy on this.
Well, I have yet to see a C, QSC, QSM, or any Cummins (or other make) engine
UNDER 500 HP need an 8" wet exhaust system to meet specs. But, I continue to
see this as a RECOMMENDATION for the exhaust size in many cases. I guess if
you have the room, the budget, and have an extraordinary amount of bends and/or
an overly restrictive muffler, then maybe you should go for it. But from a
vast amount of experience, we always stay with 6" max if under 500 HP.
When we get up into the 500 - 800 HP-ish HP range, this is when we need to
think about some, or all, of our WET side of the system being 8". Since most
of my work and the discussions on this site center around repowers of sportfishing
boats like Bertrams, Vikings, Hatteras, etc, let's confine the 8" to this general
style of boat in the 40-60 ft length range. And, with so many older Detroit
powered boats/owners still out there that are now seriously looking at repowers,
and most of these having 8-71's , 6-92's, 8-92's, etc., this is now even more
important to understand.
Usually, these boats have 8" or even 10" WET systems aft of the engine room.
This is typically where the mufflers are located too. If all is good aft of
the engine room - all piping, mufflers, outlets and hoses - (it is usually
not), then this is a good size to start with for adapting to the newer 4-stroke
engine. If some or most of this piping needs to be replaced because of age,
it's usually easier just to stay with the same size for the section aft of
the engine room. If you'd like to size down for space or cost reasons, then
a good look at the overall system and engine requirements will be needed.
Let's look at a new QSM at 670 HP as to what will easily meet the spec. The
factory supplies an 8" fabricated wet elbow or mixer and IMO is of questionable
use or quality. Yes, they do work in many applications when used correctly,
but also seem to be subject to both internal and external leaks before their
time because of the many welds and thin wall construction. Even worse, though,
are the "installers" who re-weld or bolt these units in a dangerous "up" orientation
in order to build a cheap "Mickey Mouse" type riser. I not only see this with
the QSM's but also in many other installs using the factory 5" and 6" "factory"
wet elbows. |
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| Cummins 300B's with reorientated
factory elbows - 700 hours over 5 years and water was leaking into the turbo
- Luckily it was caught before an expensive disaster struck. |
Cummins QSM 11 with "reorientated"
factory wet elbows - This owner was told at sea trial that they would fail
internally, he said he would fix them. Never did.... |
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For those who cannot grasp
what the issue is here, a simple explanation - This is a double walled 90 degree
wet elbow that was never designed to "hold water" in this orientation
but, rather, be mounted horizontally so as to exit downhill and self drain.
When installed like this, the sea water that will eventually corrode thru the
elbow will now go down the pipe into the turbo or worse. I hope you do not
recognize this type of system on your boat!
Now, back to 8" - This is what we have found with engines in the 600-700
HP range as to designing a wet exhaust system and working around the existing
8" - 10" piping or "book" recommendations. We typically
use 4" dry piping from the engine up to a well designed 6" wet mixer.
This usually includes 3 to 4 pieces of 4" pipe size "weld" 90
degree dry elbows and then a double walled wet mixer of our design orientated
on the downhill side of the system. The mixer is always pointed to a spot on
the vessel so we DO NOT have to incorporate any WET bends / "90's",
etc., BEFORE the transition to 8" (or 10") from the engine room aft.
With good design and thought, along with some first class fabrication, meeting
exhaust restriction requirements can easily be done with a mix of both 4"
dry, 6" and 8" wet which will make for a much cleaner install while
making the engine room more user friendly as to maintenance, etc.
To recap, exhaust size is something that can be adjusted to not only meet requirements
for the engine, but also fit the vessel's constraints. When sizing the system
to meet acceptable restriction requirements in order to protect the engine
from excessive back pressure, keep bends to a minimum, especially "wet" 90
degree bends. With some planning, most exhaust routing designs can eliminate
a one or two 90 degree bends by building a custom riser/mixer and make for
a less restrictive, less cumbersome and safer exhaust system. |
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