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David Pascoe's "Exhaust Risers"
BoatU.S.'s "Exhaust Manifolds"
See "The Boatowners Mechanical and Electrical Manual" by Nigel Calder
Types of exhaust systems:
- Dry (gets hot).
- Water-jacketed: water on outside, gas on inside (vulnerable to corrosion).
- Water-lift: water and gas mixed in an "elbow" (quiet, cheap, light).
Some systems are a combination of these types. My Perkins 6.354 engine has
a water-jacketed exhaust manifold, then a dry riser, then
an elbow that mixes water and gas, which then runs
downhill and out.
A dry riser is cheap (it's just a pipe), and avoids the possibility of a failure in a
water-jacketed riser that could leak water down into the cylinders.
The water-jacketed exhaust manifold could fail and leak water down into the cylinders.
I'm thinking of installing a manual drain valve to drain water out of it
after stopping the engine.
From Mo Girard's "Spring Check Up":
You should visibly inspect the entire length of the exhaust hoses,
the through-hull fittings, and any other components such as mufflers,
elbows, or straight pipes for any signs of wear, corrosion, or leaks.
You can usually spot even a very small leak by looking for an outcropping
of salt crystals or a deposit of white powder. This is a guaranteed sign
that a small leak has developed and needs to be addressed. The salt is
a clue even if the leak is only present under pressure when the
engine is running.
From Norm on The Live-Aboard List:
... the best thermo device aboard is also the cheapest (about $10) and
simplest. It is a normally open thermoswitch [Therm-O_Disc brand fan
control switch made by White Rodgers;
Grainger part number 2E246] held against the rubber exhaust
hose with a spring around the hose. The switch closes at 120 degrees. On the
genset it shuts down the engine. On the main engine it rings a bell. The
rubber hose is the first thing that gets too hot if you lose seawater flow,
the primary cause of overheating on my engines. It has shut down the genset
three times so far due to plugged sea suction strainer or thruhull. I
cleaned out the debris and was off and running again. Without that little
switch it would have meant three exhaust hose replacements, not a fun day.
Before the switch was installed I burned up a main engine hose. It is 5" id
and twenty feet long. It was not a fun day.
From Norm on The Live-Aboard List:
I have had a temperature sensor on my (water-cooled) exhaust hose on my 8 KW
genset for many years and it has saved my butt many times.
It consists of a normally-open Klixon-type thermoswitch, used by the HVAC
industry, that closes at 120 deg F. It is a small button device with two wings
to take sheet metal screws mounting into 3/4" hole in a duct. There are two
push-on 1/4" quick connectors. I attached the switch to the rubber exhaust hose
about three inches from the water injection point using a spring from one
mounting tab, around the hose to the other mounting tab to hold the button against
the hose. One electrical connection goes to ground, the other to the oil
low pressure/block high temp line.
When (not "if") the seawater flow is inadequate to keep the hose below 120
deg F, the genset shuts down. Without this shutdown, it often happens that the
rubber exhaust hose is damaged. In the case of sudden water loss, the block
will not heat up fast enough for the block high-temp switch to shut down the
engine before the exhaust hose is damaged.
I had this happen on my main engine once. That hose is 5" x 20', expensive
and a pain to replace.
The switch on the genset exhaust hose has shut it down at least a half-dozen
times, enabling me to fix the problem without damage to the exhaust hose.
From John Dunsmoor:
[Re: redesigning the exhaust system:]
It is an exhaust system NOT brain surgery.
The only reason you have water injection is to cool the exhaust. So all the
plumbing is nearly a damn foolish exercise in the first place.
At the same time the largest threat is getting water back into the engine
causing considerable damage.
Gravity ... for the most part water runs downhill. Mounting the
engine below the exhaust outlet is dangerous, and done all the time.
Hydrolift exhaust system, doing everything wrong and getting away with it.
Custom stainless exhaust riser-and-elbow ... twelve hundred
dollars ... what bullsh*t. These guys think they are brain surgeons.
You could build what you need out of galvanized steel from your local
plumbing store for $35 and it would work just fine, for years. In the old
days that is just what you had: IRON, not stainless or aluminum.
Lots of elbows are made out of aluminum. Problems: hot, corrosive, salt
water, coupled with dissimilar metals, and just the softness of aluminum and
they die. This can be especially true for poor-quality castings that might
not have the molecular durability of an extruded piece of material.
These elements can be the same for bronze, but less so.
You're a smart, thoughtful, careful, engineer type of person. Get a
flashlight, a sketch pad, a glass of ice tea and just think it out. What
you have is there due to elements such as availability of parts, what the
engine came with, the way we always did it, where the cabinet work
is ... sometimes last on the list is getting exhaust
out of the boat.
Westsail 43 did away with the long run for the exhaust and ported right out
the side of the boat. This took a normal run of twenty feet and turned into three.
From Stuart Burgess:
A couple of hints in the metal-working department. DON'T use marine
fabricators. They are a complete rip-off. Stainless 316 and aluminium tubing
is very cheap to buy. Phone a metal stockholder and you will find that they
will have bends and shapes and tube in stock and may even let you nosey
about in their warehouse. Buy yourself a top-quality metal hacksaw and top-quality
blades for cutting stainless. Then with your cheap collection of
tubing and bends, cut these by hand until you get the shape and size you
Mark each piece where it needs to be welded to the next piece (I use tipex
for this). Then phone around and find a firm or someone who TIG welds
agriculture/food machinery (this is 316 stainless mostly) and ask them to
weld it for you. It will cost you peanuts.
I was quoted £700 to produce a new stainless high riser off my exhaust
manifold with a water injection inlet.
I bought all the stainless from a stockholder for £20 and had it welded
after I cut it to shape, for £40! In fact I bought so many bends I
fabricated a spare one just in case.
If your exhaust looks bad/ropey, it will actually be much worse than its
initial appearance and you should replace it. It's a simple matter to weld a
stainless exhaust onto a cast manifold plate if you want to replace it in
stainless, using a TIG welder.
From article by Aussie Bray in 8/2003 issue of Sail magazine:
- Compared to dry exhausts, wet exhausts are cheap, cool and quiet.
But if something goes wrong, they can allow water into an
engine cylinder, damaging or destroying the engine.
- Possible failures: water level gets higher because boat gets more loaded,
leak inside water-jacketed exhaust manifold, heeling puts elbow too low, water
surges in through exhaust outlet, raw-water pump impeller disintegrates.
- Good if engine is installed on an incline, so water in manifold
tends to drain away from cylinders.
- If manifold is below waterline, siphon break will help
prevent water from coming in backward when engine is stopped.
Siphon break should be on centerline, and high.
- Risers tend to be heavy; their weight and vibration can damage the manifold.
Install a bellows to prevent this.
- In a heavy roll or knockdown, a riser can trap water
and send it back toward the cylinders.
- Close intake valve when engine is off (dangerous; don't
forget to re-open before starting engine).
- Seal exhaust outlet when engine is off (dangerous; don't
forget to re-open before starting engine).
- Drain water out of exhaust system when engine is off.
Idea from Roy Cecil:
To make a cheap elbow on top of a dry riser:
The riser is a vertical pipe; at the top, another pipe attaches at 90 degrees,
and runs downhill to stern.
At that top point, join the pipes with a tee-fitting, where
the middle part of the tee attaches to the riser pipe.
On the uphill (empty) part of the tee, insert a smaller pipe,
letting it run entirely through the body of the tee and out into
the downhill pipe. The first few uphill-end inches of the
outside of that smaller pipe have to be
threaded to seal onto the tee. You'll need a reducing plug,
because the smaller pipe has to be significantly narrower than
the tee and downhill pipe.
Clamp the seawater hose to the uphill end of that smaller pipe.
Now the tee and smaller pipe form an elbow, with the water being
injected past the tee and in the downhill section. No welding
or custom part needed, just some threading.
You could put some kind of nozzle on the end of the smaller
pipe, to make the water spray out and mix well with the gasses.
I replaced my leaking $1300 stainless-steel wet riser with a $250 galvanized-pipe dry
The dry riser works fine, doesn't sound louder, is simpler, will be cheap to repair, and
gives me confidence that water will never run back into my cylinders.
But it gets very hot. There is a bit of odor, and I don't know if
it's exhaust gas leaking, or paint being burned off (I used high-temp paint on it).
And since I did it, I've heard that galvanized pipe emits bad fumes under
high temperature; maybe I should have used plain steel pipe or something else.
As of 2/2012, the dry riser has worked fine for 9 years. I'm happy with it.
- Exhaust outlet should be well above waterline.
- Want shutoff valve in exhaust line, to prevent water intake;
must be accessible.
- Put flapper valve on outlet to prevent backflow.
- From Tom Neale: Replace exhaust anti-siphon valve (which is prone
to failure) with a tube (maybe up to cockpit and out the drain).
Water from tube also lets you see that exhaust cooling is working.
- Have engine exhaust temperature alarm installed
("120 degree Klixon N.O. thermoswitch (Grainger) attached
to the wet exhaust hose just after the manifold"; or maybe
Cummins/Onan 309-0259; or kit from Hamilton Marine; or White-Rodgers 3F01-121).
- Put wire mesh around dry riser, to avoid burning yourself on it.
Nauqualift Silencers at Boatman's I-Net Marine
Centek (available from West Marine)