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Finally, another approach to siphon prevention is to dispense with the siphon break or
vacuum breaker altogether, and replace these devices with a length of hose carried up
under the deck and vented overboard; preferably into the cockpit. The hose must be
taken high enough that the back pressure from the exhaust system does not cause the vent
to weep when the engine is running. In general, as long as the vent is above the highest
point in the exhaust hose, there will not be a problem.
The Exhaust Side
On the outlet side of the siphon breaking device, the raw-water injection line connects to
the raw-water injection nipple. This nipple needs to be inserted into the exhaust in such
a way that it projects the water down the exhaust pipe. It should also be set a few inches
below the exhaust manifold; 4" is often quoted as a ballpark figure. Most marine
engines are now supplied with a mixing elbow designed to meet these objectives.
From the injection nipple, the exhaust should drop down to the water-lift box. Eight to
ten inches is a minimum figure often given for this drop. (See top illustration, The box
itself should be installed in a fore-and-aft line with the outlet from the exhaust
manifold; any offset will require an additional drop to compensate for the effects of
heeling. For example, if the water-lift box is offset 12" to port or starboard from
an exhaust manifold, a 20 deg. angle of heel, quite common on sailboats, will raise or
lower the box 4" in relation to the manifold. The box, then, will need to be set at
least 12" below the manifold to maintain an 8" drop at all angles of heel.
From the box, the discharge hose loops up under the deck, well above where the waterline
would be at maximum heat loads and at all angles of heel, around 18" from the top of
the loop to the waterline is commonly recommended, and then drops to the exhaust
through-hull. This loop is generally considered adequate to prevent water from siphoning
in from the back of the boat. From the top of the loop, the hose should maintain a
continuous downward slope--a minimum drop of 1/2" per foot of hose run, with no dips
or hollows to the through hull, which should be at least 12 " below the top of the
loop.
To lessen the likelihood of water ingestion, the exhaust outlet should always be at least
a few inches above the waterline when the boat is at rest. This location will also make it
easy to quickly verify the raw water, flow now when the engine is running. In
practice, the outlet is frequently installed at, or below, the waterline for a number of
reasons. to improve cosmetics; to aid in the disposal of the exhaust gases; and to help
silence engine noise. If the outlet is down near the waterline, powerboats can add a
measure of protection by including a hinged flap on the outside of the hull. For
sailboats, add a seacock that can he closed in heavy following seas when the engine is not
in use.
An often-overlooked requirement is that the volume of the water-lift box should he greater
than the total volume of the exhaust hose from the water-lift box to the highest point
under the deck. Then when the engine is shut down, even if the exhaust is full of water,
and even if it all drains back into the water-lift box, the water will still not flood the
engine.
Finally, water-lift boxes should have an accessible drain at their low point to permit
winterizing or removal of excess water resulting from several unsuccessful cranking
attempts.
Variations on a Theme
The system described so far is a "typical" installation for the most common
water cooled exhaust. But, not all boats can meet these physical requirements. In
particular, there is often not enough vertical distance from the engine to the hull to
provide the necessary drop from the exhaust manifold to the water-lift box. It's possible
to circumvent this difficulty, however, by adding a riser to the exhaust. This is a raised
loop of dry exhaust pipe preferably with a water cooled jacket which can be tailor made to
provide enough height for a proper water cooled installation.
Even with adequate vertical distance, another difficulty may arise: excessive back
pressure in the exhaust. The higher the water must be lifted from the water-lift box to
the top of the exhaust, the greater the back pressure in the system. As noted above, some
back pressure is highly beneficial in damping exhaust noise. Any significant pressure,
though, can impair engine performance. Turbocharged engines and Detroit Diesel two-cycles
are particularly sensitive to back pressure. In fact, with too much back pressure, a
Detroit Diesel two-cycle simply will not start at all. As a general rule of thumb, the
vertical water lift on a naturally aspirated engine should not exceed 40"; and on a
turbocharged or two-cycle engine, 20" In any case, consult the engine manual for more
precise information.
You can minimize back pressure on any installation by using exhaust hose that is at least
the correct diameter, if not oversized. It should have no sharp bends, and be as short as
possible. The size of the hose from the exhaust manifold to the water-lift box is
determined by the size of the outlet from the water injection elbow supplied with the
engine; the hose from the water-lift box to the overboard discharge may be the same size,
but is sometimes an inch or more larger than that in diameter. The longer the hose, and
the more bends, the larger the diameter should be. You can check back pressure by tapping
a 1/8" pipe thread into the exhaust manifold (after any turbocharger), installing a
1/8" pipe nipple into the manifold, and then connecting a sensitive pressure gauge
(measuring in inches of water or inches of mercury) to this nipple. Most engine manuals
give allowable levels of back pressure for different engine loading, the most important
being the back pressure at full load.
If you find excessive back pressure, you can dramatically reduce it by replacing the
water-lift silencer with a stand pipe type exhaust. This is similar to a riser, except
that the entire water-lift box is raised, eliminating the need to lift the water at all.
The trade off, though, is that the reduced back pressure allows an increase in exhaust
noise, which may need to be further muffled with an in-line silencer. The dry section of
exhaust pipe leading from the manifold up to the standpipe will get extremely hot and
require: either an expensive water jacket, or some other type of insulation. It should be
fabricated from metal, and have some kind of flexible expansion joint to accommodate
engine vibration, as well as the changes in pipe dimensions that accompany changes in
temperature. For example, a 4' length of pipe grows 1/4" in length as its temperature
increases from 100 deg. F to 950 deg. F.
PREVENTING CARBON MONOXIDE LEAKS
Both ABYC's P-l standard and NFPA (National Fire Protection Association) standard 302 Fire
Protection Standard for Pleasure and Commercial Motor Craft, require all exhaust systems
to be gas tight to the hull interior. The primary concern here is to prevent carbon
monoxide poisoning of the occupants, which causes a number of deaths every year.
According to these standards, all exhaust hose connections must have two all stainless
steel hose clamps, each with a minimum band width of 1/2". Note, however, that many
all-stainless steel hose clamps have a band made from 300-series stainless, but a screw
made from 400 series stainless. Although these clamps still qualify as "all
stainless," the screws are nevertheless likely to corrode in a salt atmosphere,
compromising the integrity of the band. For increased corrosion resistance, you should use
all 300-series stainless steel hose clamps.
Where rigid pipe is included in an exhaust system (primarily dry exhausts), the ABYC
standard states that it must be "at least schedule-80 pipe or equivalent," and
that it must be resistant to corrosion from exhaust by products. The standard notes that
many copper alloys are not resistant to the sulfuric acid that occurs in diesel-engine
exhausts. Accordingly, the NFPA standard does not allow copper to be used in dry diesel
engine exhaust piping, "or within six pipe diameters downstream from the point of
water entry in water-cooled exhaust systems." If there is more than one metal in the
exhaust system, both standards require the different metals to be galvanically compatible.
The NFPA does not allow threaded pipe ends to be inserted into non-metallic exhaust system
components such as rubber hose; you must use the proper adapter and hose nipple for this
purpose.
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