The following article has been printed in several journals. I got this
copy from Professional BoatBuilder
Magazine. If your a Marine Mechanic and are not getting this magazine, you are missing
some of the most up to date information available to the marine repair business.
Note: the above link opens in a new window.
Most of today's pleasure boats with inboard engines have wet exhausts: Water is
injected into the system to cool exhaust gases, and then passed overboard through the
A wet exhaust has several advantages over a dry exhaust. The injected water lowers the
temperature of the exhaust gases from as high as 2,000oF to below 212oF,
cooling the gases sufficiently to allow the use of flexible hose after the water injection
point. Flexible hose is easier to run than pipe, is not subject to corrosion or stress
cracking; and absorbs the movement and vibration of a flexibly mounted engine. The
water-cooled exhaust needs no insulation, and does not pose a fire or burn hazard to
either the boat or crew. What's more, as the temperature of the exhaust gases falls, the
volume of the gases declines proportionately, reducing exhaust noise. At the same time,
the minimal exhaust back pressure generally needed to lift the exhaust water out of the
boat also muffles the sound.
This is a significant list of benefits. On the downside, ever since water has been used to
cool exhausts, there have regularly been cases of engines flooding with water from the
exhaust system. Boats have sunk on their moorings from water first filling the engine, and
then trickling out of engine apertures until the boat goes down. It's simply unacceptable
that such things still happen, because the principles of a safe cooling and exhaust
installation have been understood for decades.
How Engines Flood
On a boat with a water-cooled exhaust, the water in which the boat floats--called raw
water--cools both the engine and the exhaust. In the most common arrangement, a raw-water
pump draws water from the engine intake seacock and through a strainer. It then pumps this
water through a heat exchanger-and perhaps an oil cooler or two, into the exhaust pipe via
a water-injection nipple or mixing elbow. The water flows with the exhaust gases into a
water-lift box, also called a water silencer. The discharge pipe of the water-lift box is
set slightly above the bottom of the box. The water level rises until it blocks the
discharge pipe, at which point the trapped exhaust gases build up sufficient pressure to
lift the water up the exhaust pipe and out of the boat.
Problems occur because on almost all sailboats, and on many powerboats, the engine is
installed below the waterline. Even if it's not below the waterline when the boat is at
rest, the engine may well end up there when the boat heels, or when it is heavily loaded.
This latter case can be particularly dangerous: because the boat's designer or engine
installer may not have adequately planned for it. If the engine is or any time below the
waterline, any cooling circuit that allows raw water into the exhaust has the potential to
set up a siphon action. Water may siphon in from the water injection side; or, if the
exhaust outlet is below the waterline, from the exhaust outlet side.
Even if the exhaust outlet is normally above the waterline, water may siphon into a heeled
or well-laden boat, as mentioned previously. It may he driven up the exhaust pipe by
following seas; or, in an otherwise calm anchorage, be forced up the pipe by repeated
wakes from passing boats or water-skiers. This phenomenon is known as water hammer. I
heard of an interesting case of this recently in which waves, deflected off a seawall,
repeatedly hit the stern of a boat, eventually sinking it. If a boat has more than one
engine, but is backed down under only one, water can be driven up the exhaust of the
second engine. Similarly, on a boat with a generator, backing down can force water into
the generator exhaust.
Finally, repeated cranking of a difficult to-start engine can pump excessive water into an
exhaust. With every cranking attempt, the raw-water pump will move more water into the
exhaust. In a typical installation, this water will not be pumped out until the engine
fires by which time, there may be enough water in the exhaust to flood the engine.
Whatever the mechanism, once the exhaust fills with water, the water will back up the
exhaust pipe into the exhaust manifold and the engine. If the engine has more than one
cylinder, one or more of the exhaust valves is likely to be open. The water will run
through this valve into the cylinder below, and will then dribble down the sides of the
pistons and rings into the crankcase. Given enough time, the crankcase will fill until the
water begins to trickle out of one aperture or another, perhaps the dipstick tube, into
Recognizing these potential problems, the American Boat and Yacht Council (ABYC) standard
P-l, Installation of Exhaust Systems for Propulsion and Auxiliary Engines, says that a
wet-exhaust system must be designed to keep water out of all engines, in all situations,
including when the boat is backing down or not in use. The question is, what is the best
way to comply with these requirements?
The Supply Side
Let's look first at the supply side. Almost all raw-water pumps are the rubber-impeller
type. In theory, when these pumps are at rest, they act as a check valve, allowing no
water to pass through the pump. That's why some engine installers rely on the pump to
prevent a siphon from developing. But as we'll see, this is fool hardy in the extreme.
It's not uncommon for a rubber impeller pump to lose a vane or two from the impeller, but
still move enough water through the engine to keep it operating at normal temperatures,
particularly in colder; northern waters. The operator may well be unaware that there is a
problem. But when the engine stops, if the missing vanes of the impeller should come to
rest between the inlet and outlet ports on the pump, there will no longer be any siphon
The only sure way to protect against siphoning is to install a siphon break, also called a
vented loop, between the last heat exchanger or oil cooler on the cooling circuit, and the
point at which water injects into the exhaust. A siphon break is a very simple device set
at the top of a loop of hose. An installer takes the hose as high under the deck as
possible, to a point that will always be several inches above the waterline, at any angle
of heel and at maximum boat loads, and plumbs in the siphon break at this point. The
device consists of nothing more than an upside-down U-shaped pipe with a small valve on
the top of the U. The valve is usually a rubber flap. When the water pump is
operating, water pressure closes the flap: when the engine is shut down, the weight of
water in the hoses sucks the flap open, allowing the water column on either side of the U
to drain down, thus breaking the siphon.
A siphon break is a good idea, even on boats where the raw-water injection point is above
the waterline, making the development of a siphon theoretically impossible. As noted
previously, these "level state;' calculations often do not hold at sea. The boat may
be heavily laden and down a few inches on its waterline. There may be times when the angle
of heel puts the injection point below the waterline, a particularly likely scenario on
twin-engine boats, where the engines are offset from the centerline. Or, there may be
occasions when substantial waves surging past the boat create sufficient hydrostatic
pressure to set up an intermittent siphon, eventually flooding the engine.
Siphon breaks will be effective as long as they are working. Unfortunately, it's not
uncommon for them to foul up. What happens is this: Every time the engine shuts down, a
few drops of water are left around the valve seat. The residual heat from the engine
evaporates this water, leaving a grain or two of salt. In time the valve becomes clogged.
It may then either fail to close, allowing salt water to spray over the engine when the
motor is running; or it may refuse to open when the engine is shut down, and not break the
siphon. Sometimes a salt-encrusted valve will do both! These valves require regular
maintenance: Periodically remove the rubber flap and rinse it in fresh warm water.
A device that may require less attention than a siphon break is a vacuum breaker from the
Scot Division of the Ardox Corporation (Fort Lauderdale, Florida). This consists of a
plastic poppet valve held against a Delrin seat by an external stainless steel
spring. Since the valve is normally closed, neither fluid nor vapor in the system
can get past the seat. Salt crystals do not have a chance to form, so there is less
likelihood of problems developing than with a traditional vented loop. When the engine
stops, the weight of the water in the lines sucks the valve open. admitting air to the
system, and breaking any tendency to form a siphon. These valves work well, with just one
potential problem: Opening the valve against the spring requires a certain minimum suction
pressure. For this suction to develop, the valve must be installed 2' above the waterline
at all angles of heel. On many boats, this height may be difficult to achieve.