A well-designed exhaust system will Keep water from

drowning an engine

 
   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.
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  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 exhaust outlet.
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 the boat.
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 protection.

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.

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