By Tom Burden, Last updated 6/25/2019
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Water-cooled inboard engines inject cooling water into the exhaust pipe, which cools the exhaust and muffles engine noise. The exhaust then pushes the water out the exhaust pipe. This is known as a wet exhaust system. Wet exhaust systems have the advantage of using the heat-absorbing and sound-dampening qualities of water to cool and quiet the exhaust. Air-cooled, or dry exhaust, systems are uncommon except in performance powerboats or large work boats where excessive heat buildup and/or noise are tolerable, or in precisely engineered trawler/passagemaker exhaust systems which insulate the hot exhaust pipe with forced air.
Above- and below-waterline applications require distinctly different products to muffle engine noise. Powerboats use marine mufflers, commonly made out of non-metallic materials, to contain engine noise and cool the exhaust. Mufflers are always mounted above the waterline, and usually have some sort of check valve in-line to prevent backwards water flow, which might damage the engine.
Lift mufflers like Centek Vernalifts are used both by powerboats and sailboats. They are vertical mufflers mounted below the waterline with an outlet line coming off the top of the container. Exhaust gas and expended cooling water are mixed shortly after the exhaust manifold and travel together into the muffler's chamber where the cooling water collects before the exhaust gas pressure pushes it out the remainder of the exhaust hose. The mufflers's chamber also provides a place for seawater from following seas to collect if the boat is running under sail. For greater quieting, an inline muffler can be used downstream of a lift muffler.
For vessels with through-the-transom exhaust, there are flapper valves that prevent following seas from running back up the exhaust system and flooding the engine. They clamp around the outside diameter of an exhaust pipe to form a check valve.
Exhaust hose or tubing is used to connect the exhaust manifold to the muffler and runs from the muffler overboard. It is essential to use material which can withstand heat, oil, acids, pressure, water weight and vibration without failure. As a rule of thumb, hose runs that are longer than four times the inside diameter of the hose should use wire-reinforced exhaust hose or fiberglass exhaust tubing. Shorter lengths can use non reinforced hose. All hose connections should be made to rigid surfaces (such as tubing, pipe, or the muffler itself); hose-to-hose connections are never recommended.
The difference between a wet and dry exhaust explained!
Compare a dry exhaust to the exhaust pipe of the car: a pipe with a flexible connecting piece on the exhaust manifold, discharging the exhaust gases directly to the outside air. A dry muffler is often required to muffle the noise. Cooling water from the engine is added to the exhaust system either externally or at a much later stage. The tubes and pipes are often made of steel to prevent them from melting due to the high temperature. It is a simple system, but with two major drawbacks: it emits intense heat of up to 600°C at the source, and it produces more noise.
Rather a wet exhaust?
What is a good alternative? That’s right, the much talked-about wet exhaust system. It is recognised by the manifold that is mounted on the engine. This is often custom-made from cast iron. It is also recognised by the freezer caps in the cooling channels. The heated cooling water in the engine is around 60-90°C when it leaves the engine, so it can be perfectly used to cool the 600°C exhaust gases. Water and gases are often not yet mixed in the first section of the exhaust pipe. That is to prevent the water from entering and blocking the gases from exiting the engine. Gases must be able to exit the engine smoothly so as not to compromise the engine performance. At a short distance from the engine, the gases and liquids come together and exit the engine.
The difference in temperature of a dry and wet exhaust is noticeable and has many advantages.
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The noise of a wet exhaust is muffled by the water, but a rubber exhaust hose can also be used. This gives more flexibility and has a longer life span.
Various cooling systems
In simple terms, there are three types of cooling systems: 1. A system that pumps water straight from outside into the engine block by way of an impeller pump; 2. A separated system, often referred to as an intercooling / heat exchanger system, with coolant in the engine and an impeller pump pumping cooling water through a heat exchanger. 3. A keel cooling system, with cooling pipes containing coolant running underneath the boat. A wet exhaust is not possible in this system, unless an extra pump is installed.
From dry to wet? Convert the dry exhaust if you would like to experience the advantages of a wet exhaust system. Fully assembled exhaust pipes are available on the market. Or just add water to the dry exhaust, where only the first section is not cooled. Conversion may require a fair amount of money, but it is an easy job for the handy do-it-yourselfer and it will pay off in due course.
( wet exhaust system with waterlock and muffler).
A wet exhaust consists of the following components from start to finish:
Also, a so-called Waterlock is recommended when using a wet exhaust. A waterlock collects the water and pushes it upwards through the exhaust with the exhaust fumes by means of overpressure. A waterlock also muffles the noise, because it remains at water level. The gooseneck serves as an extra safety feature to prevent backflow of water when the engine is lower, as is the case in most sailboats. Installation of a muffler is still an extra option. A muffling and flexible exhaust hose can also be used for wet exhaust systems; it can be attached using stainless-steel hose clamps.
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