Wet exhaust systems help neutralize vibration on yachts

Jan 18, 2016 by Rich Merhige

Over the past few years, more and more frequently, exhaust systems have been identified as the source of vibration on yachts. Today, most yachts are equipped with wet exhaust systems, whereas an injection of water is used to cool exhaust gases, and then passed overboard through an outlet.  

Sea water discharged from the engine’s jacket water is used for the water injection into the exhaust. Much like humans need to exhale after inhaling, wet exhaust systems are designed to remove the gases that are the by-product of the engine’s combustion, as well as waste heat that is produced.  

The design of the system’s piping must be properly laid out and installed, and also be properly maintained to prevent noise, vibration, and damage to engines and mechanical components.  Never attach structures supporting the weight of the exhaust piping directly to the engine block or components.  

Engines are not designed to support the weight of the exhaust system and this will also short circuit the engine vibrations through the piping into the structure. To prevent vibration, the piping must be properly isolated. Isolator pipe supports should be used to minimize vibration.

The isolator supports are also most effective when spaced irregularly, rather than spaced at regular intervals to reduce pipe resonances. Oftentimes, exhaust lines are too small in diameter, designed with too many bends, or, have clogs. All of these scenarios can cause back pressure.

All engines have different specs for exhaust back pressure, so it’s important to pay close attention to the stated requirements laid out by the manufacturer.

The engine cooling water is injected into the exhaust line using a spray ring right after the exhaust riser begins to drop. As the gasses are cooled, their volume decreases, which in turn, decreases exhaust vibration.

After being injected into the exhaust, the mixture is often routed to an inline muffler or water-lift muffler for further noise reduction before being ejected overboard.  In some cases a water separator muffler is also used, which quiets the exhaust even more and allows the gas portion be discharged above the waterline and the water portion to be quietly discharged below the waterline.  

Boats with underwater exhaust have a secondary bypass to relieve backpressure from the exhaust when the boat is not on plane. On plane there is a venturi effect that sucks the gases out. Wet systems provide for better cooling, which means that flexible hoses can be utilized. Flexible hoses absorbs more vibration and movement from the engine, meaning they are also less likely to suffer stress cracks.

They are also not as vulnerable to corrosion, and are easier to run than piping.

A properly installed and maintained system will provide the proper back pressure which will muffle the sound of the exhaust water leaving the vessel. When stiff piping is used, they can have a resonance that can be excited at different speeds.

When this occurs, it’s recommended to install an expansion joint and more hangars into the exhaust system. Water must not backflow into engine from the exhaust system. One way to minimize the possibility of the water entering the engine is to have and exhaust riser. Exhaust risers are pipes that elevate the exhaust gas and typically have a deep downward slope, downstream of the engine.

The weight of the risers must be supported from the engine and/or the marine reduction gear. Exhaust noise is the main source of noise from any engine installation. It arises from the release of high pressure exhaust gas releasing from the engine cylinders, which causes strong gas pressure fluctuations in the system.

The symptoms are discharge noise at the outlet, and noise coming from the exhaust pipe and silencer surfaces. Silencers are used to reduce the noise of the exhaust before it is released into the atmosphere. As with all mechanical components of a vessel, exhaust systems need to be properly specified to the anatomy of the vessel, and complement the other machinery it interacts with. Systems need to be well designed and supported in their environment to properly minimize and deter noise and vibration. Proper maintenance of the system is just as important, to prevent the worsening of noise and damage to the system, and the engines.

Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale. Contact him through www.AMEsolutions.com.