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Yellowed piping telltale sign exhaust is too hot

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By Mike Prado

Many times when we visit yachts at the marinas and shipyards, we see that the main engine and generator exhaust wet section’s piping and mufflers, which are typically fiberglass and painted or powder-coated white, have turned yellow.

This discoloration is due to the exhaust temperature being too hot for the fiberglass and paint. Some of the conditions that can cause this overheating are exhaust gas temperatures, mixer tank efficiency, exhaust system piping geometry and muffler design. 

Calculating the velocity of the exhaust gases, exhaust temperature and waterflow from the engine water pump or auxiliary pump is critical. The manufacturer of the engine supplies the specifications for the water pump flow rate. This flow rate, along with the exhaust gas temperature and volume, is used in the design of the mixer tank. Note that it should be considered that some of this water flow could be diverted for other components, such as transmissions or shaft cooling, reducing the flow rate for the exhaust system. 

Also important: If the time/distance from the point where the hot exhaust gases come in contact with the raw water from the mixer tanks is too short, gases are not properly cooled and cause hot spots, which is the reason for the discoloration. In some cases, this can be remediated by the addition of a spray nozzle to aide in the cooling. Calculations need to be made to ensure the additional spray nozzle does not negatively impact the initial cooling by the mixer tank. 

The geometry of the piping between the turbo and the hull outlet – including the diameters, angles of elbows, length, compensators, hoses, valves and other materials – needs to be properly calculated. The diameter for inlets and outlets of all components from the outlet of the turbo to the inlet of the mixer tank-diffuser, including the diameter and thickness of the mixer tank itself, must be designed properly.

The muffler internal design calculation should have a minimal restriction for the exhaust gases, and location consideration of the waterline for the yacht. Placement too far below the waterline will increase the back pressure to the engine and also increase engine room temperature.

Proper design, calculations, engineering, manufacturing, and use of quality materials according to the engine manufacturers specifications all play an important role in ensuring a well-performing exhaust system that can last for decades.

Mike Prado is vice president of business development with DeAngelo Marine Exhaust in Fort Lauderdale. Comments are welcome below.

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