Many times when we visit yachts at marinas and shipyards, we see that the main engine and generator exhaust wet section, including piping and mufflers – which are typically fiberglass and painted or powder coated white – have turned yellow.
Exhaust Temperature, Exhaust Gases & Water Flow
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 water flow from the engine water pump or auxiliary pump is critical. The engine manufacturer supplies the specifications for the water pump flow rate. This flow rate, along with the exhaust gas temp 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.
Raw Water & Hot Gases
Also important is the distance the raw water is in contact with the hot gases. If this time and 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 aid in the cooling. Calculations need to be made to ensure the additional spray nozzle does not negatively affect 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 turbo’s outlet to the inlet of the mixer tank-diffuser, including the diameter and thickness of the mixer tank itself, must be designed properly.
The muffler’s internal design calculation should have a minimal restriction for the exhaust gases, and location consideration of the yacht’s waterline. Placement too low 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 manufacturer’s specifications) all play an important role in ensuring a well-performing exhaust system that can last for decades.
Author Mike Prado
Mike Prado is the VP of business development for DeAngelo Marine Exhaust, manufacturer of marine exhaust systems, parts, and services.
Original Article: This Cat Is Too Hot Too Handle | August 2019
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Contact DeAngelo Marine if you require an inspection / survey of your engine room, main engines or generator exhaust systems. We invite your inquiry for exhaust parts & accessories and exhaust repairs and service with your existing vessel. If you are in the design phase of new vessel construction or contemplating an engine overhaul, contact DeAngelo Marine Exhaust by email or by telephone: +1 954.763.3005.