This Cat Is Too Hot to Handle — Temperature & More

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, typically fiberglass and painted or powder-coated white — have turned yellow.

Exhaust temperature, exhaust gases, and water flow

That discoloration is the exhaust temperature being too hot for the fiberglass and paint. Conditions that can cause this overheating include exhaust-gas temperatures, mixer-tank efficiency, exhaust-system piping geometry, and muffler design.

Calculating the velocity of the exhaust gases, the exhaust temperature, and the water flow from the engine water pump or auxiliary pump is critical. The engine manufacturer supplies the specifications for the water-pump flow rate. That flow rate — along with exhaust-gas temperature and volume — drives the design of the mixer tank. Keep in mind that some of the flow may be diverted for other components such as transmissions or shaft cooling, reducing the rate available for the exhaust system.

Raw water meets hot gases

Also important: the distance the raw water is in contact with the hot gases. If the time and distance from the point where the hot exhaust gases first meet raw water from the mixer tanks is too short, the gases aren't properly cooled and hot spots appear — the root cause of the discoloration. In some cases, a spray nozzle can help, but calculations need to confirm that adding one doesn't negatively affect the initial cooling at the mixer tank.

The geometry of the piping between the turbo and the hull outlet — diameters, elbow angles, length, compensators, hoses, valves, and other materials — needs to be properly calculated. The diameter for inlets and outlets of every component, from the turbo outlet to the mixer-tank diffuser, including the mixer tank's diameter and thickness, must all be designed correctly.

Muffler design

The muffler's internal-design calculation should impose minimal restriction on the exhaust gases, with placement considered against the yacht's waterline. A muffler placed too far below the waterline raises back pressure on the engine and increases engine-room temperature. Proper design, calculations, engineering, manufacturing, and use of quality materials according to engine-manufacturer specs are what produce an exhaust system that performs well and lasts for decades.

Author: Mike Prado — VP of business development, D'Angelo Marine Exhaust.