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No smoking allowed: The Basics of Exhaust

Interview Marquip by Maria Roberta Morso

Mention “green yachting” and people usually think of fuel consumption statistics and hybrid propulsion. It is rare for people to bring up exhaust systems. Yet many shipyards already equip their yachts with sophisticated systems, which not only reduce odors and emissions, they also help minimize noise and vibrations. Such is the case on the new Danish Yachts Shooting Star, featured in Yachts International’s October edition. The yacht includes a custom exhaust system designed by Dutch exhaust specialist MarQuip. To understand how these systems work, we asked Maurice Smit, MarQuip’s sales and marketing manager, to give us a few pointers for new yachts as well as refits.

Would you please explain the different types of exhaust systems—wet, dry and wet/dry?

Let’s start with the “wet system.” In this system, water (used as a coolant) is injected into the exhaust gas just behind the engine. The gas and water mix, then the mixture runs through pipes, a water lift, silencer and separators (made out of FRP) before it exits the hull, usually above the waterline. This system is typically used on boats with a power range of up to 500 kW. For higher power ranges using water as a coolant (up to 7,000 kW), the exhaust gas runs through a combined wet silencer/bypass and exits the ship under the waterline. The system, although complex, has several advantages: It is compact, lightweight and it helps reduce noise levels and control heat effectively.

In a “dry system,” the exhaust gas is not cooled. The exhaust runs through a series of conventional silencers and usually exits the ship through a funnel. This relatively simple system is applicable for all power ranges. While it is a relatively effective and inexpensive system, it has its disadvantages. First, it is space-intensive because it requires room for the silencers and insulation around the pipes. Then, with the exhaust being released into the air, there are issues with odor and soot.

The third common system available is a combination of the two previous systems, and is often used for power plans in ranges from 100 kW to 4,500 kW. In this system, water is injected into the exhaust gas after it passes through a dry silencer. For generator applications, the cooled exhaust gas runs through pipes, a water lift and silencer/separators before exiting the hull, usually above the waterline. The exhaust from main engines tends to exit below the waterline. This system effectively controls noise and heat emissions, but it is a rather complex system, which requires a careful design.

These are the basics. In reality, there are a number of combinations and variations of all of these systems.

How can an exhaust system help build a “green yacht,” especially when you consider noise a pollutant?

An efficient exhaust system can significantly lower a yacht’s environmental impact. You can have the exhaust gas go from the engine outlet directly into a dry silencer with an integrated catalytic converter. The silencer guarantees proper noise attenuation. From there, the exhaust gas goes to a soot trap with an automatic burner. Then, an injection piece injects water into the system. Special nozzles control the water droplets’ size and help diffuse them evenly to avoid any hot spots. This helps cool the exhaust rapidly and efficiently before it enters a water/exhaust separator that also serves as an extra silencer. An exhaust system such as this one will help the yacht’s owner get the cleanest and quietest system available.

Which are the main parameters to consider when choosing or designing an exhaust system?

The main engines are the starting point, of course. Their power in kW, exhaust gas volumes, gas temperatures, decibel output and maximum allowable back-pressure will all be relevant in the design. In order to achieve the best results, it is very important to decide at an early stage the kind of exhaust system that will be adopted and where the outlets will be located (for instance, above or underwater, on the sides, aft or through a stack). This goes for the generators as well; they are used a lot during anchoring. Reducing noise and vibrations and, at the same time, cleaning the exhaust gasses are important steps toward achieving onboard comfort.

Is it possible to retrofit an older vessel with a “greener” exhaust system?

Yes, but a refit requires a different approach. We have developed a modular unit, which is extremely easy and quick to install. Its size is such that it does not require a haul-out, cutting the hull or rebuilding the engine room. After dismantling the old silencer, it is possible to get the new modular exhaust system inside the hull through existing openings and doors. It can save the yard and the yacht owner considerable time and money. Just as in a new build, though, refitting a yacht with a “green” exhaust system requires planning. It is imperative to get as much accurate information about the engine performance and the layout of the engine room ahead of time. These highly customized exhaust systems take time, so it is necessary to be ready 20 to 24 weeks before the vessel comes in for the refit. By working this way we can guarantee an exhaust system to the highest standard and satisfaction of the customer.

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