While dry exhaust systems are utilized in many commercial maritime applications, they are found on only a small number of recreational vessels. Nevertheless, the debate over the advantages and disadvantages of dry exhaust systems remains active, with ardent acolytes on both sides.
I’ll begin with what isn’t debatable: Unlike wet* exhaust systems, dry exhaust uses no seawater injection whatsoever. Instead, they convey exhaust gases from the vessel’s engine to a discharge, typically a stack of some sort, using insulated metallic pipe of mild steel or stainless steel (if the latter, it must be 316 for ABYC compliance).
The system is often combined with a hull-mounted keel cooler. Coolant is circulated through the keel cooler, making the water in which the vessel floats in effect the “heat exchanger.” No raw water is needed to cool the exhaust, making it possible to do away with all engine-associated raw water components.
*For a comprehensive article on wet exhaust systems, see “Internal Combustion’s Backside” in Professional BoatBuilder No. 170 (December/January 2018), page 60.
Why Use Dry Exhaust
The primary advantage of a dry exhaust system is that it enables the elimination of the raw water components typically found in a wet system—including seawater strainers, pumps, impellers, hoses, waterlift mufflers, seacocks, and exhaust mixing elbows—and the maintenance and potential failures associated with all of these items. While it can be argued the system requires less maintenance, it is not maintenance-free. The engine’s closed cooling system, including the keel cooler, must still be flushed and coolant periodically replaced, which would be necessary with a wet exhaust system as well, and keel coolers must be cleaned.
Most keel cooler manufacturers recommend against antifouling paint. If used regularly keel coolers tend to remain relatively free of marine fouling; however, if left idle for long periods in high-growth areas, fouling is common, making cleaning a necessity. Keel coolers are usually equipped with sacrificial anodes, which must be replaced periodically.
Dry exhaust systems should be inspected for leaks, soot, and deterioration of insulation and support mechanisms. Again, inspection would also be necessary on a wet exhaust system. Unlike a wet exhaust system, however, dry exhausts often produce soot at the stack exit, especially at start-up, which is problematic on recreational vessels when soot particles rain down on white or teak decks. Some operators resort to placing a butterfly net–like device, or panty hose, over the exhaust discharge at start-up to capture particulates.
Dry vs. Wet
While wet exhaust systems generally require more maintenance, they do have one advantage over dry exhaust systems: all of the major components are inside the vessel, particularly the heat exchanger. If it needs to be repaired or cleaned, the vessel does not need to be hauled. This is not true for keel coolers. Although they can be cleaned by a diver, to some extent, outright repairs would require hauling.
Another challenge with dry exhaust systems is that the vessel must be designed around the exhaust pipe(s), which typically pass through accommodation spaces. These must be insulated against the transmission of heat, noise, and vibration, and they occupy valuable interior space, often through the middle of the vessel for a single screw application. For this reason it’s typically not practical to convert a wet exhaust vessel to dry exhaust, or to build a vessel that has been designed for a wet exhaust system with a dry exhaust.
Heat creation within the engine room and transmission to the accommodation spaces also presents challenges for designers and builders. When water is injected into a wet exhaust system, exhaust gasses are rapidly cooled, reducing noise considerably, as well as allowing them to be transported within fiberglass pipe and flexible hose. This is not an option where dry exhaust systems are concerned; all plumbing must be metallic, which is invariably heavy. It must be properly supported, eliminating or reducing, depending on the engine manufacturer, the load imparted to the turbo-charger or exhaust manifold flange.
Supporting these loads is no small challenge. Adjustable struts can be used to support the initial section of exhaust, to the engine or transmission, in a fashion similar to a dry riser on a wet exhaust system (see “Internal Combustion’s Backside” in PBB 170). Thereafter, a flexible section of exhaust pipe, called a wrinkle belly, must be installed, allowing the engine to move independently of the remainder of the dry exhaust system, which is supported by the engineroom overhead structure, and then the cabin structure as it wends its way out of the vessel. This part of the system is insulated pipe and often includes a heavy metallic muffler, which must be flexibly mounted in order to avoid conveying vibration to the vessel. Supports here typically incorporate a spring or motor mount–like mechanism. Additionally, any supports attached to the exhaust must also include a means of isolating or insulating heat transfer from the exhaust pipe to the vessel structure, particularly if it’s FRP or timber, as well as minimizing both burn and fire hazards.
Although it comes at a design cost, when properly executed, a dry exhaust system can be robust, simple, and long-lasting, making it a good choice for the right vessel.
About the Author: For many years a full-service yard manager, Steve now works with boat builders and owners and others in the industry as Steve D’Antonio Marine Consulting. He is an ABYC-certified Master Technician, and sits on that organization’s Engine and Transmission and Hull and Piping Project Technical Committees. He’s also the technical editor of Professional BoatBuilder.