High-performance diesel engines pump their own weight in seawater through the cooling system and out the exhausts about every three minutes. And those exhausts empty through two very large holes near the waterline. “If you don’t pay attention to your exhausts, you could have a lot of water going into your bilge,” says Craig McLeod, who handles product development and sales for Marine Exhaust Systems in Riviera Beach, Florida (www.marine-exhaust.com). And that’s just one of many bad days at sea that can come from neglected exhausts, ranging from seized turbochargers to bent piston rods, or lethal carbon monoxide lurking inside your boat. Exhausts are one of the most critical systems aboard, yet they’re often overlooked.
To keep heat out of the engine room, wet exhaust systems pass 800-degree exhaust gas through an inner pipe, and that inner pipe passes through a slightly larger outer pipe that carries seawater from the engine. Wet systems were popular from the 1970s through the mid-1990s, largely for looks, because those outer stainless-steel pipes can be polished to a mirror finish. But McLeod says a sparkling engine room isn’t worth the liability. “The pipe is double-walled starting right at the turbocharger,” McLeod says. “The day will come when that pipe will fail, and it will cost a lot of money in engine repairs.” In hours, or even a handful of minutes, leaking seawater can trickle back through the turbocharger, corroding it beyond repair, or into the engine’s cylinders, necessitating an overhaul.
McLeod says the key to making wet systems last is proper drainage. When the engines are running, flowing water keeps everything cool, but as soon as you shut them down, the water trapped anywhere within the system boils. This erodes the stainless-steel pipe and welded joints in the same spot again and again. McLeod says proper design eliminates this problem — at least, until a simple ⅜-inch drain hose valve is mistakenly closed!
It’s impossible to inspect the water passage between the inner and outer pipes. Pressure-testing is unreliable, since tiny cracks may open only when the metal expands with heat. “We have no way of pushing 800- or 900-degree gas through to check,” McLeod explains. The bottom line: “If a wet system is over 10 years old, I’d get it out of there.”
Most exhaust systems built since the mid-1990s are hybrids. Insulated dry pipes rise up from the engines and then turn down toward the waterline. Water is injected after that turn to cool exhaust gas and carry it overboard. From where water enters the system, leaks can flow only out of the exhaust, not back into the engine. Two decades ago dry exhaust sections were insulated with expensive but problematic removable blankets that stained, shed their silver coating or came loose. Now, maintenance-free, hard, black fiberglass insulation looks good for the life of the exhaust system.
