Exhaust System Basics

By Tom Burden, Last updated 6/25/2019

Water-cooled inboard engines inject cooling water into the exhaust pipe, which cools the exhaust and muffles engine noise. The exhaust then pushes the water out the exhaust pipe. This is known as a wet exhaust system. Wet exhaust systems have the advantage of using the heat-absorbing and sound-dampening qualities of water to cool and quiet the exhaust. Air-cooled, or dry exhaust, systems are uncommon except in performance powerboats or large work boats where excessive heat buildup and/or noise are tolerable, or in precisely engineered trawler/passagemaker exhaust systems which insulate the hot exhaust pipe with forced air.

Above- and below-waterline applications require distinctly different products to muffle engine noise. Powerboats use marine mufflers, commonly made out of non-metallic materials, to contain engine noise and cool the exhaust. Mufflers are always mounted above the waterline, and usually have some sort of check valve in-line to prevent backwards water flow, which might damage the engine.

Above water line exhaust schematic
Below water line exhaust showing water lift muffler and anti siphon loop

Lift mufflers like Centek Vernalifts are used both by powerboats and sailboats. They are vertical mufflers mounted below the waterline with an outlet line coming off the top of the container. Exhaust gas and expended cooling water are mixed shortly after the exhaust manifold and travel together into the muffler's chamber where the cooling water collects before the exhaust gas pressure pushes it out the remainder of the exhaust hose. The mufflers's chamber also provides a place for seawater from following seas to collect if the boat is running under sail. For greater quieting, an inline muffler can be used downstream of a lift muffler.

Additional System Components

For vessels with through-the-transom exhaust, there are flapper valves that prevent following seas from running back up the exhaust system and flooding the engine. They clamp around the outside diameter of an exhaust pipe to form a check valve.

Exhaust hose or tubing is used to connect the exhaust manifold to the muffler and runs from the muffler overboard. It is essential to use material which can withstand heat, oil, acids, pressure, water weight and vibration without failure. As a rule of thumb, hose runs that are longer than four times the inside diameter of the hose should use wire-reinforced exhaust hose or fiberglass exhaust tubing. Shorter lengths can use non reinforced hose. All hose connections should be made to rigid surfaces (such as tubing, pipe, or the muffler itself); hose-to-hose connections are never recommended.