Back Pressure: It's Exhausting

Have you ever wondered why so many vessels in the marina have fuel and oil floating around them and a dark discoloration near the exhaust port on the side of the hull?

One of the most common causes is too much back pressure on the diesel engine or generator.

No two engines or gensets are alike — every brand and model has its own specifications for air and water velocity. Those variables drive the mechanical-design configuration for the route, length, and diameter of the exhaust system, all within the back-pressure limits set by the engine manufacturer. Restrictions of the exhaust-gas flow are made worse by adding too many elbows, water mixer tanks, and mufflers. The result is poor engine performance and economy, premature turbo wear, exhaust-valve damage, and increased exhaust temperature.

We often see piping and mufflers positioned below the waterline, which lets water accumulate, slows the velocity of the exhaust gases, and raises back pressure. Even a through-hull fitting that's too small or too far below the waterline drives back pressure up. All of that shortens the time between engine overhauls, lowers combustion-chamber temperature, and produces unburned hydrocarbons and soot — out of the boat and into the air, the water, and onto the hull.

Reducing back pressure

The solution starts with the engine manufacturer's specifications and then comes down to proper engineering: piping diameter and length, minimizing elbows, keeping the exhaust system dry and above the waterline through risers to the water mixer tank just before a good-flowing muffler, and a properly sized and positioned through-hull connection.

Beyond design, working with a well-known marine diesel exhaust manufacturer matters. Look for in-house engineering; certified welders and fabricators; pressure testing of all systems; passivation of metal for longevity; hard-coat or blanket insulation to reduce engine-room temperatures; and highly experienced mobile service teams. Those factors not only guarantee a correctly designed exhaust system but also minimize downtime and produce an installation that will last for many years.

Diesel Particulate Filter (DPF)

If the system has been checked for back pressure and it's within limits — or you've replaced the system and soot is still landing on the hull — the next best move is adding a DPF. DPF systems burn off particulates after the turbo, catch the ash in the filter, then catalyze it and expel water vapor out to the atmosphere.

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