All Engines Produce Soot

• Internal combustion engines, whether inboard diesel engines or gasoline outboard motors, produce the gray-black particulates we know as soot.
  
  Usually, the soot is in the form of particles so small, they form a nearly colorless cloud. Over time, this cloud may leave a light streak--a mere discoloration of the deck or transom--that you can wash off quickly with a hose. Other times, though, the streak on the deck or transom may be thick and greasy, and look as if it's discolored the gelcoat.
  
  When the cloud is no longer nearly colorless or the streak becomes pronounced, an appraisal of the engine, exhaust system and power train is in order.
  
  

Diesel Engines and Soot

• In years past, the sulfur in diesel fuel might have appeared to be the culprit behind the soot, but that is no longer the case. The U. S. Environmental Protection Agency (EPA) implemented more stringent diesel fuel rules that required 90 percent reductions in sulfur levels for non-road-use diesel fuel by 2007. The reduction of sulfur in diesel fuel may may help the situation somewhat, but the primary reason for soot deposit lies in the exhaust system.
  
  Boat diesels have one of two types of exhaust systems: a dry exhaust or a wet exhaust. Dry exhaust systems expel exhaust gases directly from the engine into the atmosphere and produce a fine cloud of soot that grows heavier when the boat is accelerating or maneuvering at low speeds. In wet exhaust systems, the exhaust gases are mixed with seawater and pumped from exhaust ports on the sides of the boat, minimizing soot deposits.
  
  

Outboard Motors and Soot

• Outboard motors are relatively low-horsepower gasoline engines fitted with wet exhaust systems. The appearance of soot is usually the result of overloading the engine, not by weight but by an incorrect choice of propellers.
  
  You can recognize an outboard motor that is producing soot almost immediately. At full throttle (the maximum throttle position recommended in the engine operator's manual), the boat moves far more slowly than expected, or the motor "lugs" at less than full throttle or under acceleration but has an unexpectedly high top speed.
  
  If you bought the engine used, the former owner may have changed the standard prop to a one with a lower pitch, which produces lower top speeds and overloads the engine on the high end of its operating range--like driving a car 60 mph in first gear.
  
  If the former owner opted for propeller with a higher pitch, you'll notice that the top speed at full throttle is indeed fast, but the engine strains at lower speeds--like a car starting off in top gear.
  
  In either case, the motor is overloaded and produces heavy soot deposits.