Recently a friend mentioned his buddy experienced black smoke coming from the engines of his new-to-him Downeast cruiser. It got us talking about how to potentially troubleshoot engine and running gear issues from the amount and color of the exhaust smoke.
In traditional, mechanical diesel engines, some amount of white smoke at start up is normal, as accurate metering of the fuel is difficult to achieve with cold mechanical engines. The more modern electronically-controlled engines are far better at adjusting fuel metering and delivery to match the conditions and loads so these engines don't smoke as much.
Once an engine warms up, however, most white smoke goes away, as this "smoke" is really atomized, unburned fuel. You may even see a slight oil slick of fuel on the water's surface. Starting a cold engine, especially in cold weather, does not fully burn the metered fuel, as cold air and cold combustion chambers are not particularly conducive to proper combustion, so some amount of unburned fuel exits out of the exhaust. This is why some engine companies add electric preheaters to get the incoming air up in temperature for better fuel combustion.
Blue smoke at start up or when increasing the throttle after prolonged idling often indicates worn and leaking valve seals, allowing crankcase oil into the combustion process. This is especially true for older diesels that regularly burn oil, such as the sailboat engine in Captain Ron. If that is what you find, the likely suspects are your valve seals or piston rings, which do wear out. If it is a newer engine, did someone overfill the oil level in the crankcase? It happens.
If your fuel injection pump timing is off, or your older engine has faulty injectors or worn piston rings, evidence of these issues often results in white, gray, or blue smoke.
Black smoke indicated the engine's fuel-to-air ratio is off, with too much fuel going to the engine, or not enough air due to some restriction. The throttles were pushed suddenly forward perhaps, sending lots more fuel to the engine, but the engine has not had time to reach the higher rpms, so black smoke. The engine could be struggling under load, such as from an improperly sized or pitched propeller, or as is often the case, the running gear is fouled.
In some cruising areas, a propeller can foul in a relatively short time, especially if it has not been properly coated with one of the newer anti-fouling products, such as Propspeed. If you have this black smoke at your normal cruising speed, and you haven’t seen it before, it is very likely that your problem is underwater. It is amazing how barnacles can affect your propeller’s efficiency. But it may also be a clogged air filter, throwing off the fuel-to-air ratio.
In addition to having the right tools and parts for your cruise, getting to know your engine is also a good thing to develop. And to better understand potential issues, learning your smoke signals is a good way to anticipate maintenance before things stop working, or these issues become disabling. That’s not something you want to have happen when you are off cruising.