Transom Diesel Soot

I recently had a question on how to eliminate diesel-engine sooting at the transom.  It’s an interesting topic because almost everyone is convinced they have a solution. These solutions run from expensive diesel fuel additives to passing the diesel through permanent magnets on the way to the engines.


Overall, I’m pretty resistant to paying $300 for a simple permanent magnet even if it is packaged in a nice machined aluminum case.  I’m a believer in simple systems and solutions. Generally, my preference is to start with looking at why the engine is smoking in the first place.  One common cause of excess sooting in marine environments is engine overload.  Boat builders specify props that allow the boat to produce the best speed possible when new and lightly loaded, and the engine manufacturer will ensure that configuration doesn’t overload the engine. But, as boats get older, more equipment is brought on board and boats typically get heavier.  Dirona is perhaps an extreme example, but it makes the point clearly. Bayliner advertised the 4087 at 24,000 lbs and when it was last pulled out of the water, it was over 29,000 lbs.  For those whose boat manufacturer props for maximum speed, problems can develop as the boat gets older, the tanks are filled, and the bottom becomes less than perfectly clean. The boat ends up dangerously over-propped and the engines will be overloaded under many conditions.  Again, using Dirona as an example, Bayliner shipped the boat with 22x21x4 props. We’ve reduced pitch twice since it was new in 2000 and are now using 22X19X4 (see Avoiding Diesel Engine Overload for more details on how to know if you are correctly pitched).


When diesel engines are overloaded, they emit large amounts of soot. Black clouds aft are a sure sign that something is wrong and needs quick attention. I took the picture on the right back in 2004 at the end of the Memorial Day weekend. We were part of the usual stampede back to the Seattle area from the San Juan Islands, and I was amazed at how much smoke many of the boats were producing.  The boat pictured below is a Bayliner 4788 and its engine is seriously overloaded. The best thing the owner of that boat could do is remove 2” to 3” of propeller pitch. If they did that, they would find they spent less time cleaning soot off the back of the boat and the engines would be under considerably less stress. Our Bayliner 4087 produces no visible smoke when under power and its engines will likely last much longer than the engines powering the boat in the picture.   


Check to see if you are over-propped.  It’s surprisingly common and, if you are, reducing pitch is easy and cheap, will reduce or eliminate transom soot, and your engines will have a much better chance to living a long and healthy life. It’s nice not having to clean the transom at each stop and potential longer engine life is an additional benefit that is hard not to like. Dirona’s engines have just crossed over 3,600 hours and we’re hoping for many more smoke and trouble free hours in the years to come.


James Hamilton


If your comment doesn't show up right away, send us email and we'll dredge it out of the spam filter.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.