Question: Over the winter while cruising the boat shows I noticed a system at one of the booths that interested me. The photo I sent in shows the system, a diesel fuel polisher. I’ve always wondered if this is a gimmick or if there is merit to a set-up like this.

Fuel polishing system

Fuel-polishing systems aren't cheap, but they can make sense depending on your circumstances.

My boat has two medium-sized diesel engines and spends more time at the dock than underway, so I’m a little concerned about the diesel fuel in my tanks getting old and stale since it spends quite a bit of time just sitting there. What’s your take on this?

Answer: This is a good question that I think will have a slightly different answer for different people, depending upon how they use their boat.

Diesel fuel is quite different from gasoline in that it can actually support microbial growth. It can happen fairly quickly too. I’ve done some of my own experiments with this and actually seen the microbes begin to appear in the fuel after about a six-week period of time. One of the keys to success with diesel fuel is keeping it fresh, so if you are buying your fuel from a source that is constantly being replenished with fresh fuel, and it is being filtered before it ends up in your tank, you are probably getting good fuel. The problem in your case is that you are letting the fuel sit around in your tanks.

The bottom line in your case is that your use habits are not conducive to keeping the fuel in top condition. Condensation and quite possibly microbial growth are real possibilities in your case. Boats that get used frequently typically have fewer problems as long as their supply source is reliable.

Fuel polishing systems are not cheap, so you really need to do a little cost-benefit analysis. Systems I’ve checked out range in price from a low of about $520 to around $6,500 depending upon system sophistication and fuel volume requirements, and those prices do not include installation costs.

Keep in mind that as long as your boat is already equipped with good primary and secondary fuel filter/water separators and you use a biocide additive when you fill up the tank(s), the worst-case scenario if you don’t add a polishing system is the possibility of plugged fuel filters and the engines shutting down. No walk in the park on a dark and stormy night, but no permanent damage done. If the thought of going through that experience seems a bit frightening, I do recommend investing in a polishing system.

Written by: Ed Sherman
Ed Sherman is a regular contributor to, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.