Q: I've just purchased my first diesel-powered boat. The engine has a filter on it, but there's no primary filter. I plan to install one. How do I choose the filter element for each one? I know how important clean fuel is to my engine.
- Tom Stutz, Milwaukee, WI

A: Filters are ranked by the size of the particle they can pass. A micron is a millionth of a meter. A lot of people put the finest filter (under 10 microns) first. The idea is to trap as much of the crud as early as possible. However, putting the finest filter first means that it plugs quickly because everything big or small gets stopped there. As a result, the second filter has nothing to do.

Filter manufacturers recommend installing a 10- to 30-micron primary filter. This stops the big stuff. Then, install a 2- to 7-micron secondary filter on the engine to get the rest. This setup allows both filters to share the load so that each lasts as long as possible in service.

Be sure to carry spare primary and secondary filters in your spare parts kit. Otherwise, you may be stuck somewhere waiting for the right filter. Learn how to change both filters, as well as prime and purge air out of the fuel system. That way, a load of bad fuel won't leave you stranded.

Send your BoatCraft Q&A questions to Dave Brown c/o Boating World , 2100 Powers Ferry Road, Atlanta, GA 30339. You can also e-mail your questions to editor@boatingworld.com. Due to the volume of inquiries, Dave Brown will be unable to answer every question.