DayTank » Page 1 of 5

November 22, 2006

A few years ago the fuel system on our 1979 44’ trawler, Francesca, suffered with a tiny air leak. It was so small that problems only arose after many hours (50 or more) of operation and when rolly seas would cause accumulated air bubbles to dislodge somewhere in the system. At such times the engine would stumble for a minute or two, but eventually recover. It was something that would set your heart a pitter when the timing coincided with critical maneuvering. All attempts to fix the problem were unsuccessful; the periodicity was too long and unpredictable. It was a nagging reliability issue that, along with other fuel system design deficiencies, needed to be sorted out with certainty. Considering our cruising itinerary it was doubly important.

Beginning in 1998 we have done some serious cruising with Francesca including 5 trips to Central America. In 2003, with our planned and much anticipated Amazon River Trip soon to begin, I set about to improve the fuel system for greater reliability, fuel use calibration and to reduce the number and types of spare fuel filters required. The project was not an easy one, but at the end of the day it was well worth it.

Let’s begin with a general discussion about fuel systems and the merits of a day tank. On Francesca we have twin Lehman’s (120hp). Just after buying the boat, we added two Racor® 900 series filters, but left the primary and secondary filters mounted on the engines as originally installed. More precisely, each engine had one Racor® and two engine mounted filters. One can argue about whether that was a good configuration or not, but let’s skip it because it’s old hat.

Regardless of how the filters were plumbed, the system had many faults. First, there were too many different filters to maintain in the spares kit. Second, the engine-mounted filters were designed by an evil mind. Changing them out was difficult, a mess, required air purging and one was never certain if they were truly purged and leak free. If it became necessary to change one of these filters at sea, you really had your hands full. Keep in mind the potential for fowled fuel filters is highest when rolly seas stir up whatever goo there may be in the bottom of the fuel tanks. A third issue was that changing the Racor’s also had the potential of entraining air in the fuel lines. As a general statement, it was a reliability nightmare.

There was yet a fourth concern associated primarily with the Lehman engines. Most diesel engines pump copious quantities of fuel to the injector pump and return the unused larger quantity back to the fuel tank(s). Over time this has the advantage of cleaning up fuel tanks and