Hints and Tips



Fuel Tank Dredging and Fuel Polishing

© Karl D. Lahm, April 2010, 2011


Late in the 2008 season, I left my marina to take my SO34.2 on a Sunday afternoon sail with three passengers, one an infant. The waves had heaped up from high winds the day before, though the winds had dropped into the teens by the time we got underway. As the boat was raised and lowered by the short-interval Lake Michigan swells leaving the harbor entrance, her engine lost and regained RPMs. A week later, we attempted to take her down to the yard for winter lay-up, but were forced to limp back to port when the engine couldn’t hold 2000 RPM and barely kept running at 1200.


Of course, both her primary and secondary fuel filters were clogged. They were replaced and she made it to the yard the following weekend with no issues. Two years ago, I decided that a fuel polishing was in order. My initial attempts to do this were almost comical and largely unsuccessful. We suffered another fuel-starvation incident later that summer. So, after a fair bit of grief and mis-steps, a scheme for dredging the sludge and debris from the bottom of the fuel tank and “polishing” the fuel has been developed. This involves under US $200 of pieces and parts, most of which can be reused every two years or so.


Here’s the general scheme:


a) Intake fuel into the primary filter using a piece of externally-reinforced plastic tubing that can be directed into the lowest corner and other readily-accessible locations of interest inside the fuel tank.


b) Temporarily install a filter element in the primary position that has the porosity of the secondary filter.


c) Pull fuel out of the primary filter using a low-velocity pump and direct it back to the fuel tank filler port astern.


When the “wand” created by the tubing and its reinforcement is brought along the bottom front and side corners of the fuel tank, accumulated debris will be pulled toward the cleaning filter. This may cause tubing clogs, which must be dealt with promptly. Once one is confident that all significant larger particulate debris and bottom-sludge has been extracted, overall “polishing” of the fuel in the tank can be performed using the “wand” or the normal fuel intake pipe.


A. Primary Fuel Filter

Two years ago, I replaced the OEM primary filter with a Racor 230R2 series filter system and relocated it into the prop shaft space where it was more easily accessed for sludge drainage and element replacement. Though normally equipped with a 30 micron element, 10- and 2- micron elements are available for this filter. During the dredging/polishing process, one of the latter elements is highly recommended. Additionally, the normal intake and outlet hoses are removed and plastic hose barb fittings are substituted.


B. Pump System A

Jabsco #17860-0012-3 flat tank oil changer system is used to pump fuel through the filter and direct it back into the fuel tank. The pump itself features 3/8” inlet and outlet quick-connectors that directly mate with 3/8” plastic tubing. The tubing reducer at the pump inlet and the 90 deg elbow into the oil changer tank are temporarily removed. This pump pulls fuel oil through at a reasonable pace, far faster than ordinary engine consumption.


C. Interconnecting Tubing

The scheme was implemented using ordinary 3/8” OD plastic tubing, which is readily available in the US and metric approximations (10 cm) are undoubtedly available worldwide. This tubing is not softened by the fuel, has a bend radius small enough, and is inexpensive. The downside is that frequent disassembly of tubing/hose barb junctions to remove particles will necessitate cutting off the tubing end, as it tends to crack/split longitudinally after the second removal/reattachment cycle.


D. Putting it Together

The following photo illustrates the general connection scheme, with the aft berth cushions removed (my boat is the “owners version” with one aft berth).

The “wand tube” is shown running from the yellow funnel at the lower left-hand corner of the picture, dropping down into the prop shaft area, and connecting into the Racor filter. A short tube connects the filter outlet to the pump inlet, with the last piece of tubing going from the pump outlet up through a port, across the cockpit, and into the fuel filler port.


The “reinforced tubing wand” is illustrated below. I simply doubled some coathanger wire and used nylon cable ties to secure it to the tubing, thereby taking the curl out of the latter and making the whole thing bendable to a specific shape. Others might prefer more substantial materials, such as steel or aluminum rods, etc.


The fuel level indicator float assembly is removed from the tank and its port used to gain access for the tubing wand. See the picture below.


The tubing from the pump outlet to the fuel tank filler port will carry vibrations from the pump and must be held in the filler port, lest it vibrate itself out onto the deck and cause an expensive fuel spill. This happened to me, though “on the hard” with absorptive crushed rock below the boat and not for very long, thereby keeping the cleanup cost minimal. In the slip, this would have caused harbor cleanup costs and fines. I simply placed a winch handle over the tubing to keep it in place just prior to the filler port, not the most sophisticated method, but it works. You do not need to “prime” the Racor filter using its built-in pump mechanism. The Jabsco pump will suck fuel into the filter and, once that’s full, send it on to the tank.


E. Operation

The initial scheme is to “dredge” the corners of the tank accessible from the level float port. If your tank contains large particulate matter, as mine did, such granules will get stuck either at the tubing wand inlet end or at the inlet to the filter, at the plastic hose barb. In either case, one must shut off the pump and then drain the fuel in the tubing into a plastic bowl. The debris may also fall into the bowl. More likely, you will need to shake or pry it out of the tubing after the fuel is drained. Do be aware that more than one tubing removal-restoration cycle at the hose barb will likely cause a longitudinal split in the tubing, which, in turn, necessitates cutting the last quarter-inch (6 mm) off the end.


Once you are sure that all of the substantial debris in the tank’s lowest corners has been dredged up, it’s a good idea (and a necessity) to drain the Racor filter to remove debris blocked by it.


Next, you can proceed to fuel polishing. What I did was to leave the tubing wand in the forward port corner of the tank, its lowest point, and just let the pump run for two or so hours while attending to other maintenance chores. You should ensure that the pump outlet tubing remains in the fuel tank filler inlet at all times, with checks not less than every few minutes, as pump vibrations can carry down the tubing and shake it out of the filler inlet. A heavy winch handle helped keep the tubing in its place. Some might prefer to run the pump all day.


The picture below shows the crud dredged and polished out of the system, the Racor filter bowl (after drainage) to the left and the plastic bowl into which the granular and other crud was drained, in the plastic cup to the right.


© Karl D. Lahm, April 2010, 2011