What Should A Diesel Tank Look Like after 40,000 Gallons (170,000 liters)..


Corey McMahon and cohorts have been working on “the list” and this afternoon’s feature was an inspection of the diesel tanks. After 5700 hours of engine time over 6.5 years, and more than 40,000 US gallons (170,000 L) of diesel, sourced all over the world, we were more than a little curious as to what we would find.

The photos above and below are of the forward center (1000 gallons/3850 liters) fuel tank.


The lead photo is the starboard side, the port is above.

Several things are obvious. First, the walls of the tank are totally clean, not a sign of bacteria. Second, there is minimal debris in the bottom.


A close up of the tank sump.  There is perhaps a cupful of junk, a little tar, and no water. If we were giving grades for best kept tanks, this would get an A+.

You might be wondering the features which lead to such a felicitous outcome. Some of these are:

  • A special design venting system that minimizes the amount of humidity in the tanks.
  • Turning the air vent off when the boat is not in use.
  • Not returning hot fuel to the main tank.
  • The use of a large day¬†tanks.
  • Sufficient capacity so we can be selective about where we purchase.
  • A fuel magnet to kill bacteria (controversial and maybe real or maybe not).
  • Periodic fuel polishing.

Tomorrow we shall stir the debris up with a Scotchrbite pad on a stick, and then see if we can suck this through the main filers

Posted by Steve Dashew  (November 18, 2011)

11 Responses to “What Should A Diesel Tank Look Like after 40,000 Gallons (170,000 liters)..”

  1. Jerry S. Says:

    Interesting article. Thank you.

    Could you please describe the humidity limiting vent system? Also, does closing the vent cause a pressure differential problem as ambient air temperature changes?

  2. Steve Dashew Says:

    Hi Jerry:
    Basically, all of the main tanks air vents feed to the day tank. The day tank vent then services the entire system. There are some nuances, and care needs to be taken with the plumbing runs.

  3. Scott M. Barnett Says:

    I stand educated! Thank you.

    Scott M. Barnett

  4. Chris Says:

    Wondering why I never thought of closing the vent lines, love that idea. Add 2 more valves to the shopping list.

  5. Norm Moore Says:

    Steve, I feel.compeled to say this as a great admirer of your designs. Using the day tank as a hub for all the individual fuel tank vents is absolutely brilliant. I hope others realize the advantages of this arrangement. Just had to say that.

  6. Victor Raymond Says:


    Oddly enough my boat does not have a day tank although it seems to have everything else. There certainly is room and could be retro fitted should I decide to go that route. The previous owner did 20 years 70,000 miles without it so one wonders if it is necessary but I like the idea. My question is: Is the day tank placed high enough to gravity feed the engine fuel pump or is there another concept going on here?

    I hope I have the courage to look at mine someday. I am able to pump out sediment and water from the bottom of each tank and like you had very little to show for the effort. Nevertheless, one still is curious what it looks like after all those years. I will have to think about how I will seal of the air vents.

  7. Steve Dashew Says:

    Hello Victor:
    There are two schools of thought. Having the day tank above the fuel inlet reduces load on the engine lift pump, and insures good flow and easy bleeding. On the other hand, a leak with the fuel level high can be a real problem. We keep our day tanks high, but draw out of the top with a pipe to the bottom.

  8. John Says:

    Hi Steve,
    It would also be interesting to see a shot from the water tanks. I have integral aluminium tanks and I am interested to know if corrosion is a problem.
    thanks John

  9. Steve Dashew Says:

    Hi John:
    We’ll have a look and shoot some photos for you.

  10. Bob Says:

    Very interesting article. In looking at your pics, it seems your tanks are fairly tall with angle baffles rather than typical fuel tanks with a large footprint and not very tall. Its difficult to get a 3D view in 2D. What are their dimensions if you don’t mind.
    Your logs are very illuminating about some of the nuances in building boats!
    Thanks for taking the time and effort to put these on your site and answering questions.

  11. Steve Dashew Says:

    Howdy Bob:
    We typically don’t like the structural grid – which forms the tank baffles – to be more than 60cm/two feet on center in the dominant direction (in Wind Horse his is transverse, The FPB64s longitudinally). On both designs the grid in the opposite direction is similar below the tank tops. Depth on the FPB 83 in the photos is about 75cm/30″. The tight baffles/structural system has several major advantages: it is much stronger as the span of the plating is reduced (see Iron Lady after pounding on a reef) and we do not reduce plate thickness accordingly, the motion of liquids within the tanks is suppressed which helps with motion in heavy weather, and it is much quieter. The ability to baffle like this, indeed have a double bottom, is one of the reasons we are so fond of aluminum construction.