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Question: Does an empty marine fuel tank condensate when left empty and accumulate water?


For many years, likely exceeding 30, I have always drained our boats fuel tank each winter and re-filled it in the spring with fresh fuel. I would simply burn the old fuel, or what was left, in our homes oil fired boiler. The fuel never went to waste and it never got a chance to absorb moisture by sitting in the tank over the winter. Storing the boat empty also gives me an opportunity to see the inside of the tank, check the bottom for debris and look for any signs of pitting or corrosion. I can also effectively clean it as needed. In all those years I've not once accumulated any water in the empty fuel tank over the winter.


So what prompted this experiment? This statement:


"An empty tank will condensate and fill with water, a full tank is far better than leaving it empty."


Seeing as I already knew this to be untrue, as related to the marine fuel tanks on the boats I have physically owned, I figured I would spot check it and see if there was even a shred of truth to the tank accumulating water in an empty tank theory so factually stated in the quote above..


FACT: Even the fuel in a full tank can reach a "saturation" point where it can absorb water into the fuel. With no fuel, this can't happen because there is no fuel in the tank to absorb any water..


I know it is not realistic for most boaters to empty a tank in the winter, but for me it was a no brainer and easy. I now have an H2Out vent filter and Racor Lifeguard LG100 installed to keep my fuel contents drier when the tank does have fuel in it, but my #1 preference is to 100% drain the tank each winter. At the behest of H2Out and Practical Sailor magazine I purposely left my tank with fuel in it (partial fill) for two winters. I never noted any visually quantifiable change in color of the H2Out beads. I have now gone back to an empty tank each winter even with the H2Out filter.


Not being an expert in this field, I did as I usually do, I created a "real world" experiment to see what happens. What I used for the experiment.


#1 An empty, clean and dry 20 gallon aluminum marine fuel tank


#2 All ports plugged except for 5/8" vent line


#3 Hole drilled in my barn to outside for vent


#4 Environment where temp changes rather dramatically = second story of barn



The barn certainly sees temp swings. The second story is uninsulated and has a black roof. In the winter I often heat the barn by 60 or more degrees in just a few hours and in the summer temps on the second floor can exceed 120F and nights might be 45-50F. The swings are actually far wider than that of our boat seen right outside the window, and this is due to the black asphalt roof.



In order to make this as real world as possible I used a real boats fuel tank, a standard 5/8" vent hose and vented it to the exterior. Forget jars or simulated tanks, when you have a real tank, why not use it. I also placed it as close to our actual boat as I could.


Over the last 14 months the low temp was -17F and the highest recorded temp was 131F. The barn has no insulation on the second floor thus the 131F temp.


Our boat (when stored), barn and home are approx 100' from Casco Bay/ the Atlantic Ocean so the humidity here is pretty intense.