Does an empty marine fuel tank condensate/fill with water when left empty?

Image: On 5/31/2013 This tank was installed on the second floor of my barn with a vent to the outside. The vent was installed just feet from the fuel tank vent on my own vessel as can bee seen out the barn window.

For many years, likely exceeding 37, 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 from an internet boating forum member:

An empty tank will condensate and fill with water! A full fuel tank is far better than leaving it empty.

Having been doing this for years, I already knew this statement to be untrue, as related to the marine fuel tanks on the boats. I figured I would just set up an experiment to see if there was even a shred of truth to the accumulating water in an empty marine tank theory, that was 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 inside the tank, this can’t happen, because there is no fuel in the tank to absorb any moisture/water.

Is an Empty Tank Even Realistic?

I know it’s not very realistic for most boaters to empty a tank in the winter. For me, with a sailboat, I also did this with my commercial lobster boats, it was a no brainer and is actually quite easy. For many boat owners this will require a fuel pump and pick up that can reach the lowest part of the tank as well as a place to use or get rid of the fuel that came out. The oil fill fitting for our homes heating system is just feet from our stored boat and is able to be pumped directly from the boat into the 275 gallon oil tank, so for me this is a very easy task.

At the behest of one of the investigators at Practical Sailor Magazine, who were testing the H2Out vent line filters, 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.

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.  Still, for me, my #1 preference is to 100% drain the tank each winter.

Not considering myself 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 this experiment:

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

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

#3 Hole drilled in my barn to outside for the vent line

#4 Environment where temp changes rather dramatically = Second story of the barn with a black asphalt roof

The barn certainly sees temp swings. The second story is not insulated and has a black asphalt 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, is approx 100′ from Casco Bay/ the Atlantic Ocean so the humidity here is pretty intense.


Tank Vent on outside of Barn:

Yes, the tank is still sitting there in the barn. It’s been over 8 years since I set this up. I’ve randomly spot-checked the tank on 7 separate occasions since setting this experiment up. To check it I remove the fuel sender and send some colored paper, on a stiff but bendable wire, into the tank. I rub all the walls and tank floor/ceiling areas that I can get to. As of yet, every single attempt at finding condensation has come up bone dry.

Does an empty marine fuel tank condensate…..?? This one doesn’t, at least for the first eight years. (grin)

Spot Checks:

EDIT 9/13/2013: Checked tank for the first time: Results = Bone Dry

EDIT 5/18/14: About two months ago I set it on some 2″ thick stone pavers as some surmised that the tank needs to remain cool as it would if in the belly of a boat at ocean temp. Up here in Maine, when hauled out for the winter, there is no “ocean temp” but I placed it on the “slower to change temperature” stone pavers anyway to satisfy a few doubting boat forum members. Results = BONE DRY….

EDIT 6/25/14: Just checked it again, after setting it on the pavers. Not even a hint of moisture. I chose to check it yesterday because humidity levels have been in the 90%+ range and night time temps have been dipping to the high 40’s 47F – 49F and day time barn temps have been reaching 118F! That is approximately a 70F swing with 80-90% humidity. Results = Bone Dry

EDIT 10/22/15: Topic just came up on one of the sailing forums and it prompted me to go check the tank again. Just finished a very hot summer and now we are seeing temps into the 20’s at night and 60’s during the day. Tank interior is still 100% bone dry. Considering the tank has been here since March 31, 2013 and today is October 22, 2015 I think it is safe to say an empty sailboat fuel tank will not “magically” fill with water. Results = Bone Dry

EDIT 7/24/17: Cleaning the barn and noticed the tank. Figured I would do another spot check. Results = Bone Dry

EDIT 6/3/2019: Looking for parts, saw the tank, and gave it a spot check. Results = Bone Dry

EDIT 5/27/2021: Believe it or not, this tank is still here and has been since 5/31/2013 or just over 8 years! Results = Bone Dry

I suspect 8+ years is a pretty solid data-set for illustrating that an empty marine fuel tank does not magically make its own water. This is why I personally prefer to leave my fuel tank empty, even while some owners prefer full.

Only you can decide what works best for your boat…

Like What You Saw Or Read?
Would you like to see more articles like this? Is so feel free to donate, support the site and keep it growing.
Please DO NOT feel obligated at all. If you like it and want to make a small donation than that’s all I ask.
Your donations help keep the content coming and also help keep it free.

Click the DONATE button below if you would like to make a donation via PayPal.

Donate To Marine How To!