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Keep fuel tank full?

Nov 16, 2020
C&C 37 San Diego, CA
I was reading the owners manual for my old Yanmar 3HMF and saw the recommendation to refill the fuel tank at the end of each day, with the intent to eliminate condensation on the inside of the tank.

Is this something that I should be concerned with? I usually think about refilling once I hit half full....maybe 6 months or so.

My boat is in San Diego....so fairly mild conditions both summer and winter.

Feb 21, 2013
Hunter 46 Point Richmond, CA
Suggest topping it off when convenient to mitigate condensation and add diesel biocide fuel treatment to mitigate bacterial growth (bugs, algae, bacteria, yeast, molds and fungi) in diesel fuel and prevent their growth that can plug fuel lines and filters. You have 20 gallon fuel capacity. In my case, I top off my diesel tank (100 gallon capacity) every 3 months with typically about 5 gallons of fuel. That has worked well for all my boating life without any fuel issues.

More information on why you should keep your tank topped off: Preventing and removing diesel algae - BoatCoachBob
Last edited:
Sep 25, 2008
Alden 50 Sarasota, Florida
The consensus among the professional crew on some of the big boats around our marina, many of which have tank volume in excess of 1000gallons, is keep the fuel clean and fresh. None keep their tanks full unless they need the range.
Feb 21, 2013
Hunter 46 Point Richmond, CA
Don.......good recommendation but unlike my diesel truck and skiboat turing over my boats diesel tanks, one with 100 gallons at 20 gallons per year and another with 530 gallons at 200 gallons per year, is impossible so I opt to keep the topped off.


Jan 6, 2006
Beneteau 423 Mt. Sinai, NY
Sort of like the anchor question. I keep my tanks as empty as possible until I am going on a trip..... then I fill them. No fuel just sitting there. I have a great water separator if water gets in, but have never had a need to drain any from my racor. My personal experience only here in the ne for the past 25 years.

Location and tank material make a difference.

Good Luck!


Aug 24, 2005
Nassau 34 Olcott, NY
The only time I worry about filling my tank is when cruising or for winter storage. No issues with this boat for over 15 years not keeping it full during the season.
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Mar 26, 2011
Corsair F-24 MK I Deale, MD
Topics never to be discussed: politics religion and condensation in fuel tanks. Some say it's impossible for the air in the average tank to contain enough water to do any harm.

The problem with this link is that he does not actually understand the math or the phenomenon. Some is correct, a few things he misses.

Example. One of the first things a large diesel tank inspector is trained to do, before entering a tank, is to check the roof for serious internal corrosion and unstable roof rafters. The cause is condensation, which is COMMON. I've seen a LOT of internal roof corrosion in tanks.

But yes, this is an interesting debate. Condensation vs. new fuel. The obvious answer is an absorbant vent filter, if you are going to worry about it. They do work.
Mar 26, 2011
Corsair F-24 MK I Deale, MD
Size matters.
Perhaps that was a joke.

No, in this math it does not, really. I have also inspected a few marine tanks, same problem. I've see condensation inside downward sloping fuel hoses. That fact that it is in the bilge of a boat does help. The fact that boats are within a foot of the water all the time (high humidity) makes it worse.

One obvious thing that is missing in the math is that once a drop of condensation falls, it is trapped under the diesel and cannot escape. It's a one-way trip. Another thing he misses is that the steel tank roof and upper walls can be considerably cooler than the air and are not buffered by the temperature of the fuel.
Jul 7, 2004
Hunter 30T Cheney, KS
I never fill up my tank all the way. Besides adding stabilizer and Biocide, I added an H2Out fuel vent desiccant dryer. I can see that it works. Over time the (reusable) crystals turn pink.
Oct 22, 2014
CAL 35 Cruiser moored EVERETT WA
Somethings you can control and some things just are.

Water in diesel is only something I can possibly influence, cope with, and attempt to treat.

I have a good filter. I only fill the tanks (2, 85 gallons each) if I am going on a long trip. I flush some fresh fuel into the tanks along with biocides as identified in Practical Sailor magazine. I carry replacement filters.

The challenge is your fuel source can be everything from wonderful pristine to disastrous. Look for fuel in places that pump a lot of fuel too get the freshest and hopefully with the least amount of dissolved water.

Change your O rings on your intake pipes regularly.

When heading out on the water into a lot of wave chop, expect any crud in your tanks to be stirred up and sucked into the fuel lines.

Be ready to shut down if your engine suddenly races into high RPM. You could be experiencing fuel starvation. A quick shut down can save you from having to prime your engine's now emptying fuel lines.
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Dec 28, 2015
Laser, Hunter H30 Cherubini Tacoma
Keeping your tank full to decrease the potential of water condensation is the most correct tactic but life and logistics doesn’t make this a hard fast rule. If the tank is poly then the need is even less
Oct 26, 2008
Catalina 320 Barnegat, NJ
Short answer ... I don't know anybody who tops off their tank at the end of each day sailing. Most people fill up only when they need to. MIld, stable temperatures, mostly dry humidity ... I can't think of any reason why you should be concerned about condensation in your fuel tank. Do you have a clear bowl with your primary fuel filter? Have you ever seen water in the bowl?
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