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Old 08-16-2019, 11:02 AM   #1
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Tanks Full or Empty during Offseason?

I am curious what others are doing during the offseason - are you keeping your diesel fuel tanks full, empty or partially full? What are you doing and why?
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Old 08-16-2019, 11:23 AM   #2
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Theory says warm moist air in your tank will condense on cold surface leading to water droplets. A full tank won't have any air to hold the moisture. In practice I have never noticed water in the racors during spring commissioning. I typically top off and do a thorough pump out in the fall. I do put biobore in the tank just in case.
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Old 08-16-2019, 11:29 AM   #3
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I am curious what others are doing during the offseason - are you keeping your diesel fuel tanks full, empty or partially full? What are you doing and why?
Mostly empty - did not desire older fuel and never wanted an extra ton and a half of weight when on the hard.
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Old 08-16-2019, 11:29 AM   #4
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I've had several boats over the years of various types and I've tried all the mainstream strategies. I don't feel there's a material difference between full, partially full, and empty. However, all else being equal I'm inclined to fill up before storage just because the fuel seems to be cheaper than in the following spring.
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Old 08-16-2019, 11:36 AM   #5
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I have 2 - 300 gallon steel diesel tanks. I never fill them completely since I only use about 250 to 300 gallons in a year. The tanks are 43 years old.

I leave around 50 gallons in the tank in winter. We use the boat 3 to 4 times during the winter.

I keep the engine room at a constant 50 degrees all winter so no damp air entering tanks.

My fuel tanks have sumps so any water will accumulate there to be pumped out.

I check for water monthly with KolorKut. No water ever found.

I open the inspection ports in the tanks every 5 years for a look and pump any debris out if found. I spray the sides of the tank with diesel with a wand and wash everything towards the sump where it gets pumped out, filtered and sprayed back out of the aimed wand.
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Old 08-16-2019, 12:06 PM   #6
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nearly empty so the amount of old fuel will be minimal in spring. Then I only add as much as I will use soon so that I use up most of the old fuel in the first runs. Also because the fuel at the dock might be old if I am early.

I rarely full tanks unless Ill definitely be using it soon. Some sloshing around keeps stuff in suspension and low levels cause fuel return to increase the filtering rate for a little self polishing.
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Old 08-16-2019, 01:05 PM   #7
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I've had several boats over the years of various types and I've tried all the mainstream strategies. I don't feel there's a material difference between full, partially full, and empty. However, all else being equal I'm inclined to fill up before storage just because the fuel seems to be cheaper than in the following spring.

That is the best reason Iíve ever read for either full or empty.

Of course, it is all academic to me. What is this ďoff seasonĒ of which you speak?
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Old 08-16-2019, 01:06 PM   #8
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The insistence on full tanks comes from gas. That's where you have the condensation issues. I've known many gas tanks to have water in them from being left empty over winter, but I've never known it to be a problem with diesel.
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Old 08-16-2019, 04:32 PM   #9
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I start watching diesel prices in March and when the price is low I buy fuel. There is usually a short period between the winter heating season and the summer vacation driving season where the fuel will be low - usually. Sometimes not.
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Old 08-16-2019, 04:39 PM   #10
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The insistence on full tanks comes from gas. That's where you have the condensation issues. I've known many gas tanks to have water in them from being left empty over winter, but I've never known it to be a problem with diesel.
I agree.

I've been leaving diesel fuel tanks nearly empty during the winter season for over 20 years and have never found water in my tank. I do have the boat in a boathouse where the temperature is not extreme and no wind. That and engine room heat prevents the tanks from breathing in moist air.
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Old 08-17-2019, 09:21 AM   #11
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Usually around 3/4 full. No particular reason, it's just what I do.
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Old 08-17-2019, 11:53 AM   #12
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Usually around 3/4 full. No particular reason, it's just what I do.
We have no offseason, so full.

Now when we lived on a lake in NC and had gas powered engines, we kept full for multiple reasons. First, condensation. Second, finding open marinas with fuel over the winter. Third, several marinas we wouldn't trust in the spring. Smaller marinas would frequently have water in their fuel for the first customers of spring. Not as bad as they once were. When I was a kid, tried to warn customers and speak to marina owners where I knew there were issues. Also, carried additives to try to help those with water in their tanks. 95% of the time it would work.
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Old 08-17-2019, 02:50 PM   #13
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We usually have them full but this year ai am not going to fill them because I am going to replace my port tank so I want it as empty as possible. However there will still be at least 150 gallons that I will have to pump out.
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Old 08-17-2019, 05:24 PM   #14
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CMS did some tests, you will find them on his website. No difference in condensation between empty and full.
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Old 08-17-2019, 06:01 PM   #15
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CMS did some tests, you will find them on his website. No difference in condensation between empty and full.
Who's CMS?

Link to the test?
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Old 08-17-2019, 07:34 PM   #16
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Who's CMS?



Link to the test?

CMS is a member here. CMS is Rod Collins, a ABYC certified marine electrician. He posts on many forums as ďMaine SailĒ. He is an independent contractor who has provided boaters helpful advice for years as well as running his own marine services business in Maine.

I think Rod got tired of answering the same questions repeatedly but he has posted tons of articles on a variety of topics on his website www.marinehowto.com. I believe the article describing his fuel tank tests are on his website.

The website is a great resource. Rod also sells some gear from his website and Iíve found his prices to be very competitive, plus I much prefer to support someone who has given me, and so many others, excellent advice over the years.
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Old 08-17-2019, 08:15 PM   #17
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I agree his website and tests are first class.
My only issue with his winter fuel tank test was it was empty so condensation could evaporate. I'd like to see it repeated w a partial fill of gas or diesel instead of empty. That way any condensation would be covered by the fuel and no / little opportunity to evaporate.
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CMS did some tests, you will find them on his website. No difference in condensation between empty and full.
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Old 08-17-2019, 09:37 PM   #18
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CMS is a member here. CMS is Rod Collins, a ABYC certified marine electrician. He posts on many forums as ďMaine SailĒ. He is an independent contractor who has provided boaters helpful advice for years as well as running his own marine services business in Maine.

I think Rod got tired of answering the same questions repeatedly but he has posted tons of articles on a variety of topics on his website www.marinehowto.com. I believe the article describing his fuel tank tests are on his website.

The website is a great resource. Rod also sells some gear from his website and Iíve found his prices to be very competitive, plus I much prefer to support someone who has given me, and so many others, excellent advice over the years.
Thanks Dave.
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Old 08-17-2019, 09:49 PM   #19
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For the last 4 years we are leaving the tank the way they are for winter, it was almost empty (less than 1/4 left), half empty, or almost full (3/4 full). Never seen any difference. No more or less water in the separator, no difference in fuel quality (or ability to burn), just no difference.
Not sure if keeping some fuel 4 to 6 months in a tank would make a difference. There are many diesel usage where fuel is kept more than 6 months sitting. Maybe some factors may make a difference but for now did not notice any.

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Old 08-17-2019, 10:29 PM   #20
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I donít have enough experience with ďlargeĒ diesel fuel tanks. Iíve only had this boat for 3 1/2 years. Most of the time, my tanks are about 1/2 full. We use the boat all year, but obviously more in the warmer months. It is cool and wet here in the winter and the boat stays in the water all year. So far, Iíve never found any water in my Racors and I run all my fuel through one primary Racor 10 micron filter and a 2 micron secondary filter. My primaries are a darkish grey when they are changed. I donít use any fuel treatment.

On my sailboats, I used even less fuel. On my last sailboat most years I would top off its 50 gallon tank in July. Not a plan, it just seemed to happen that way. Sometimes I would put some Stabil in the tank when I filled it up, but most of the time I would simply forget. Again, never any water or crud in the fuel despite the cold and wet winter months.
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