[C320-list] Fuel tank not full

[Scott Thompson]

I've heard this argued both ways with no definitive answer. There are 
risks from storing with the tank completely full if you top it off when 
the weather is cold. The fuel expands when temperatures warm, and I've 
seen evidence of this causing fuel to overflow out of the fuel vent on 
boats that are stored for the winter with the fuel filled to the top. A 
bit of room for expansion is a good thing. And when I've asked mechanics 
about this they generally say its best to not start the season with a 
tank of old fuel. Those are the arguments for not topping off. You 
already know the arguments the other way. I've generally not worried 
about it and haven't had any problems with either a mostly empty or 
mostly full tank here in the Annapolis area.

Scott Thompson


On 11/19/2017 8:04 AM, Troy Dunn wrote:
> David-
>
> Not sure why no one has responded.  As I'm sure you know the conventional
> wisdom is that leaving the tank partially filled with a hygroscopic fluid
> like Diesel fuel in the tank leaving room for condensation you may end up
> with water in your fuel, only IF there is in fact condensation, and then
> the condensation is subsequently drawn into the Diesel.  Mainesail did a
> fairly extensive experiment with an empty tank (
> http://www.pbase.com/mainecruising/fuel_tankt_condensation ) but others
> would point out that without a hygroscopic fluid in the tank, perhaps...the
> accumulated moisture was just evaporating once the humidity dropped again.
>    I have not seen a more controlled and well documented experiment, than
> Mainesail's.
>
> My opinion is that as long  as your deck plate isn't leaking water into the
> tank, and you put some sort of fuel conditioner in your tank for the winter
> along with a biocide...you should be fine.    You could always run an
> experiment and let us know your results.   Draw off a pint of fuel every
> month from your Racor filter and see if you get any water in the fuel.
> The water in your fuel tank will sink to the bottom, so it should be the
> first thing that is drawn into the Racor each time you do the experiment.
> You could also check the Racor before draining to see if any water has
> settled there but I think you need to keep drawing off the fuel to get to
> what's in the bottom of the tank.
>
> At the end of your winter, my prediction is that you will find no water in
> the fuel.   It's a total guess, but I'm thinking that generally in most
> areas, winters are dry not humid and without high humidity in the air and
> without the fuel in the tank warming sufficiently for the tank to
> breathe...condensation will be minimal.   Keep in mind that a 20'x20'x10'
> room (4,000 cubic feet) at 100% humidity has roughly 1/2 gallon of water
> content.   You'd need a LOT of air exchange and high humidity to get to
> that much water.
>
> On the flip side...if you can't sleep at night, by a diesel fuel can, fill
> it at your local gas station and top of your tank.
>
> Fair Winds-
>
> Troy Dunn
> S/V Wonky Dog
> Hull#514
>
> On Fri, Nov 17, 2017 at 3:34 PM, David Nolte <dcnolte at mac.com> wrote:
>
>> I always top off my fuel tank heading into the winter. So when we went to
>> do our final pump out our marina had run out of diesel fuel. My 20 gallon
>> tank is 3/4 full. Should I worry about condensation and getting water in
>> the tank? Has anyone had negative consequences in a similar situation?
>>
>> Thanks!
>>
>> David Nolte
>> Beach House 0004