[C320-list] Fuel tank not full

[Jeff Hare]

I used to make sure the tank was topped up.  I try to make sure it's close to 3/4 full or so.  I've seen a number of boats over the years that fill their tanks full in the fall trying to avoid every bit of moisture, only to have them start leaking diesel fuel in the spring when it warms up and the system expands a bit.  Pretty much happens each spring around here that some boat or another is getting a nastygram from the yard master saying their overflow is dripping.

With that said, I haven't had any difficulty with water in the fuel since I've owned the boat.
-Jeff Hare
#809


-----Original Message-----
From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf Of Doug Treff
Sent: Monday, November 20, 2017 7:45 AM
To: c320-list at lists.catalina320.com
Subject: Re: [C320-list] Fuel tank not full

Troy is correct that the amount of water is small, however it varies by temperature, so the calculations are a little more difficult. The other issue is that the tank will "breathe" throughout the winter as the temperature of the environment changes. The fuel and the air inside the tank will expand and contract, causing small amounts of air in the tank to exchange with outside air. Any water that "precipitates" into the fuel will sink to the bottom of the tank and will be trapped below the fuel, and not able to evaporate back into the environment. This is what people worry about. But I feel that's overblown, and not really an issue if you plan on using your boat every year. Especially with diesel. I think we've become hyper-sensitive to water in the fuel ever since the gasoline using boaters started having more problems when E10 fuel was introduced.

IMHO, and from my personal experience in the Chesapeake bay area, 3/4 tank is nothing to worry about. I've never had a problem with that amount of air in the tank. In fact, I had that same amount of fuel in my tank last winter and there were no issues in the spring. 

3/4 full in a 20 gallon tank means 5 gallons of air which is only about
.66 cubic feet of air. Also remember that the moisture holding capacity of air at 100 degrees F (38 deg. C) is 10 times the moisture holding capacity of air at 30 degrees F (~ 0 deg. C). This is a very important observation. It means that as long as your fuel is clean and does not contain much water in the fall, the amount of water vapor that could be introduced into the tank from the outside air after the seasonal temperature drops is 8-10 times less than the amount of water in the summer. Nobody bats an eye at the tank sitting 1/4 full in July when the temperature swings between 75 and 90 degrees on a daily basis, at 70-80% humidity.

Troy brings up an interesting point. I wonder how many people with water in their fuel are blaming condensation when it's really a leaky deck fill plate. 

I don't think there will be an issue.  Make sure your deck plate doesn't leak, treat the fuel, and sleep well this winter.

--
Doug Treff
September Song #350 - 1996 model
doug at treff.us

On Sun, Nov 19, 2017, at 08:04 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