full tank vs. empty tank for storage

109jb

Lieutenant Commander
Joined
Jul 15, 2008
Messages
1,590
Re: full tank vs. empty tank for storage

My opinion (Worth exactly what you are paying for it) is that storing empty is not at all practical for the vast majority of boat owners and here is my reasoning.

1. it is nearly impossible to totally empty a fuel tank, so for most we would have to assume a small quantity of fuel remaining. I'll call this "nearly emptied"

2. Condensation can occur in the tank and if it is "neraly emptied" there is a large volume of air and also a large surface area of tank walls to promote the condensation. The larger volume and area mean that MORE condensation can collect in the tank.

3. Since the small quantity of fuel remaining in the tank would take less water to cause phase separation you are actually increasing the likelyhood of getting phase separation because of the greater quantity of condensation that can occur too. It takes more water to cause phase separation of 20 gallons of fuel than 2 quarts of fuel.

I personally store full with stabilized fuel in everything I own including boats, ATV's, Motorcycles, snowmobiles, lawn mowers, weed whackers, etc. I also don't drain and replace the fuel when a season starts and I have never had a problem with stale fuel.
 

45Auto

Commander
Joined
May 31, 2002
Messages
2,842
Re: full tank vs. empty tank for storage

Dang Phil, usually when I post something like post 36 above I never bother to look at the thread again cause no one ever pays any attention to them or replies to them! Now you?ve been all gracious and everything, and I feel guilty and have to back my crap up! :rolleyes:

Pascoe has a mixture of gallons, meters, grams and ounces. Easiest thing to do to start is convert his 200 gallon tank to cubic feet. Since there are 7.48 gallons in 1 cubic foot, a 200 gallon tank = 200 / 7.48 = 26.78 cubic feet.

He claims this 26.78 cu_ft tank contains .81oz of water at 86F. Ok, I can live with that. We?ll determine what he used for Relative Humidity from the above two pieces of info, then we can compute the Dew Point. The Dew Point is the temp at which condensation starts taking place.

Since there are 16 oz in a lb, .81 oz of water equals .81/16 = .0506 lbs of water in the tank.

Next we need to know how many pounds of air are in the tank.

The air density calculator here will tell us how much a cubic foot of air weighs at the 86F temp:

http://www.denysschen.com/catalogue/density.asp

86Fdens.jpg


Since it weighs .0723 lb/cu_ft, and our tank is 26.78 cu_ft, that means we have .0723 x 26.78 = 1.933 lbs of air in the tank.

The Humidity Ratio is the amount of water in each pound of air. To find that, we divide the weight of water in the tank by the weight of the air: .0506 / 1.933 = .02618.

Now we can find the Relative Humidity he assumed, which will give us the Dew Point also. Easiest way is to adjust the Relative Humidity number in the psychometric calculator below till the Humidity Ratio equals the .02618 we calculated above. The only numbers I entered were the dry bulb temp of 86F, and then I played with the Relative Humidity entry until the Humidity Ratio equaled .02618 as we calculated above. Everything else was calculated by the psychrometric calculator webpage.

http://www.sugartech.co.za/psychro/index.php

initialhumidity.jpg


What this means is that the ONLY way possible to have .81 oz of water in a 200 gallon tank at 86F is to have a Relative Humidity of 96.4%. Nothing wrong with that, he's free to pick any starting point he likes.

Now comes the interesting part. ?Dew Point? is the temperature at which condensation will start forming. Notice from the psychrometric calculator webpage what the Dew Point is at 86F and 94.6% relative humidity? It?s 84.84 degrees! This means that under his conditions of temperature and humidity, condensation will start forming on a surface that is 1.16 degrees colder than the air temperature! Hardly what I would call a ?much colder? surface!

Next, he states that at 50F the tank would contain .46 oz of water.

It?s real easy to do the same calcs as before. .46 / 16 = .0287 lb of water.

Air density at 50F is .0774 lb/cu_ft

50Fdens.jpg


So the tank has .0774 x 26.78 = 2.073 pounds of air in it.

.0287 / 2.073 = .0138 humidity ratio.

Now go to the psychrometric calculator, put in a dry bulb temp of 50F, and adjust the Relative Humidity until you get a Humidity Ratio of .0138.

50Fhum.jpg


As you can see, it would require a relative humidity of 180% to meet his condition that at 50F a 200 gallon tank contains .46 oz of water. Ain?t no way in this universe. Hopefully everyone here is aware that 100% relative humidity means that the air is completely saturated and can hold no more water. It?s possible to get briefly above 100% in a supersaturated state, but I very seriously doubt if many boaters have a Wilson Cloud Chamber and do nuclear physics experiments in their fuel tank ?.

From
http://en.wikipedia.org/wiki/Relative_humidity

The statement that relative humidity (RH%) can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function.

To cover the points from post 36:

1) His stated condition of .46 oz of water at 50F in a 200 gallon tank is impossible.

2) He states that a surface must be ?much colder than air? for condensation to form at 86F with .81 oz of water in his 200 gallon tank. Reality is that he?s at a Relative Humidity of 96.4%, and it requires a temperature differential of 1.16F degrees for condensation to form under those conditions!

3) He claims that aluminum is second only to copper for heat transfer properties. Several things are better than copper for rapid heat transfer. At least he?s right in that aluminum is below copper. Graphite is 1950 Wm/k (we use it in rocket nozzles because it conducts the heat away so fast it doesn?t melt), diamond is 895, silver is 429, copper is 401, aluminum is 237.

http://en.wikipedia.org/wiki/List_of_thermal_conductivities

4) According to him, tank sweating is either ?rare? or ?likely to occur with boats in very cold waters when warm days are encountered?. All it takes is Relative Humidity and Temperature to tell when sweating will occur. At a high Relative Humidity, it takes a very small temperature drop for condensation to occur. Note: This is why it rains a lot in areas with high humidity. Raindrops require a microscopic piece of dust, smoke, etc to condense around (nuclei). A small drop in atmospheric temperature is enough to cause this condensation to begin, so it rains.

In summary, Pascoe calculates nothing but the amount of moisture in a closed system (he even gets that wrong 50% of the time).

A fuel tank is NOT a closed system. As simply as possible, to calculate the amount of condensation in a fuel tank would require determining the condensation rate by estimating the heat flow into the tank both from the atmosphere and from the condensation itself (btu/hr), then use the Enthalpy (btu/lb) from the psychrometric charts to determine the amount of water that is pulled out of the air in a given amount of time. That would give you lb/hr of water into the tank from condensation. You would also have to determine how much flow you have into the tank through the vent maintaining atmospheric equilibrium.

Good luck with that. I know some very smart people who have been working on it for over 25 years. It gets very critical at cryogenic (-238F) temperatures, and there?s a whole lot of stuff that goes into it:

http://wins.engr.wisc.edu/teaching/mpfBook/chapter9/node2.html
 

Mark42

Fleet Admiral
Joined
Oct 8, 2003
Messages
9,334
Re: full tank vs. empty tank for storage

My opinion (Worth exactly what you are paying for it) is that storing empty is not at all practical for the vast majority of boat owners and here is my reasoning.

1. it is nearly impossible to totally empty a fuel tank, so for most we would have to assume a small quantity of fuel remaining. I'll call this "nearly emptied"

2. Condensation can occur in the tank and if it is "neraly emptied" there is a large volume of air and also a large surface area of tank walls to promote the condensation. The larger volume and area mean that MORE condensation can collect in the tank.

3. Since the small quantity of fuel remaining in the tank would take less water to cause phase separation you are actually increasing the likelyhood of getting phase separation because of the greater quantity of condensation that can occur too. It takes more water to cause phase separation of 20 gallons of fuel than 2 quarts of fuel.

I personally store full with stabilized fuel in everything I own including boats, ATV's, Motorcycles, snowmobiles, lawn mowers, weed whackers, etc. I also don't drain and replace the fuel when a season starts and I have never had a problem with stale fuel.

I do respect your opinion, but I want to comment that (my observation over the years on this forum) for most members of iboats, their boats are trailered, probably have less tan 40 gallon tanks, and siphoning these boats dry is not an issue. With the boat at an incline, the hose is easy to snake to the rear of the tank and siphon all buy maybe a quart or two. I would bet the reason my tanks are dry as a bone in the spring is probably because that small amount of fuel just evaporates. Maybe along with any condensation that is happening, and that is why I never have water in my tanks come spring??? Think that is plausible or is there another reason there is never any water in my tanks come spring? Maybe I'm just lucky?

But I do see your point of view, especially relating to big boats with a couple of 100 gallon saddle tanks or a 120 gallon belly tank. They could be a little more difficult to siphon because all that gas has to go somewhere and most people don't have storage for that much gas. Besides, storing it in the boat, or storing it in a shed, what's the difference?

Being a small boat guy, I don't know the issues that big boat owners have, but I can tell you that small boats are easy to empty the tanks.

I love these issues/topics that never go away....
 

WIMUSKY

Moderator
Staff member
Joined
Sep 26, 2009
Messages
20,087
Re: full tank vs. empty tank for storage

After 40+ posts, the OP should have a good idea on what to do. Either that, or, now he's totally confused. Especially after post #42, no offense...........

beating_a_dead_horse.jpg
 

109jb

Lieutenant Commander
Joined
Jul 15, 2008
Messages
1,590
Re: full tank vs. empty tank for storage

I do respect your opinion, but I want to comment that (my observation over the years on this forum) for most members of iboats, their boats are trailered, probably have less tan 40 gallon tanks, and siphoning these boats dry is not an issue. With the boat at an incline, the hose is easy to snake to the rear of the tank and siphon all buy maybe a quart or two. I would bet the reason my tanks are dry as a bone in the spring is probably because that small amount of fuel just evaporates. Maybe along with any condensation that is happening, and that is why I never have water in my tanks come spring??? Think that is plausible or is there another reason there is never any water in my tanks come spring? Maybe I'm just lucky?

But I do see your point of view, especially relating to big boats with a couple of 100 gallon saddle tanks or a 120 gallon belly tank. They could be a little more difficult to siphon because all that gas has to go somewhere and most people don't have storage for that much gas. Besides, storing it in the boat, or storing it in a shed, what's the difference?

Being a small boat guy, I don't know the issues that big boat owners have, but I can tell you that small boats are easy to empty the tanks.

I love these issues/topics that never go away....

And I too respect your opinion. If it works for you then I am all for it. For me I don't drain. As I mentioned earlier I had the tank out of my boat when I re-decked it and even out of the boat I couldn't get all the water out. BTW, my boat is trailered and only has a 20 gallon or so tank. Now I will say that I didn't even take the fuel sender off when trying to get the fuel out. I would estimate there was about a quart or two still in there during the rebuild. If you do take the sender off you may be able to get most of the fuel out with a siphon hose but it is more work than I am willing to do. Especially considering the number of gasoline powered things I have around.
 

dockwrecker

Lieutenant
Joined
Mar 10, 2006
Messages
1,392
Re: full tank vs. empty tank for storage

After reading all of this, I feel like the horse!
 

Ike-110722

Chief Petty Officer
Joined
Sep 3, 2007
Messages
408
Re: full tank vs. empty tank for storage

OMG Animal cruelty. I'll have to report that to the ASPCA or Peta or somebody! LOL
 

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
8,282
Re: full tank vs. empty tank for storage

I ALWAYS keep my tank full. I fill it up at the gas station on the way home. Starting in late September I also add Stabil every time when I get home. That way if the weather turns early or I get sent out of town on business and the boat doesn't go out again before winter, all I have left to do to winterize is fog, change the oil, etc. My boat is always in the garage covered when not in use. I also do routine tuneups and the like even though the boat may be running perfect.

On the other hand......my buddy keeps his boat outside all year. Always some water in the bilge. Puts a $1.99 KMart blue tarp on over winter and by May its all chewed up and leaking. His tank is always empty. He only puts in the gas he needs for the day. He DOES use stabil, but only at the end of the season and just pours some in. If he does change a sparkplug, its probably because he had to be towed in.

I have 100% success with my procedures. Never been towed, never had a failure because of gas and never saw water in my tank.

My buddy has also had good success. His boat runs every time. Starts up fine in the spring. Two years ago he didn't use the boat that summer and did no special treatment, and it started up fine after 2 years. He says all this hype is just myth.

I'm not making any conclusions or offering any suggestions. Just stating some experiences from a snow bound winter climate. Maybe he's running on borrowed time. I guess time will tell.
 

191Seanymphstriper

Petty Officer 1st Class
Joined
Aug 23, 2010
Messages
265
Re: full tank vs. empty tank for storage

Some storage or marinas etc ask that the fuel tanks are empty because they are going on hangers high up and do to wieght and leaks etc on other vessels they say empty....


The fact is the tank should be stored 95% full with stabil added and thats it! End of story
 

rbh

Fleet Admiral
Joined
Mar 21, 2009
Messages
7,939
Re: full tank vs. empty tank for storage

Some storage or marinas etc ask that the fuel tanks are empty because they are going on hangers high up and do to woeght and leaks etc on other vessels they say empty....


The fact is the yank should be stored 95% full with stabil and thats it! End of storey

WHAT THE HE## has a YANK gotta do with it???
(why don't they have hockey sticks somewhere on the putter keyboard) :D
 

Biged007

Petty Officer 2nd Class
Joined
Sep 9, 2010
Messages
176
Re: full tank vs. empty tank for storage

This part of the year is hurricane season around here. Therefore, I keep it full of stabilized gas so I have a good supply for the generator when the lights go out. ;)

With the Puke we call gas today that would be the only reason for keeping a Full tank. I had to reverse my thoughts also about 2 years ago, this is not my myth but a fact that comes from a motorcycle lecture I attended. Gasoline is by nature hygroscopic and worse yet with the addition of ethanol suspends it, Now add to the fact that some time ago the refineries started adding a chemical that will actually kill it as to discourage "hording" the only gas that this does not apply to is in fact AV Gas
Now to end my rant for the past 2 years I have been draining all of the GAS tanks on the farm equipment I have the charge of approx. 11 pcs. ttl and both of my bikes force drying all of them with air and "fogging" them with with WD-40. I can honestly tell you folks the spring wake up has actually been easier.
Just mt .02
Ed
 

Fed

Commander
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Apr 1, 2010
Messages
2,457
Re: full tank vs. empty tank for storage

Did you have to drain any condensation out of the tanks for the spring refill Ed?
 

45Auto

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Joined
May 31, 2002
Messages
2,842
Re: full tank vs. empty tank for storage

191Seanymphstriper said:
Some storage or marinas etc ask that the fuel tanks are empty because they are going on hangers high up and do to woeght and leaks etc on other vessels they say empty....

The fact is the yank should be stored 95% full with stabil and thats it! End of storey

Hopefully your credentials for making such a sweeping statement are much better than your spelling and grammar! ;)
 

eli_lilly

Chief Petty Officer
Joined
Dec 22, 2005
Messages
435
Re: full tank vs. empty tank for storage

When you go to use your boat the next time, would you rather have 100 gallons of ethanol with 10 gallons of water sitting at the bottom or 10 gallons of ethanol with 1 gallon of water sitting at the bottom?

That, right there, answers the question of whether to leave it near-empty or near-full.

-E
 

18WCmerc

Petty Officer 2nd Class
Joined
Sep 7, 2010
Messages
193
Re: full tank vs. empty tank for storage

How many innocent horses must die before we can peacfully resolve this conflict!!!!!! God have mercy!!!!!
 

45Auto

Commander
Joined
May 31, 2002
Messages
2,842
Re: full tank vs. empty tank for storage

What makes you think it'll ever be resolved? There are lots of people out there who don't understand the physics behind certain events, and who believe that if something hasn't happened to them, then it must not exist.

Look up the old "Airplane On A Treadmill Thread" if you want some laughs. If you don't understand that the force from a planes' prop is independent of the wheels, then you won't understand how the speed of the treadmill is irrelevent to it.

About a third of the US population doesn't believe we went to the moon. It's pretty easy to calculate the power, speeds, and trajectories it would take to get there. Then it's just a question of building something that would deliver the power required.

High school robot team at the base of one of the unused Saturn V moon rocket first stages:
DSC05153.jpg


The information is out there for anyone willing to put forth the effort to educate their self. Unfortunately, I believe that the US ranks at the top in education spending, but somewhere around 25th in the world in results now ....
 

18WCmerc

Petty Officer 2nd Class
Joined
Sep 7, 2010
Messages
193
Re: full tank vs. empty tank for storage

lol are those four guys who have never kissed a girl? sweeet
 
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