Topping-Off Fuel Tanks.....Maybe NOT.
Michael T
Member Posts: 7,227 ✭✭✭✭✭
No, I am not rushing winterization topics but I heard an interesting discussion at the slips a few weeks ago by a group of boaters about what level of fuel was best to leave in your tanks over the winter. I thought I'd better post it or I'll forget about it. The discussion was about 100% similar to our lengthy posts on it every Fall. Guys owning the boats included a couple of Rinker 342s, a Rinker 360, A Rinker 280, some Sea Rays, a new Cruisers 350 and a big Chaparall - all gassers. A professional looking guy walked by with a bunch of gear and identified himself as a marine surveyor. He was headed up the docks to spend two days on a Sea Ray 540 or something like that. He gave some business cards out and asked if they would be interested in his opinion. I believe "**** Yes" was the reply from the group. He said he recommends running the fuel down to as low as is practical and adding a bottle of fuel stabilizer, he liked STA-BIL Fuel Stabilizer AND a bottle of Gasoline treatment, he liked Star Tron Gasoline Additive, to EACH tank - even if it was 7/8 empty. That way he said it would highly protect the fuel and tanks over the winter and would be there for the first fill-up in the Spring. No big news right? BUT ......He said worrying about the fuel quality or water getting into the fuel had very little to do with his advice to keep as little fuel as possible in the tanks. He said when he sees a cruiser with a set of blocks under the stern at the drain hole, a set of blocks up under the bow and a stand on each side of a boat under the hard chines that's not bad - although He'd like to see two stands per side BUT...if that same boat has full gas tanks there would be far too much weight on the stringers, cross members, bulk heads and hull which will most certainly over time result in bulkhead and stringer separation and hull deflection. It's not a question of fuel quality for him but a huge amount of unsupported weight. One of the things he was going to look for on that Sea Ray (which he does on every boat he surveys) was any signs of cracking in the gelcoat or stringer/bulkhead de-lamination around the fuel tanks. I used to believe in topped-up and additive conditioned tanks over the winter. I've changed my opinion - this coming winter I'll run 'em down and condition the remainder like crazy.
Comments
I normally store with a pair of stands at the stern and a second pair near the forward keel block. Maybe I should put the second pair under the fuel tanks instead.
2007 280 Rinker Express 6.2L B3
Dream 'Inn III -- 2008 400 Express
The engine room of a big express cruiser has 6000 pounds of machinery (engines, fuel, genny, batteries etc.) being supported by the hull. If the tanks are empty vs. full, the weight might be 5000 lbs instead of 6000 lbs.
The hull is designed to support that 6000 lbs while it's getting jolted around in wave impacts that probably reach 10 g or more. That's a dynamic weight of 60,000 pounds that the hull is designed to support. I can't imagine the hull cares whether the static weight is 6000 vs. 5000 lbs during storage.
Having said that, I imagine there are situations in which the fuel weight matters for some boats. For example, if it's a displacement hull that's not designed for heavy impacts, and the boat has really big fuel tanks, and the placement of keel blocks and jackstands is really bad -- maybe it could be a concern. But not on an express cruiser, unless I'm missing something.
Imagine the condensation, phase separation associated with a LARGE volume of fuel. 100+ gallons could easily produce a gallon or two of water in the bottom by springtime. All ready to be sucked up into the motors.
I run almost empty + Marine Sta-Bil or equivalent. Do the same with all my stored equipment for years with zero issues. Boats, sleds, cars, etc.....
MichaelT's point about excess weight during storage is likely valid. Just another logical conclusion to support empty tanks during storage.
They don't ship them from the factory with full tanks.
my 2 cents.......
If you're interested in the math and real facts read this:
STORING YOUR BOAT ON AN EMPTY TANK
First off, let me refer you to a website which has the MAX water content of air at various temperatures:
http://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html
So you will see at 86F, the max moisture content (100% humidity) is 30.4 g/m3.
You will also see at 14F, the max moisture content is 2.31 g/m3.
I would call this a worst case scenario - somehow 100% humid 86F air completely filled your gas tank and all the air in that tank was cooled to 14F.
This means that a maximum of 28.1g of water could condense out per cubic meter of tank volume. My tank is 285L, or 0.285 cubic meters. So at most 8 g of water could condense if I had EVERYTHING working against me. That is 1.5 teaspoons of water.
STORING YOUR BOAT ON A FULL TANK
Now for the other alternative - you have to look at two scenarios (1) storing ethanol containing fuel, and (2) storing "ethanol-free" fuel (in brackets because "ethanol-free" doesn't necessarily mean it is free of ethanol, but meets a minimum standard.
I am going to refer to this paper, for details - it's a good read if you have time:
epa.gov/oms/regs/fuels/rfg/waterphs.pdf
1) Ethanol containing fuel. This is about the worst scenario possible. See this website:
http://ttypes.org/ttt2/ethanol-blended-fuels
The problem is phase separation. If there's enough water (at 14F it needs to be higher than 0.35% water by volume), then every 5 gallons of fuel turns into 1/2 gallon of alcohol/water mixture, of which 1/4 is water and 3/4 is alcohol. That's 2.5% of the gas turning into water! In my tank, if it was full (285L), I would need around 1L of water in the fuel before phase separation would occur at 14F, then it would make over 7L of water!! Boom!
However, note that we calculated above that an empty tank can only produce about 8 mL of water, so that's not enough to cause phase separation for a full tank (and you wouldn't get that much condensation to begin with anyhow with a full tank). Not on it's own anyhow. I would say it's only a problem if the gas has been in the tank for a long time and has already absorbed lots of water from ambient, and it only needs that last 8mL to push it over the edge.
Verdict - I wouldn't keep the tank completely empty, as doing so (a) increases the amount of condensation, and (b) with less gas in the tank, it takes less condensation for phase separation to occur. On the other hand, if you have a full tank and can't consume it quick enough, you actually increase the chances of phase separation, as water will absorb over time (over many months). Don't take my word for it, but I would probably leave 1/2 tank of gas, unless I knew I would consume more in the first month after coming out of storage. Note, some sites do talk about concern of gas tank corrosion if phase separation does occur - so really you want to avoid this. The most important thing is to stabilize that gas, and use it as quickly as possible to avoid phase separation. I personally would be more worried about not using that gas for a long time, as that is the biggest risk of phase separation.
2) "Ethanol-free" fuel
Pure gas does not have the ability to absorb water (well not that much). In essence, when it reaches it's water saturation point, then any extra water just won't absorb and it settles out. It doesn't phase separate (at least that's what the sites I have read have said).
So essentially the water you will get in the gas is the water it can't absorb. If the gas was saturated with water, and the tank was essentially empty, then that means about 8mL of water in 285L can be water. I'd imagine if you fill the tank with fresh gas in the spring, some of the water would re-adsorb into the fuel, and the turbulence would disperse it fairly quickly.
I can tell you first hand that last winter I stored with a full tank of ethanol-free fuel, added sta bil as well, and I basically didn't have to fill my tank all that much this summer (didn't do enough boating). I did notice more black fumes from the exhaust, especially on starting the engine. There was also some minor carbon deposits on the water surface (not a lot, but it was different to last year, when I had fresh gas).
Verdict - chances of a water problem are a lot less with ethanol-free fuel. This small amount of water (8 mL at most) the fuel/water separator will easily handle that, especially because it won't come in as a slug (not that little in so much fuel). Still stabilize the gas, as "ethanol-free" fuel may not be totally free of ethanol. In my opinion, I don't see a big issue having the tank 1/4 to 1/2 empty. If you plan on consuming a full tank in the first month after it comes out of storage, then sure, I would go ahead and fill the tank.
Boat Name: King Kong
"Boat + Water = Fun"
They don't ship the tanks full of gas in a boat for the same reason they don't with a car: COST.
PC BYC, Holland, MI
Boat Name: King Kong
"Boat + Water = Fun"
All I've wanted was to just have fun.
It would sound reasonable to an insurance adjuster that knows nothing of boats, and a well compensated surveyor. No?
As far as heavy loading of fuel additives on low fuel level- A) after so much, the rest is useless... B ) non-ethanol fuel doesn't hold water, water lays below it as it's heavier, but what your adding may very well hold water... So.... Choose carefully and avoid anything with alcohol... C) if you have ethanol fuel, you're going to want to use an imulsifier, if you have rec gas, you're going to want to use a DEmulsifier... One allows water to attach to the chemical and fall to the bottom, and the other disallows it to combine at all... It's a big deal which you choose and why.
Mike: I know you canooks are different :-) , But down here in your basement it makes no matter the rand count (octane compound rating) whether 99 or even 100 octane, it could still have alcohol. on an aside, alcohol is actually more stable than gasoline, and by itself if using the same ratings test and it being pure, would register around 115 octane... The problem with it is that it tends to explode instead of burn really fast like gas, and it doesn't pack near as much energy punch when comparing like volumes.... Hence, you burn more, it burns hotter, and it's corrosive.
"They don't ship them from the factory with full tanks."
Obviously my comment was meant as a joke. Skipped the lol....
Nonetheless, personally - I believe full tanks are a total waste of time & effort.
I would much rather be using fresh gas at start up....than 6 month old gas that sat all winter. (actually 8 month old gas in my case.)