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Old 10-07-2011, 13:21   #1
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Gas / Petrol Evaporation in Dinghy Tank ?

My plastic dinghy fuel tank bulges in the heat of the day. Significantly.

If I leave the air port open during the day how much fuel would be lost to evaporation?

Or just leave it shut and let the bulge bulge?

My tank is 25 litres (6.6 US Gals)



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Mark
PS My dinghy is a monohull. I just put that in so we don't get into irrelevant arguments. No anchor and obtained outside the USA without a broker.
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Old 10-07-2011, 13:36   #2
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Re: Gas/Petrol evaporation in dinghy tank?

i found more salt water coming into the tank than evaporation of fuel out of it when i left my vent open.
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Old 10-07-2011, 13:51   #3
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Re: Gas/Petrol evaporation in dinghy tank?

On my 3 gal plastic tank I just crack the seal very slightly on the air vent. I am more worried about a seam spiitting and gas leaking out, though I have never seen it happen. These plastic tanks have a rough life, out in the sun most of the time, bulging, contracting, etc. Sorry, no idea how much would be lost by evaporation.
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Old 10-07-2011, 13:56   #4
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Re: Gas/Petrol evaporation in dinghy tank?

Quote:
Originally Posted by MarkJ View Post
My plastic dinghy fuel tank bulges in the heat of the day. Significantly.

If I leave the air port open during the day how much fuel would be lost to evaporation?

Or just leave it shut and let the bulge bulge?

My tank is 25 litres (6.6 US Gals)



Thanks,


Mark
PS My dinghy is a monohull. I just put that in so we don't get into irrelevant arguments. No anchor and obtained outside the USA without a broker.
I bet you were too tight to pay for a survey as well, and you wonder why you have problems? BTW, is it a production dinghy or true Blue Water?

Greg
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Old 10-07-2011, 14:03   #5
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Re: Gas/Petrol evaporation in dinghy tank?

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Originally Posted by Eleebana View Post
BTW, is it a production dinghy or true Blue Water?

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Its an AB made in some 3rd World country. I don't know if I've assisted the wages of a family or supported slave labor?


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Old 10-07-2011, 14:04   #6
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Re: Gas/Petrol evaporation in dinghy tank?

Since you probably don’t have an “FDV” on your tank, I’d open the vent slightly (except when it's raining), and expect negligible fuel evapouration. Don’t over-fill the tank, allowing ample room for expansion.

See ➥ Portable Fuel System | Mercury Marine
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Old 10-07-2011, 14:17   #7
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Re: Gas/Petrol evaporation in dinghy tank?

Yo Mark, If you have an ethanol blended gasoline leave the vent CLOSED. Shade the tank if possible to minimize expansion. With the tank vent open and as the tank breathes you will expose the ethanol in the fuel to the humidity in the air. When the ethanol absorbs 2% by volume “water” it will phase separate and the ethanol will fall out of the fuel with the water and end up at the bottom of the tank. This were the fuel pick up is and your asking your OB to run on essentially 98% ethanol. This will rot any and all rubber parts, fuel lines, diaphragms ect.

Here’s a good read www.uscgboating.org/assets/pdf/recalls/BSC85_1.pdf Look at the section “Pain in the gas”

Regards,

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Old 10-07-2011, 14:20   #8
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Re: Gas/Petrol evaporation in dinghy tank?

Mark the fuel level. Open the vent. Let it sit all day in the sun. Report back.
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Old 10-07-2011, 14:37   #9
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Re: Gas/Petrol evaporation in dinghy tank?

Quote:
Originally Posted by MarkJ View Post

If I leave the air port open during the day how much fuel would be lost to evaporation?



.
Most airports are open during the day and most planes dump a lot of fuel as they approach. There is a lot of evaporation there from a lot of fuel. It depends on how busy/big your airport is though.

I don't think Mother (earth) will mind your small amount of pollution from your dinky tank.

The airport however..................
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Old 10-07-2011, 15:15   #10
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Re: Gas/Petrol evaporation in dinghy tank?

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... I don't think Mother (earth) will mind your small amount of pollution from your dinky tank ...
Mark is a prototypical cruiser - he's concerned about the cost of the evapourated fuel.
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Old 11-07-2011, 11:35   #11
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Re: Gas / Petrol Evaporation in Dinghy Tank ?

I just had a nasty experience with a quarter full tank. I had the vent closed and it bulged up in the sun. The small valve on the fuel line started to drip fuel. I only noticed when it had run all the way down my side deck (tank is stored on the boat, not the dinghy). Think I'll leave the vent cracked now.
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Old 11-07-2011, 12:30   #12
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Re: Gas / Petrol Evaporation in Dinghy Tank ?

the vent should be up at the tank top so why would fuel come out??
(oh, I suppose this is a boat fuel tank, not a regular gas can, then that might happen.)
Ethanol gas sucks water from the air. The water causes alcohol to sink to the tank bottom. Takes only a tiny amount of water to ruin the gas and cause phase separation.
Plus you loose volatile components out of your fuel which make engines start easier. Today's gasoline is lousy, imo.

Quote:
As an example, at 60 degrees F, water can be absorbed by a blend of 90%
gasoline and 10% ethanol up to a content of 0.5 volume percent
before it will phase separate. This means that approximately 3.8
teaspoons of water can be dissolved per gallon of the fuel before
the water will begin to phase separate.
so say your fuel is slightly wet yet still ok, the temp drops, the fuel will then phase separate right in the tank.
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Old 11-07-2011, 12:42   #13
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Re: Gas/Petrol evaporation in dinghy tank?

Quote:
Originally Posted by GordMay View Post
Since you probably don’t have an “FDV” on your tank, I’d open the vent slightly (except when it's raining), and expect negligible fuel evapouration. Don’t over-fill the tank, allowing ample room for expansion.

See ➥ Portable Fuel System | Mercury Marine
So many funny posts. However, this post is more to the point.

* If the vent is open during rain expect trouble the next day. Sustained humidity is bad too. When I started ALWAYS closing the vent my water troubles went away completely.
* Always fill the tank after use. Though Gordmay is correct that you need room for expansion (do not fill it slam full), it is primarily the air/vapor that expand. If the tank is nearly full it will not bulge; near empty tanks and gerry cans bulge the most, as we have all noticed. A full tank also absorbs less water (less breathing).
* Full tanks don't heat so much. A nearly empty tank can easily reach the initial boiling point of e-10 (120F), while a full tank will only warm a few degrees in the sun.
* Close the vent when the tank is full and the engine is tilted. At least on mine, it will spill into the tender.

Problem solved.

Post script: Though we are trained to leave room for liquid expansion, I'm not sure a liquid CAN expand enough to do any damage to a plastic tank. Slam full is probably safest situation for the tank (or gerry can) if the vent is closed. Of course, when you open the tank fuel will come out, which is bad, and pouring out of a full can is a trial. But full is probably safest for the can (metal tanks are different and so are tanks with open vents--please don't extrapolate what I said). I'm wonder if this has been studies with e-10..... time for Google.
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Old 11-07-2011, 12:54   #14
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Re: Gas/Petrol evaporation in dinghy tank?

Quote:
Originally Posted by cwyckham View Post
. I had the vent closed and it bulged up in the sun. The small valve on the fuel line started to drip fuel. .
yeah, I have to unplug the fuel from the engine as the conection to the filter leaks under preasure. Floods.

Quote:
Originally Posted by GordMay View Post
Mark is a prototypical cruiser - he's concerned about the cost of the evapourated fuel.
Yes, its the cost I'm worried about. If one cup evaporates per day thats 7.5 liters per month / 2 Gals = $11.00 per month / $132 per year. Sounds chicken feed, but its 2 dinners out and a mid-range restaurant.

Thats a lot of money in Cruiserville.
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Old 11-07-2011, 12:59   #15
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Re: Gas / Petrol Evaporation in Dinghy Tank ?

Thinwater, the reality is that cruisers fill their ob tank and run it till its near empty. Its not like a car where you can just pull into a fuel station. Especially 2 strokes where you need to mix oil etc. Getting the gerry out to refill the in dink tank is a pain. So fuel level is never consitant.
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