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How long does gasoline stay usable in a boat tank?

Whidbey Island Cruiser

Whidbey Island Cruiser

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I have a Bayliner with a very clean tank that holds 75 gallons. 305 engine, with water separator filter.
Generally, we keep it at half a tank, and add 25 gallons (yes, with ethanol) or so, then run it, then add more as we have money available. This fall, I poured in a 1/2 quart of blue Stabil marine additive.

I've never filled it completely. My son thinks I should keep it filled "to eliminate water". It's at about 1/3 tank again for the fall now, before 5 months of non-use to come. Zero easy ways to drain the tank. Thoughts?

Any good additives to "fix ethanol effects"?
 
You can also find articles that explain the Myth about keeping a gasoline tank full during the lay up period.

Your worst enemy is the Ethanol in the gasoline!
Second enemy is warm weather during the time the boat is not being used.

If you add fuel stabilizer, you need to shake it up... in other words, tow the boat a few miles and let the short trip help mix the stabilizer with the fuel..... or, better yet...... add the stabilizer prior to your last outing!
 
Re: keep changing filter...they'd like you to do that!!!! The current crop of fuel filter/seperetors contain two compartments. One is a media filter for particles. The other to trap water. If the media portion of the filter is clean, then dumping the contents of the filter into a large glass or fuel resistant transparent plastic container will allow you to see if there is any water in the fuel.. ( also see some of crap trapped by media filter) just wait about 20 mins for it to settle. Having done this, the filter is reusable... not indefinitely as the media will eventually plug up with crap, but to trap water in the fuel, totally reusable. To make restart easier, prefill the filter with clean fuel and smear a thin film of engine oil on the rubber seal ring, and reinstall... DO not over tighten. Regardless, I replace the filter every other year.
 
Gas only lasts a week in a boat's tank. Fish often! Just kidding, but I do burn about 1/2 a tank every week.
Seriously, though - I agree with your son about keeping the tank full. Unlike our cars & trucks, boat gas tanks are vented & exposed to the atmosphere. As temperature fluctuations occur, air moves in & out of the tank - bringing in humid air. When that humid air cools at night, condensation occurs. The ethanol in gas "mobilizes" the water, helping it move through your entire system. So, a partial tank of ethanol fuel is the worst combination. Using marine fuel will help minimize the effect of water in the tank, as they pretty well separate from each other.

As for pumping the gas out - it should be pretty simple, provided you get some kind of pump. I have a pair of these, that I use:
https://www.amazon.com/Moeller-Marine-Products/dp/B07DQYT7V7
This model is a little expensive, but they work great for draining a gas tank and also for draining oil through the dip-stick, when you do an oil change.

As for additives to neutralize ethanol effects - I don't know of any, and everything I've heard and read says there is no way to make it truly safe for older marine engines.

Best of luck!
 
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TipDS said:
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Seriously, though - I agree with your son about keeping the tank full. Unlike our cars & trucks, boat gas tanks are vented & exposed to the atmosphere. As temperature fluctuations occur, air moves in & out of the tank - bringing in humid air. When that humid air cools at night, condensation occurs.
The fuel would need to expand considerably during the day, and contract considerably during the night.
Then we must factor in;
...... just how much gasoline expands/contracts via temperature differentials.
One search suggests: When gasoline rises from 60 to 75 degrees F, for instance, it increases in volume by 1 percent.
...... how long does it take for the fuel to absorb and react to temperature differentials.
..... just how humid or dry the ambient air is
The space above the fuel would then need to move enough air (in/out) to create a pull/push via the fuel tank venting system.
Do the math....... this equates to an extremely small amount of air movement.

Here is an interesting article by David Pascoe.
http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm

Read what David has to say about the more common means of water intrusion (deck fill neck caps, no high loop in the vent line, etc.)

IMO, we should be more concerned about the degradation of the fuel regarding the Octane rating.


The ethanol in gas "mobilizes" the water, helping it move through your entire system. So, a partial tank of ethanol fuel is the worst combination. Using marine fuel will help minimize the effect of water in the tank, as they pretty well separate from each other.

As for pumping the gas out - it should be pretty simple, provided you get some kind of pump. I have a pair of these, that I use:
https://www.amazon.com/Moeller-Marine-Products/dp/B07DQYT7V7
This model is a little expensive, but they work great for draining a gas tank and also for draining oil through the dip-stick, when you do an oil change.
That is very similar to the original Tempo Oil Boy...... and they work great for starting a siphon.
Remove your Anti-Siphon valve, temporarily replace it with a standard barbed fitting, and siphon away!



As for additives to neutralize ethanol effects - I don't know of any, and everything I've heard and read says there is no way to make it truly safe for older marine engines.
I would agree! It would be nearly impossible to eliminate the effects of Ethanol by adding chemicals.
Click to expand...
 
RicardoMarine said:
...just how much gasoline expands/contracts via temperature differentials...
Click to expand...

It's not the expansion and contraction of the gasoline, but that of the gaseous mixture above, which creates the flow. A 40 degree swing in temperature, starting at 20 degrees will create more than an 8% swing in volume. That isn't insignificant, especially over several thermal cycles.

RicardoMarine said:
...just how humid or dry the ambient air is...
Click to expand...

I suppose you could ask yourself the same question any time you look over your lawn on a cool morning & see it covered in dew.

In the end, it's a "best practices" issue, not a "make or break" issue. Some boats are more susceptible to the issue, some less. If you have a metal fuel tank, no fuel/water separator (or one that's already full of water) and a cast iron engine with no knock detection/mitigation, then you want to take every reasonable step to avoid the issue. On the other hand, if you have plastic fuel tanks, multiple fuel/water separators and an aluminum engine with a sophisticated anti-knock system, you may be less concerned.

To each, their own.
 
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