I want to redesign the tank vent system

If you really want to seal this tank up then seal it with a vent you only open for fueling and running. But beware, you are going to encounter some pretty large pressure build-ups on warm/hot days. Perhaps an adjustable vent is your answer... Large when fueling, small when running and miniscule when tied up. Sounds kind of high maintenance to me.

-Tom

 

The outlet is always going to be venting outside of the cabin.
The towel is not wrapped tightly, just loosely on the deck around the fill pipe.

well in a sense, yes, that is the idea, but passive with no user intervention. Just adjusted by design to do what is needed when needed.

The problem with ethanol laced fuel is only going to get worse. E15 is coming, then maybe E20. Phase separation and tank acids are going to be eating the metal tanks from the inside out, all cars now designed for E10 use plastic tanks. And getting stranded on the water means an expensive tow or much worse, a loss of the boat, crew, anything may happen when you loose your motor on the seas. Imagine a large metal fuel tank made of aluminum eaten on the inside by corrosive byproducts of ethanol gas and water springing a leak.

Ethanol fuel in boats has only been here a few years, too short of a time to see metal tanks destroyed, but I think it is coming.
Fiberglass tanks are already wrecked.

 

Personally I hate deck fills, as a poor O ring or seal can add mucho water to the tank, every rainy day or wash down.

Our solution is a bronze 6in flat deck plate , and the fill hose is under, with a simple plumbing 2inch nipple and cap at the hose end.

Even if the deck plate leaks ,or is lost, there is no way for the water to get past the cap.

FF

 

I read about this ethanol fuel but have no experience. I understand that it doesn't like aluminium and some grades of rubber. Is it possible to use copper fuel lines with ethanol fuel ?

 

http://www.theepicenter.com/tow021799.html
I have found this while searching for copper and fuel. Fuel is degraded in copper, whether copper is degraded by good fuel, maybe not, but bad water in fuel does form acids that eat metals. Metal deactivators in a fuel preservative are designed to keep the metals from degrading the fuel. Also it mentions corrosion inhibitors for metal tanks, so the damage must go both ways.

http://www.chemistryexplained.com/Fe-Ge/Gasoline.html

 

Just a wild thought here.
If you wished an entirely sealed fuel system, why not consider the use of a sliding displacement piston and cylinder. OR an expansion bag of some type. Using this, setup the device on top of the tank. System could be designed passively to react to fuel volume pressure changes. As the pressure would want to rise as when temperature rises, raise the piston in the cylinder to create more volume in the tank for fuel expansion, as temperature dropped, the piston would drop down to create less volume in the tank.
Most changes in tank volume will be rather minor unless the temperature experiences a large swing, at that point, design the system to include a compensating reset to reestablish a new equilibrium.

tank draw down usage would be compensated for as if the temperature had plunged, contracting the fuel volume in the same fashion, by using the vent system reset.
The goal is to reduce the exposed fuel to moisture in the air. The current vent system is wide open, therefore constantly exposed to moisture in the air and is wrecking havoc with todays ethanol fuels.
Using a double springed vent, 2 one way valves, set at 2 PSI would be a help, but that vent design will open much more frequently that using a more closed system. Think of the piston-cylinder or bag idea as a buffer, sealing the system from minor changes.
Boats just sit at the dock for much of the time with no engine running. Fuel in tank constantly exposed to the air, steadily sucking in moisture.

 

What happens as the engine uses fuel volume and it cant be replaced by air volume. ? Vacume ? A siphon breaker ?

 

yes the vent system will have to reset itself to allow for the fuel to continue to flow. Could even be as simple as a spring loaded valve set to open as the fuel is drawn out passively or actively with a solenoid that opens the vent when the engine runs.

The boat sits around at the dock without the engine running most of the time. And that is where you need to concentrate the protection of the fuel system from constant exposure to moist air and minor temperature fluctuations causing the tanks to breathe, wind air currents flowing into and round the tank vent etc....

I wonder how much water vapor gets into diesel and causes the bacteria-algae growth in the tanks due to wide open vent systems.

 

A standard syphon break could be used. Valve to shut off, on the big "taking on fuel "vent, then run on the small syphon vent. The problem is that you are using fuel. The concept will work but you better think it thru...

 

http://www.yachtsurvey.com/myth_of_condensation_in_fuel_tanks.htm

 

Yes and no to the yacht survey report.. I never fill tanks for layup because Im CHEAP and fuel is expensive. I never..repeat NEVER have detected any water in my fuel tanks from storage over a lifetime on yachts. Water in fuel always comes from improper or leaking installations.

Alcohol based fuel is a different matter. I have no experience with it , but Im led to believe that Alcohol fuel and atmospheric exposure are bad. Is this true ? Is it worthwhile to take defensive actions ?

 

michael Do not use copper. Copper is very susceptible to vibration and cracking. The USCG and ABYC standards require that if you want to use copper, you use copper nickel or nickel copper. These copper alloys are much better and will last a long time. They are also a lot more expensive than marine fuel hose.

Ethanol per se does not eat tanks. It is when ethanol fuels sit in the tanks for long periods of time that the problems occur. You get phase separation (the gas and alcohol separate ) and the acids that exist at the boundary layer between the alcohol, gas and water, eat the tank. If people used their boats as much as they use their cars you would not have a problem. Newer cars don't have these problems because their fuel systems are designed for AND the fuel doesn't sit in the system for months at a time.

Yes their have been problems with ethanol resulting in leaky tanks, and it has been going on for over 30 years. What made it more prevalent is the EPA requirement for 10% alcohol in all of our gasoline. This causes problems with metal and fiberglass tanks and with gaskets and hoses.

See Much Ado About Ethanol http://newboatbuilders.com/docs/Ethanol.pdf

Actually many of us old farts remember the days of Gasohol in the 70's. Gasohol IS ethanol. Gasohol is gasoline with 10% ethyl alcohol. Ethanol is gasoline with 10% ethyl alcohol.

The USCG and the industry changed the composition of marine fuel hose and gaskets in the mid 1980's to deal with this. We did a two year test of hose and plastic tanks for susceptibility to alcohol and worked with the industry to develop alcohol resistant parts.

As for unvented tanks; you do not ever want an unvented system in a boat unless your engine and fuel system are outside. Most fuel leaks are at fittings. If you have a pressurized system, even as little as 2 or 3 psi, and you get a leak, it empties all of the fuel into the boat.

As for the idea of a sealed system, see http://www.rbbi.com/folders/show/bw2000/exhibits/access.htm and http://vaporlessfueltank.com/default.aspx

I spent a number of years as the USCG liaison working with Top Dog to develop this system. It is very similar to fuel systems used on the space shuttle. Unfortunately it never succeeded on the market. It is a totally sealed system. It is also very expensive and complicated.

So it can be done, but you have to account for things like:

If you get a leak in a sealed system where does the fuel go? In a car it just goes on the ground.

As fuel is used how do you compensate to keep the pressure the same?

How do you prevent vapor from forming? There has to an equalization of volume (essentially the tank has to shrink as the fuel is used) to prevent vapor.

Sealed fuel systems make it almost mandatory to use a pump at the tank to "push" the fuel to the engine, meaning pressurized fuel lines all through the boat. Current regulations require the pump to be mounted on the engine or within 12 inches of the engine. This is so the pump "pulls". If a leak develops the negative pressure drops to zero and the engine quits. With a pressure system the engine keeps running until you run out of fuel and the whole time the fuel is being sprayed into the boat.

A major manufacturer of ski boats developed a system like this and spent years working with us at the USCG to make it work. It works, but they had to jump through a lot of hoops (like using a special hose that is essentially a hose within a hose that senses a leak and shuts the engine off). And the deal breaker for most boat owners is, they are prohibited from working on it. You know those labels that say No User Serviceable Parts? This engine and fuel system has a similar label.

I'm not saying it can't be done because I have seen it done. But it won't work with your simple system. But if you can come up with a simple system that works you'll probably be able to patent it and make some money on it.

 

I agree Ike.... a conventional, USCG appraoved, fuel vent system is fool proof and it works. Every system Ive ever used on commercially rated yachts follows the USCG method. I see no need to change unless you have very special, demanding circumstances.

 

PS: Just try not to blow yourself up in the process