Why are gas tank vents BELOW the fill port?!?

[IDBoating]

Why are boat fuel tank vents BELOW the fill port, where you insert the gas pump handle? This design almost insures that you'll spill gasoline since it will back up and out of the vent before it reaches the sensor in the handle. If the vent were above the fill port, the fuel level would reach the handle first and shut off the flow before getting up to the vent.

There has to be some reason for this, because it's so obvious that it would be better the other way. Anyone know?

[da.bell]

First, Tige has been know to spit gas. Just ask anyone here and we all know that you fill the tank slowly.

As for the air vent, the RZ's seem to be at or above the gas cap nozel. I don't remember the 24Ve that I had but I thought that it was at the gas cap level also.

[IDBoating]

As for the air vent, the RZ's seem to be at or above the gas cap nozel. I don't remember the 24Ve that I had but I thought that it was at the gas cap level also.

Yes, but why? Why not just put the vent above the fill port and solve the problem permanently?

I wonder if a loop in the vent hose would achieve the same effect. The entirety of the loop would have to be above the port to prevent fuel from pooling and blocking the vent, but if the top of the loop was above the fill port it might work.

[chpthril]

Why are boat fuel tank vents BELOW the fill port, where you insert the gas pump handle? This design almost insures that you'll spill gasoline since it will back up and out of the vent before it reaches the sensor in the handle. If the vent were above the fill port, the fuel level would reach the handle first and shut off the flow before getting up to the vent.

There has to be some reason for this, because it's so obvious that it would be better the other way. Anyone know?

The USCG strongly urges boaters to NOT fill the tanks to full as you need to allow room for expansion. Having the vent below the filler neck helps prevent this.

[IDBoating]

The USCG strongly urges boaters to NOT fill the tanks to full as you need to allow room for expansion. Having the vent below the filler neck helps prevent this.

Better to spill at the gas station than at the lake or in your driveway, I suppose {grin}. It just predictably defeats the purpose of the auto shutoff in gas station pump handles, which I find odd.

[zad0030]

All the boats I have filled up spit out gas regardless of the auto shutoff. The gas fills the hose too rapidly.

[Ewok]

I have never known an automobile gas station to auto shutoff without spitting or spilling gas in any of my boats. I open both fill ports and listen very carefully and fill very slowly to avoid gas spillage. I also try to keep track of how much gas it will take to fill the tank so I can expect when it will be full. I have no idea why boat gas tanks are so different than car gas tanks but relying on the auto shutoff from the pump will always result in spillage.

[IDBoating]

All the boats I have filled up spit out gas regardless of the auto shutoff. The gas fills the hose too rapidly.

That's hard to believe considering how large the fill hoses are. Those hoses have to be at least two inches ID. By contrast, the gas pump nozzle looks to be less than an inch. That means the hose is 4x the cross sectional area. Hard to imagine a two-inch hose being filled too rapidly by a nozzle 1/4 the size.

[chpthril]

I have never known an automobile gas station to auto shutoff without spitting or spilling gas in any of my boats. I open both fill ports and listen very carefully and fill very slowly to avoid gas spillage. I also try to keep track of how much gas it will take to fill the tank so I can expect when it will be full. I have no idea why boat gas tanks are so different than car gas tanks but relying on the auto shutoff from the pump will always result in spillage.

Boat fuel systems are an open system, i.e. the separate vent, where as cars have a closed (sealed EVAP) fuel system and the vent is actually inside the filler neck. That extra pressure of the venting air the filler neck of a car is part of what shuts the pump off before the fuel gets that high in the neck.

Next time you are filling the car, keep resetting the handle after is kicks off. Before you know it, you will think you are filling the boat because you just got a gas shower.

[chpthril]

That's hard to believe considering how large the fill hoses are. Those hoses have to be at least two inches ID. By contrast, the gas pump nozzle looks to be less than an inch. That means the hose is 4x the cross sectional area. Hard to imagine a two-inch hose being filled too rapidly by a nozzle 1/4 the size.

The pipe nipple for the vent hose at the tank is probably a .5" ID. So the incoming fuel is exceeding the ability to vent the trapped air.

[IDBoating]

The pipe nipple for the vent hose at the tank is probably a .5" ID. So the incoming fuel is exceeding the ability to vent the trapped air.

If the vent were unable to keep up with the incoming fuel, I would expect to see fuel backing up in the filler tube (which, by the way, would cause the shutoff to work as intended). Instead, what I'm seeing is fuel spurting out of the vent itself, which prevents the shutoff from firing before fuel starts dumping out of the vent.

[chpthril]

If the vent were unable to keep up with the incoming fuel, I would expect to see fuel backing up in the filler tube (which, by the way, would cause the shutoff to work as intended). Instead, what I'm seeing is fuel spurting out of the vent itself, which prevents the shutoff from firing before fuel starts dumping out of the vent.

As noted above, that is by design. It is recommend to NOT fill the tanks full, so you should be shutting the nozzle off long before fuel should be reaching the vent hose or filler neck.

This is from "Boat US"

Fueling your Boat
Proper Fueling Procedures

Proper fueling procedures are very important in preventing onboard fires. Gasoline vapors are heavier than air and
can spread rapidly into enclosed spaces. You should check the bilges and all closed compartments for gasoline vapors. The sniff test is the most effective method for detecting fuel leaks.

The proper way to fuel your boat is as follows:


•Secure boat to the dock
•Switch off engine(s)
•Extinguish all open flames
•Do not use electrical switches
•No smoking
•Ports, hatches, and doors closed
•Make certain all passengers are ashore
•Determine quantity of fuel required and make sure it is the proper type of fuel
•Hold hose nozzle firmly against fill pipe opening
•Do not overfill
•Wipe up all spillage
•Open ports, hatches, and doors to ventilate
•Turn blower on for four minutes minimum
•Do the sniff test
•Start engine(s)
•Re-board Passengers
•Untie from dock and cast off


Proper fueling techniques also keep our water clean and safe, and help marine life thrive in a healthier environment. You are legally required to report fuel spills, and to take prompt action to clean up any spills that may occur.

Under federal law (the Oil Pollution Act of 1990 and the Clean Water Act) it is illegal to discharge any petroleum
product into the water. By law, any oil or fuel spill that leaves a sheen on the water must be reported to the U.S.
Coast Guard at 1-800-424-8802 1-800-424-8802

I am searching for the info, but IIRC, USCG says to not fill the tank past 90%

[IDBoating]

I am searching for the info, but IIRC, USCG says to not fill the tank past 90%

It would be nice if there were a convenient way to know that you're close to that level. On my existing boat, it's easy because I can just open the engine hatch - the rear side of the fuel tank is clearly visible under the rear seat and you can just eyeball it. On the 24Ve, the fuel tank is under the floor and there is no visual indication. And the fuel gauge is no help - even the manual admits it's a rough estimate "to provide a margin of comfort" (?!?). Furthermore, the gauge swings wildly. There are obviously no baffles in our tanks!

I guess we DO have a "nearly full" indication - when the vent starts leaking gasoline. Not exactly ideal, though.

[Timmy!]

I'm starting to notice WABoating that you waaaay overthink things. Sometimes you just have to go with it.

[IDBoating]

I'm starting to notice WABoating that you waaaay overthink things. Sometimes you just have to go with it.

Guilty as charged {grin}, but that comes with being an Engineer. When getting into new things, I have a super steep learning curve where data flows "in". Then I start being able to help people, and the data flows "out".

On my existing boat, I've become something of a reference for people with similar boats and engines. Hopefully I'll become useful to the Tige community that way too. But so far, I'm still in learning mode. I appreciate your patience.