Fuel Tank Vent question

[DavidS]

I'm getting ready to install a permanent fuel tank in the Rascal and have a question on the vent:

http://www.moellermarine.com/sites/moellermarine/images/fuel_systems/035732-10_07194-72dpi.jpg

Is this to be mounted vertically as shown? Any chance of water, rain, etc., getting in?

Or should it be mounted horizontally?

Is it OK for the vent to be 6 inches, or so, from the fuel deck-fill ?

Thanks,
Dave

[Jim Ledger]

It's supposed to mount horizontally, through the coaming or hull side, with the vent pointing down. The hose should, ideally, loop up to prevent water or spray from entering and fouling the fuel, a serious consideration. Nearness to the fill shoudn't be a problem so long as the fumes vent overboard.

There might be other considerations or regulations beyond these that I'm not aware of, so, stay tuned.

[Henning 4148]

Also, it should be installed higher than the filler.

[DavidS]

Interesting.....

I can mount it horizontally, but that probably means that it will be in the motor well area at the back of the boat. Don't know where else would work for this small boat.

Also, I wish it was longer, there is a limit on the hull thickness for this vent. Maybe I need something that is longer for a thicker hull.

Thanks,
Dave

[Gary E]

My boat had the vent line made from a 1/2 in copper tube. The end was flared and came out aprox 6 or 8 inches below filler level on the side decks. There was a copper screen in that tube to prevent bugs etc from getting in and it was covered with a clamshell so spray would not get in... The tube looped up as high as it could before turning down and connecting to a fitting on the top of the fuel tank under the deck.... This same arangement was on all the tanks..

If your instalation is like this, I'd return that fancy dodad and use copper tubing.

[Henning 4148]

As far as I know, at least in Europe, the thing isn't intended as a through hull but is intended to vent into a well ventilated area that can be cleaned in case of slight overspill without the fuel getting into the water (so, for instance the cockpit). So, normal assembly position would be a bulkhead or something like that. Now, with gasoline, this can be nasty if your cockpit drains into the bilge and there is a cockpit floor which prevents good ventilation of the bilge, in that case I would not vent into the cockpit, but if the cockpit is self draining or you are using Diesel as a fuel, it should be ok.

Back in them long gone days, I just took copper fuel pipe and squeezed its end until the opening was no longer round but a thin slot, then bent the tube for this opening to face down and hey, presto, the vent was ready. Worked for me ... But I have to confess that one reason for doing this was, that I was keen to get the job done and didn't want to wait for a fitting (and also didn't want to pay for one).

[pcford]

Interesting.....

I can mount it horizontally, but that probably means that it will be in the motor well area at the back of the boat. Don't know where else would work for this small boat.

Also, I wish it was longer, there is a limit on the hull thickness for this vent. Maybe I need something that is longer for a thicker hull.

Thanks,
Dave

I would be surprised if it is too short. The hull on your boat should not be much more than 3/8s. There are other types though.

Advice above is good. For a speedboat: Usually mounted as high as possible about foot or 18 inches forward of transom. On CCs the fuel vent is usually on starboard side. The discharge for bilge is in the same place on other side. _Never_ put both on same side.

[Gary E]

_Never_ put both on same side

WHY?

I have twin engines... and twin fuel tanks with fills on each side
Now where do you put that vent?

[DavidS]

I would be surprised if it is too short. The hull on your boat should not be much more than 3/8s. There are other types though.


Thanks for the answer. Yes, that's correct the hull is 4mm plywood and covered by 1/4" mahogany planks. However, the back end of the Rascal is curved inward w/ tumblehome so I was thinking of putting the vent in the motor well area where the outboard is mounted. I built that w/ mahogany hardwood that is something like 3/4" or 13/16".

Thanks,
Dave

[JimConlin]

In picking a location for a fuel tank vent, i'd have a number of requirements.
The vapors that come out of the vent should go overboard. You don't want them in your cockpit or bilge.
The vent should not be in a location subject to flooding or heavy water. You don't want water in your tank.
Inboard of the vent, there should be a loop, to keep out water.
Screened openings are a good idea, too. You don't want inhabitants in your vent.

The type of vent shown above can only go up to about 1" thickness. I recently installed a couple in a thicker hull by epoxy bonding them in place.

[pcford]

WHY?

I have twin engines... and twin fuel tanks with fills on each side
Now where do you put that vent?

Assuming you are talking of a speedboat, one on each side.

(?)

In a case like this put the bilge pump thru hull forward...just forward of windshield.

[pcford]

Thanks for the answer. Yes, that's correct the hull is 4mm plywood and covered by 1/4" mahogany planks. However, the back end of the Rascal is curved inward w/ tumblehome so I was thinking of putting the vent in the motor well area where the outboard is mounted. I built that w/ mahogany hardwood that is something like 3/4" or 13/16".

Thanks,
Dave

What does tumblehome matter?

[Peter Eikenberry]

The vent should be, as was said, were it vents to the outside of the boat.

It should also be where, if any gas is spit out the vent, the gasoline will not enter the boat.

The screen is not for bugs (although it will keep most out). It is a flame arrestor to prevent any flame from igniting the fumes in the vent hose.

ABYC standards call for the vent to be mounted at least 15 inches from any opening into the boat so fumes won't get sucked back in. So it shouldn't be near vents, windows, hatches etc However mounting it near the fill is ok. Some fuel fills have the vent inside the fill.

Actually the vents are usually mounted on the side of the hull (horizontal) but can be mounted on the transom. However you want it where water is not going to enter it, so if you put it on the transom make sure your wake when you stop or slow won't go in the vent.

Also, as was said, put a loop in the vent line to keep the water out. By loop I do not mean 360 deg. I mean, just inside the hose should go up a few inches then turn and go down to the tank. (like an upside down p trap)

You also want the vent where you can get at the inside of it so you examine the hose and connections from time to time.

After you install the tank, vent, and fill, you should have the fuel system pressure tested to make sure there are no leaks. Most marine repair shops can do this.