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Gas Bag Setups
Collapsible bags make excellent gas tanks. They may carry different names – IV bag, saline bag, bladders, etc. We'll use the term "bag", so no one will take offense to differing terminology. This How-To is intended to help you make the switch from the old style hard tanks and even the newer "made for gas" tanks and cells, to an excellent alternative proven by racers and recreational boaters alike.
Advantages:
- No vent is needed for Bag gas tanks, which eliminates the chance of water being drawn into your gas tank through the vent.
- The bags are less smelly and there is no way for fumes to escape since no vent or external connections are present to leak gas or fumes.
- Bags keep the gas oil mix from foaming, which can be a major performance limiter with standard tanks.
- Bags are lighter in weight than the majority of standard nitro fuel tanks adapted to gas tanks.
- Center of Gravity (CG) adjustments can be made easily because the bag doesn't have to be permanently mounted like standard tanks, and can be moved based on wind or water conditions.
- Bags can be easily removed from the boat after running and stored separately, almost eliminating the gas smell from your boats completely. I store my boat in my office where I can look at it throughout the day now that it does not stink!
- It is very cheap and easy to retrofit your boats with Bags. New installations are even easier.
- Racers like to keep the changing weight (gas tank) as near to the fore-aft CG as possible. This way the boat handles consistently regardless of the gas level in the tank . You don't want your boats bow to get lighter as the water gets rougher in a race.
- IV Gas bags allow the CG of the gas tank to be placed very low in the boat. Hanging a bag in the boat may negate this advantage depending on the placement of the bag and how much fuel is used.
Disadvantages:
- A fuel pump is needed to fill the Bags. I purchased my Dave Brown "6 Shooter" for around 15 Dollars. Nice electric pumps are available for a few dollars more. Other hand crank fuel pumps are also available. Make sure that the pump is designed to handle gasoline (most nitro fuel pumps are not compatible with gasoline). In a pinch, you can convert a soda bottle (well rinsed) into a pump; just drill and attach a barb fitting to the cap. Fill the bottle, cap it, attach your fuel line to the fitting, tip it over and squeeze to pump fuel.
- Bags may be more prone to rupture and harder to keep away from sharp or hot objects like your pipe or engine. This is dependent on your setup, and can be almost completely negated by thoughtful installation. Note that a bag with gas in it can contact the engine crankcase and won't melt; the fuel acts as a heat sink to keep the bag from melting. To be safe, you can also use a 1 liter soda bottle split lengthwise to cradle, or even cover the bag.
- You must be aware of how much reserve fuel is left in your bag; there is no advanced warning before running out of gas, like the sputtering of a sloshing almost empty conventional gas tank.
You will need an appropriate length of Tygon tubing, a Bag of suitable size, a short length of 5/32 brass or aluminum tube, and a zip tie (or similar tubing clamp). Note: It is very important that you do not use silicon tubing for gasoline! Silicone should only be used for water lines on your boat!
Once inserted, secure with a zip tie wrap. Make absolutely sure the zip tie is as tight as you can get it, because if the bag is going to leak, it will do so at this connection! Your bag turned tank is now finished and should resemble the photo.
Plan for success, then mount the bag. Mounting methods basically fall into two categories; Lay it flat in the hull, or hang the bag from a hook mounted under the deck or side of the boat. If the bag is laid flat in the bottom of the hull, it may be necessary to remove all the air from the bag – called "burping" the bag. A flat bag install does allow the bag to be positioned under the motor, closer to the CG of the boat.
if the bag is hung at an angle or vertically, with the outlet facing down, there is very little risk of having any air ingested into the fuel line. This can eliminate the need to burp the bag, and simplifies filling. But, it might mean positioning the bag in a traditional location (similar to a hard tank). A good combination is to place the bag under the engine , outlet facing the transom, and elevate the bow end of the bag. A 1000ml bag works well in this case.
In the photos above, you can see a flat mount, using a zip tie with a lock on each end between the stringers and through the hanger hole in the top of the bag. This particular install uses a quick fill device, but this is for convenience only and not a necessity.
In these photos you can see two different "hanging bag" installs to get a good idea of what it takes. This one uses a rubber band to keep the bag from swinging around while the boat is under way. The dark barbed tubing coupler makes a convenient place to hook up the filler line from your gas pump when filling the tank.
ed. Brad Mahs (www.modelgasboats.com)
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