Location of fuel tank (gas)

My plans call for the placement of a 100 gallon gas fuel tank pretty much in the exact center of the boat. It will be an aluminum tank or a composite one. The fittings will be grounded and no electrical lines will run through this vented space.

I have extended the cabin and this space is now under part of the enclosed living space. Should I worry about this and relocate the tank position? Thanks much.

 

What do you mean by "this space is now under part of the enclosed living space"?

Will there be any openings between the enclosed living space and the space the fuel tank is in?

 

The enclosed living space ends 3' aft of the fuel tank placement with a 26" door.

 

Will the fuel tank be accessible or visible from the enclosed living space?

If so can you build a box around the fuel tank to seal it from the enclosed living space?

 

I have some questions:

Are there any openings between the tank compartment and the living space? Are all parts of the fuel tank enclosure sealed to the living space?

How are the fuel fill and tank vent routed to the tank?

 

As long as venting and fill requirements are met, you'll be fine, assuming the tank is well secured. The more important issue is, what will the placement of this tank do to your trim. If the tank is basically in the stock location, you'll probably be fine. If the tank is positioned well forward or aft of the stock location, you will need to look at how this will offset your trim. If you have the spec's for your design, this is fairly easy to figure out and adjust for.

 

In ships that are very sensitive to trim, I mean that a small variation in the weights does change their trim, it is convenient to place the weights consumables near the longitudinal position of the water plane area cog. imo.

 

The tank will be sealed from the cabin. There will be a sealed hatch to get to the top hardware on the tank. The vent comes down the port sidewall and the fill down the starboard side wall. Tank is well secured. All passing electrical wires are up high in the side walls and all are in solid conduit. I do not believe weight and balance are a factor because the hull under is a Dory hull and flat.

 

I think everyone agrees that a gasoline tank that is placed in the living space (where there are often sources of ignition) should be sealed from that space. Understanding the USCG rules is not always easy.

Such sealing often prevents fore and aft ventilation and water flow. One solution to that is to mold in a path under the tank to provide that free flow of water and air. A half section of PVC pipe glassed over running completely through the sealed area works well. Another solution is to install a second bilge pump forward of the sealed space and vent the air is some other way.

Aluminum tanks are fine but do often have corrosion problems, especially with ethanol. An accepted practice is to not have the tank resting on anything that will allow accumulation of moisture against the exterior tank wall. In addition it is very good to seal the aluminum tank with at least two coatings of protection like Interprotect or similar.

Composite tanks are a very iffy proposition with ethanol and even USCG approved plastic tanks allow some penetration of vapor which can smell so these practices are good whenever a gasoline tank is installed below a boat sole.

 

Thanks everyone for your comments. I,m going to relocate the tank 3' aft under the outside deck. I will feel a little safer doing that and I do not think weight and balance will be much of an issue with this hull type. Again, thank you all. Stan

 

And when shopping for tanks be sure to review the oem section of Moeller

http://www.moellermarine.com/oem/fuel_tanks/fuel-tanks/
And oceanlink
http://www.oceanlinkinc.com/marine-tanks/

 

Michael,

Thanks for the heads up on changes to Moeller tanks.

Glad to see that Moeller is making some changes to help with the vapor transmission problem of the older tanks. Now, if they would exchange the one I replaced in my boat a couple years ago that does smell. A couple builders of my powerboats did not report any smell and perhaps they got the new nylon laminated designs.

Still good practice to isolate the tanks though, whether inside the cabin or under the cockpit..

The old, and very popular with manufacturers, practice of encapsulating aluminum tanks with foam is a bad one and discredited now due to many failures.

 

I you are considering an aluminum tank, do not use brass fitting into the aluminum as the dissimilar metals will cause corrosion. Use stainless fittings.
Build the tanks with larger fittings that are needed. Ie if you are using a 3/8 pipe, weld in aluminum female flanges in the 3/4 inch range. Install a 3/4 inch to 3/8 stainless reducing bushing and consider that once you have done this, it will probably not come out without galling the aluminum which, when done, would need another fitting installed on the tank.
So when the bushing is in place, you can install or remove the 3/8 fitting to which you will attach your hose.

Also, ABYC has very specific guidelines for which to install an aluminum fuel tank.
I do not have my standards handy, so perhaps another builder who knows these requirements can offer this information. Basically, the tank should be on a material that will not get wet and stay wet, normally in strips so if water gets between the strips and the tank, the water can egress. Something like a series of urethane belting should suffice. Additionally, I believe that the tank has to be painted so corrosion can not start due to standing water. The top of the tank should be slanted so water can not stand on it.

ABYC standards calls for draw tubes as well instead of a draw off the bottom of a tank so that in case of a fire, if a fuel hose melts, the tank will not fuel to the fire under the pressure of the tanks head.. I would find a spot where you can actually run the draw line up above the tank say 12 inches then back to the engine

I would use a 2 inch filler pipe as compared to 1 1/2 which is quite standard to help with easy filling.

The breather fitting must come off the tank and the hose be slanted up toward the breather fitting on the boat hull. If your tank configuration is long and narrow, consider putting a breather fitting, stainless, on each end of the tank, join them with a T then up to the hull breather fitting. With this config, you will ensure that vapor can escape in any bow up or bow down attitude with a full tank

When mounting the tank, strap the tank, do not weld a flange to the tank and bolt it in place. Build the straps to suit the cross section of the tank, glue say a 1/8 - 3/16 inch urethane strip onto the strap or tank and attach the straps to the boat. The straps will allow the tank to flex a bit when the hull moves without causing a stress concentration which can cause work hardening and cracking

Tig weld the tank if possible or at least ensure that when the welder has finished a mig bead that he grinds out the crevice crack that will occur when he stops a bead. With Tig, a welder should be able to run a continuous weld without this problem. You can probably google, crevice cracks in aluminum at weld bead termination or something like that. Some welders will just weld over top of the crack hoping that the weld will hold. Of course, they will not be out in a 50 mile an hours storm, 20 foot waves with you.



Re baffle baffle baffle
Aluminum work hardens very easily. Work hardening means that if a material is repeatedly flexed back and forth, even within its elastic range that eventually it will harden and begin to crack. If weight is not a factor, I would build it out of 3/16 inch, tig weld, and baffle baffle baffle so the panels do not have the ability to flex, work harden and crack

 

Actually stainless steel and aluminum are not friends.

If possible always use normal steel fittings with aluminium. .

 

Michael
AYBC's standards state that 300 series fittings are approved to attached sending units and act as a galvanic coupling barrier on aluminum tanks
Upon which standard do you say that this is not the case