Confused about plastic tank install.

[mogfisher]

So I went with a plastic fuel tank for my 20 foot center console. 25 gallons. On moeller's website, in the same paragraph they say to mount it securely and to make any brackets 3% larger to allow for expansion. Seems like if you mount it snug there's no room for expansion and if you leave 3% then it will flop around. What do I do! I plan to lay a sheet of 1/4" neoprene rubber down under the tank, have 2 stainless straps made and line those with neoprene too. How do I mount this thing? I only have 1 shot as its getting glassed in with no access hatch except for the fittings and such. My original plan was to foam it in but moeller specifically says not to because it wont be able to expand. Thanks for any advice.

 

[DeepBlue2010]

There is a mounting kit from the same company. Just Google "Moeller Permanent fuel tank hold down kit" [h=1][/h]

 

[mogfisher]

Unfortunately, this tank doesn't have the nifty grooves for those little brackets. It uses their strap brackets that are made of steel. Hence my desire for custom stainless straps. I'm just confused about this 3%. Are they saying the tank expands once and does not contract? That's what's implied. I think I read that is the gasoline that causes the expansion. If that's the case then it would be snug after expansion if I leave 3%tolerance? I just need to clarify before I glass a deck over this thing.

 

[Jeep Man]

If the straps are up the sides and across the top, then, when expansion occurs the mounting screws will be pulled out. Is there a ridge near the bottom whereby it can be held in by dogs? Other than that, by making up solid brackets that go up the sides and lip the top but not go across would hold the tank in place securely and allow expansion.

 

[mogfisher]

Unfortunately this tank doesn't have those ribs. Otherwise this would be easy.

 

[zool]

Hi Mog,

I have a plastic water holding tank for a pressurized system...the factory used stainless straps with 1/4" neoprene strips underneath, which held it snug, but didn't allow the strap to cut into the tank during expansion...worked just fine for decades..the water tank has no ribs either.

 

[JoshOnt]

I have no idea about the expansion but it sounds like the gas causes it, So what I would do is fill it with gas and it should expand, then measure it before and after letting it sit in a warm area for a day or two. Not sure if that might help or not.

 

[DeepBlue2010]

You are right, the factory says that the fuel will expand it once it soaks into the material that the tank made out of. What I am not sure if the expansion is permanent or not. I would try to get in touch with one of their tech support guys

 

[52FordF2]

I have a plastic tank for my boat. When I did a PSI test, I left it outside. The sun light caused it to keep expanding. The top ballooned up.
I'm planning on putting foam in the corners, with concrete blocks to hold it down until the foam sets up.

 

[Grandad]

I found that my Moeller plastic tank did expand as stated and it was from fuel, not sunlight. I was worried about this just as you are, but since my tank's mounting is accessible, I had the advantage of adjusting the fit a couple of years afterward. I made my own "Z" shaped hold down brackets. When I did my rebuild, I made the mounting holes oval to accept the wider tank and shimmed them slightly higher from the floor. - Grandad

 

[mogfisher]

I wish I had something to grab onto like your tank does. My tank has nothing. Just a rectangular tank. I just got a call back from Moeller and they said to put it on a neoprene pad, make my straps that go over the tank completely and line them with neoprene. They said just to make the straps so that they touch the tank but do not clamp down on it. The tank will expand when fuel is added and will stay at this new dimension. I guess thats what I'll do. I just hope there is no problem because I don't want to cut the floor up because of this thing flopping around!! Its 4 feet long.

 

[gm280]

mog, yes plastic gas cans and fuel tanks do expand but that is because the plastic material is easily pushed out when fuel pressure builds up in them. Look at most any red gas can for yard equipment and there is a size difference between empty and full. But it isn't a huge amount. So install your tank and just allow the straps to just touch the tank when its empty. It won't flop around because when its empty there is little to move it. And then when it is full it will push against those straps and stay in place. One other option is to install a backer plate under the floor and then make an access plate/panel to screw into the backer plate so if you ever have to gain access to the tank. you don't rip up the floor. And it can look very nice if well designed. It won't distract from the boat design either. JMHO!

 

[GA_Boater]

It's not pressure causing the expansion. It's how the gas interacts with the poly-whatever plastic used for the tank. A vented tank won't expand from pressure.

That's one of the things I don't like about the EPA gas cans, you have to keep the cap cracked to bleed the pressure or I fear the tank will split.

 

[Grandad]

I wish I had something to grab onto like your tank does. My tank has nothing. Just a rectangular tank. I just got a call back from Moeller and they said to put it on a neoprene pad, make my straps that go over the tank completely and line them with neoprene. They said just to make the straps so that they touch the tank but do not clamp down on it. The tank will expand when fuel is added and will stay at this new dimension. I guess thats what I'll do. I just hope there is no problem because I don't want to cut the floor up because of this thing flopping around!! Its 4 feet long.

Wow. A 4' tank could expand in length up to about 1-3/8" according to Moeller's instruction spec of 2-3%. But that's about the percentage that my tank expanded. It's about 18" wide and I had to oval both tie-down screw clearance holes about 1/4" each for a total increase of 1/2". That confirms their estimate. - Grandad