Fiberglass gasoline tanks, can they be done "right" ?

I am going to need an approximately 180 gallon above deck gasoline tank (fuel is carried under the washrail at the transom basically).

Historically as we know, ethanol has caused a variety of failures in gasoline tanks, both in terms of leeching dissolved resin into the fuel system, coatings aimed at protecting the resins flaking off, and the tanks losing integrity due to being eaten away by the fuel basically.

Aluminum tanks, which are the typical alternative, have their own issues. Namely corrosion related leaks and static related safety concerns being metal.

Today the standard in below ground tanks, like for gas stations, has become fiberglass. The correct resins done right have been proven as the best option, I assume largely due to the cost of metal tanks and their failures.

I'm aware of some recent gasoline tanks in boats holding up well and my fiberglass supply person has pointed me to an ISO resin rated for nearly pure Ethanol at high temperatures, ie 120 F, and without any fancy cure.

Theoretically it can be done well with the most important part being to start with a resin designed for the task, but I am not sure that I trust it. What does everyone think?

 

There is no prohibition on fiberglass for gasoline tanks. But they do, as you said, have to be done right. The unfortunate thing is in the past many people used plain old polyester resins which do not play well with gasoline. Of course I can point you to lots of examples of failed fiberglass tanks but none of those use resins designed specifically for gasoline If you use a resin that is gasoline resistant you should not have a problem. Usually the determining factor is cost and typically those resins cost more that resins used in production boats.

 

Quite a bit of stern weight. What type of boat?

maybe for starters a name change might be a good idea!

 

I've made a couple of tanks using Isothalic Polyester resins, they gave no trouble.

 

This is the same boat I have been finishing myself for some time, picked it up right about the end of 2016 and got going on it in the spring of 2017. Hull & top were laid up in Maine during 2016. Before moving it into the tent in the background, backwards so I could get the engine in later, we took it off the trailer in the road to pick it up the other way.

A Downeast, 32 Holland. Round chine semi displacement keeled single inboard of slighter dimensions than some, 32 x 10.5, coming in 1 ft at the stern with a flat area for planning aft. 9k lb ballpark and on the fast & efficient side of these hulls (attain cruise speeds of 18 to 22 fairly effortlessly in a 2 MPG ballpark, can typically reach 30 with 400 HP).

There is a 2 ft trap deck at the transom, basically the gunnel there is very deep, intended to house the fuel in a rectangular above deck tank, fairly common in these designs until you get up to slightly larger boats where they are deeper.

I'm not sure why the balance works out but the engine weight is set at about half the length of the boat normally, sometimes further aft using a V Drive, sometimes further forward to place the engine all the way into the cabin for a commercially used one. The builder typically puts 100 gallons in them, and not uncommonly has put 150 gallons in the same place; I raised the deck over standard and am pushing the limits with 180 or possibly 200 even, but will not use all that capacity if the boat is unhappy with it. Sometimes they have saddle or below deck tanks, for me the saddle tanks I feel get in the way more than a big single at the transom and being the uncommon design I wanted to create less balance questions; the below deck tanks in these end up being rather shallow and long, very non ideal as far as having the fuel running forward at low speeds and aft at high speeds. So in short I did not stray for the original design and it will have a single tank at the transom.

 

Thanks guys!

Do you have any feeling or guideline for the schedule on a ~ 7 ft X 2 ft X 2 ft rectangular tank in this ballpark?

I feel like I would probably go at it shooting for 5/16 or greater thickness, some alternating CSM & woven like any layup, with a rich layer of waxed CSM on the inside.

Is there a requirement or standard that the bottom 5 sides of the tank are molded in one piece (with the top as the only secondary bond) or is it as sane to build such a tank out of 6 independent fiberglass panels that are then tabbed together (and maybe only then built up to the full intended layup in the process). After having built a couple smaller non-gasoline tanks I realize when using a mold the trick is to be able to see any quality control issues ("air") since it's hard to get the right perspective looking down into a box, but then when building a tank from independent panels there is a tendency there for the panels to develop camber along any unsupported span as you build them up.

 

Thinking about this some more I suppose I could build a mold as a 2 part plug for the bottom 5 sides. Then I could both be able to remove the plug and could build it up to schedule in a single piece fairly rapidly. Is that most ideal?

 

I have no experience with the tank building, but I think you are missing baffles. If you don't baffle; won't she get a little wild on a half tank in a turn?

And since I would baffle, the only way to build it would be as described. The bottom sides are all laid; the baffles inserted and bonded; then pop it out and cap it off.

I would also guess you want to tape the cover on and I would probably mould the cover the same way; just bigger. Then bond the cover on and tape the seams well. Depending upon space considerations, you might want to put a step in the top of the big mould. Then the bottom of the big mould would be usable. Think like a 1/2" step with a 45 degree angle. Keep in mind, the demoulding might be more difficult this way...might need to make it breakable so the big mould becomes the cap mould or some such.

Hopefully others with experience will coin in.

Baffles also reduce the laminating needs..

Also, if I confused you, I can draw the idea. I was sort of posting while thinking there.

 

Thanks and oh no doubt I understand it will need baffles. It may even be dangerous without them.

I am not picturing the idea of the step in the mold and such, sounds like it addresses problems so I definitely would appreciate any further description there.

What I have done in installing a top so far (this is a small diesel tank) is I layup the material on a waxed table, then I set the tank down into the wet resin. Once it cures I remove it and use a sander to trim the excess edges and prep the outside of the tank, then by hand I round off the top edges to permit tabbing the top down nicely and I tab it on.

The pictured tank is made from separate glass sheets and was tabbed on the inside already but not the outside yet in the picture. I will round all the outside edges and then tab it 360 degrees, so basically other than the top it will be tabbed both inside and out.

 

This is a 10 second sketch that shows the bottom and one wall of a mould. The top of the mould has a piece of say 1/2" (tbd) thick material so when you mould the bottom; you create a place for the lid to fit. By doing this and putting a break at the red arrow; the tank won't be stuck in the mould and can be demoulded. Then the bottom of the big mould is used to build the cap, and the tank is well sealed by the bond and some tape on the outside. You could go further and relieve the tape seam with more fillers if you are tight on space. I can draw that as well.

Clear as mud?

 

Ahhh ok no I get it now. Then you could install the cap so it overhangs the edge and would be able to fill and then round the top edge.

I have been able to get a bit of a rounded edge due to the resin kinda pooling up on the inside when I set the tank into the wet glass that makes the top, and careful hand sanding. First layer of glass tabbing is a harder angle and I just use a layer of csm & let it cure, then sand hairs off. Then its fairly round and smooth and I can nicely run 1708 over it.

 

I'd go with a VE resin recommended for this purpose, they tend to do better in just about every way when compared to an ISO.

 

Not following your reply at all.

You would just sit the cap over the top. So, if you have a 1/4" thick tank; you allow say 3/8" thickness for the step; then maybe an inch for the vertical join; then fill the inch with thickened resin and place the cap. Some grinding perhaps..dryfit, etc. Fill the cover seam; bond a tape and done. A flat lid doesn't have much bonding surface at 1/4" is all...

There would be no need for any outside radius work. You would do that on the mould one time with thickened resin..sand it smooth...paint it maybe or duratec.

 

Oh ok so the lid you are speaking of would actually have a flange, it would be say the bottom 1 inch of the original mold.. Now I think get it.

I thought you were adding the step as a rebate to hide some filler or tabbing. Btw I think what you call taping I keep referring to as tabbing. Am not sure of the difference in any context but if there is one point it out ..

 

Thanks!

I am open to that, I really like VE, especially when there will be these secondary bonds like the top and fittings (that's another question, normally I would do PVC fittings glassed on, is that still ok w/ gasoline? The fill for instance I want to be on the actual tank).

Only reason I was onto this ISO DION is the fiberglass supplier stocks and recommends it. Any recommendation on one to seek? Vipel & Interplastics are two manufacturers where the whole line is probably available easily.