Composite Gasoline Tanks

Does anyone here have experience making composite gasoline tanks for trailer boats? I'm interested in learning anything/everything about it. Resin types, cloth types, etc etc.

Full background details below.

Why am I even contemplating a composite Gasoline tank?
I found this YT video, How I Made My Kevlar Fuel Tanks - YouTube where a boatbuilder called Dan Lee makes a pair of Kevlar tanks for his trailer boat.

He made them in min-2017 and commented in 2022 that they were still serving their purpose. He says in the comment that he made them using plain old West 105/206 Epoxy Resin. I was surprised by this as I would have thought the gasoline would attack the epoxy. However, Dan's experience suggests it was fine and he seems to be a credible boatbuilder.

Why not just get a metal tank?
I have a 6m aluminium trailer boat powered by a 175 Suzuki. I need to fit a new underfloor tank, ASAP. To avoid galvanic corrosion if it's meatal it needs to be made of aluminium, not stainless steel. The trouble is that aluminium plate is super expensive, these days. Tank fabrication has a long lead time too where I live in a small town in New Zealand. Off the shelf plastic tanks aren't a suitable fit for my boat and the only custom plastic tank place I know of is a full day's travel from where I live. So ... I'm, up the creek without a boat. Literally - I live beside a creek and I can't use my boat at the moment.

Yeah, but can I really make a fuel tank?
I'm comfortable that I could make a tank, like the ones in the video I've linked to above. I've made composite parts before using epoxy and fiberglass as well as epoxy and carbon fibre, including conventional hand lay, vacuum moulding and resin infusion. Mostly this has been things like fish boxes for my fishing kayaks, and storage boxes for my boat. I've never made anything as big as fuel tank (I'm think of a 150L tank) but in some ways bigger is easier than smaller.

I wouldn't use the disappearing foam approach, myself. That's just too scary for my liking (flammables etc). I'm confident that I could successfully fabricate a tank in a slightly different way, that would allow me to remove the EPS plugs physically.

 

Check with your local resin suppliers, plus the websites of the resin manufacturers, they'll have lists of gasoline approved products. VE resins can do a very good job if you want to go that route.

You don't need to use foam, there are many simple ways to build a tank.

 

If I hadn't found a heavy guage aluminum tank that served my needs, I definitely would have built a pair out of plywood, glass, and ethanol compatible epoxy.

 

Make sure the local authority or surveyor is also okay with the idea. Doing a lot of work and finding out the tank is not allowed or does not meet other requirements is misadventure.

 

I'm in NZ, where there is no applicable regulation for permanent tanks in recreational vessels. It's the Wild West!

 

In that case, safety should be your No.1 priority.
If you decided to fit, like the tank in that video you linked, make sure you have a layout of fire protection around it..and a means to remove spillages etc...as well as easy to clean surfaces/voids.
Will you fit a vent too?

 

I'd be interested to hear how you'd approach.

I have couple of ideas, but I'm always keen to hear other people's approaches.

I'm trying to avoid using board (heavy/reduces internal volume) and an external flange (reduces overall volume).

For me, the complicated parts are incorporating vertical baffles, and enclosing the final side(s), after removing any plug(s). Without support, the final side(s) will want to sag. But any support left leave in will be enclosed.

I've landed on laying out the final side(s), including tabs onto a piece of melamine, pressing the rest of the tank onto it and folding up the tabs. Once it's cured, wrap with more layers

 

Draw it

 

No fire protection around the current tank (alloy). The possible source of fire is the bilge system. Nothing else sparky under there.

I could build a box to receive the tank with separate drainage (not via the bilge). My chance of making that fume proof are slim though - no way to seal it at the top

Vent: If you mean the underfloor space, no. It's a 6m trailer boat. I don't think anyone has ventilation in those spaces (passive or active). If you a mean breather for the tank, yes. I have two breathers (front and back).

I could put some passive ventilation into a fuel compartment. Active seems too complicated as it would need to be intrinsically safe.

 

I think the 'hint' there is the "alloy"... i.e. non combustible.
Unlike the composite!

 

My company has built a lot of aluminum fuel tanks to USCG requirements. As evolved and detailed as those requirements are, I could see building an approved tank in composites to be a challenge.

I'm in the midst of repowering a 1962 41' Hatteras sportfisher. We removed and scrapped the original composite tanks as part of that. Very well made pieces - thick layup with multiple internal baffle bulkheads - but the ethanol compatibility was always a known concern and we wanted to eliminate that concern for the future. Interestingly, perhaps, the market value of the older Hatteras and Bertram sport boats that both had built-in composite tanks originally is negatively affected if the oem composite tanks are still in place.

 

I would think metal tanks are a better idea, but not because of concerns about ethanol. After all, they have used GRP tanks to contain bulk acids before now. It surprises me that fuel is so cheap that running a 175 Hp engine is actually viable, is there any kind of insurance penalty? Here in Europe you would be paying $5.20 to $7.93 per US gallon.

 

Ethanol in gas has wreaked havoc with the composite fuel tanks in older vessels (and subsequent engine damage often resulted). There are many, many reports available on the subject. Ethanol in gasoline is nasty stuff. Yamaha went so far as to put out an official position when it looked like E15 was going become the new "standard": no warranty for you if E15 is found in your engine that's in the shop for any repairs.

Results of BoatUS Sponsored Fuel and Fiberglass Gas Tank Tests https://www.boatus.com/expert-advice/expert-advice-archive/2020/january/results-of-fuel-and-fiberglass-gas-tank-tests

https://www.thehulltruth.com/boating-forum/67197-ethanol-dissolving-fiberglass-fuel-tanks.html

 

Caveat: haven't done it

I'd build the tank as a shoebox, bottom, 4 sides, baffles.

Relieve the top edges of the sides to allow for tabbing. Make inside bonding flanges for putties @top.

Bond the top, then lay 2/3 600/225 tapes over the top radiused edge and into the relief. Sand and pressure test 2-3psi.

btw, the top can have all the connections machined into it before gluing, and you can verify the sending unit fit, etc..

I see no reason to laminate the entire box after putting the top on; this would result in poorer work as laminating the corners is difficult and can take days. Your bottom fillets and tabs on the inside will need to seal any leaks anyhow!

The external laminate only serves to strengthen the box and protect any core from boat impacts unless solid frp..

If it were for diesel; a cleanout would allow you to fillet/even tab the inside with difficulty..

 

Chuckles. Only 175HP?? Monsters like this one are becoming common over here..and similar craft with two or three 300HP engines are almost the new "standard" these days.