Material for diesel fuel tank

One of my up coming projects is to add 2 aux fuel tanks to my Gulfstar 37. These tanks will only be used for long passages or if I run across a good price on diesel and want to fill up. The tanks will be all the way aft and be just under the deck. One on each side of the boat. The capacity will probably be 18 to 25 gallons each.
I could buy ready made plastic or aluminium tanks made to put under the gunnels on an open boat. Size 18 gallons. But the shape would take up alot of space under the deck because it wouldn't conform to the shape of the hull. Cost around $150 to $200 per tank.
I would rather make myself or have made a tank that conforms to the shape of the hull. So the question is what are the pros and cons of the different types of material that the tanks could be made out of?
Steel tanks would be way too heavy. So I have crossed steel off the list. That leaves aluminium, fiberglass or plywood/epoxy tanks.
Aluminium I would have to have made or at least welded by a professional welder if I cut the pieces out.
Fiberglass and plywood/epoxy I could make myself. I made my 23 gallon holding tank out of plywood/epoxy so I know what is involved to make a plywood/epoxy tank. I also have friends with a home made plywood/epoxy fuel tank and they have not had any problems with it.
So what can you guys tell me of the pros and cons of my choices.
Thanks for any help you can provide,
Chuck

 

Ordinary grp is ideal. They have been in service for getting on for 50 years and I've never known one fail that was well built.

 

If you send a PM to Greg and invite him to the thread he might find it interesting to make you a quote for 2 titanium tanks.

It would be interesting to know how this compares to the other materials as Allied Titanium says they're almost price competitive* with other materials in the custom market if I understood it well.

Good luck !
Angel

P.S. - * that's compared to 316 stainless steel IIRC, it might be different for other materials.

 

Angel,

I found it myself, but thanks!


If you are interested I can certainly have a titanium tank priced out for you, but for fuel tanks because they have to be completely welded our price will be higher than aluminium. We have made a few in the past, and they are pretty awesome but this is one of those places where fabrication costs are really high.

The flip side though is that because titanium is completely non-corrosive in the marine environment it will last just about forever. So issues like strapping, air space, corrosion tolerances, ect don't need the same considerations. You will also get a larger tank for the given cubic footage. Because we can go with relatively thin plate as compared to wood/epoxy you effectively get a larger tank for the same area.

Just as an example... If you were to build a 2'x2'x2 outside dimension box out of wood and glass, assuming an average wall thickness of 3/4". The tank would hold roughly 54.4 gallons, if you make the same thing out of 1/16 titanium it would hold 59 gallons. The lost space becomes more important as the size of the tank gets smaller, so in a 18gallon tank instead of 10% it might be 15% more fuel storage for the same outside dimension.

To play with tank size and thickness of he walls check out this calculator http://www.angelfish.net/tankvolcalc.php it only works on rectangular tanks though.

Personally though I think the best option for you sounds like fuel bladders. Not that I love talking someone out of my products, but they are cheap, will form to the boat shape well, and since these are not normal use tanks, the fact they have baffling problems isn't a huge issue.

 

Titanium fuel tanks. Interesting but don't think that it would work for me. The boat is in Mexico and the tanks will have to be made in Mexico. I am leaning to fiberglass. How thick would the fiberglass have to be? The tank will be around 36 to 42 inches long and 10 1/2" wide at the top. The depth of the tank and the width of the bottom are dependent upon the curve of the hull. The 10 1/2" top width will just barely pass through the cockpit locker opening and the length is limited by the existing deck fills for the water tank and the main fuel tank.
Still looking for more info on the pros and cons of using aluminium, fiberglass or plywood/epoxy.
Chuck

 

Polypropylene would be my choice although not many people are familiar with custom made tanks. Welded in place it would give you a nice fit and keep the cost down for small tanks like that.

 

Thanks for the polypropylene suggestion. Don't know if the material is available in Mexico where the boat is. Also finding someone who could weld it might be a problem. Did a google search and found that 3m has developed a structural adhesive, 3M DP8005, for gluing polypropylene together. Anybody have any experience with 3M DP8005?

 

Chuck,

I don't know anyone who has ever built their own polypropylene fuel tank. Typically they are extrusion welded, which is a pretty specialty process, not glued. I can't say they couldn't be built by glueing, but it would be a first for me.

 

I had not even considered a polypropylene tank for the reasons that you mention. I like to research things and just happened to run across a reference to 3M DP8005. I found another reference where a guy had used it to repair a water tank. A repair is a lot simpler than building a tank. So I don't know if it is feasible. Also doing it in Guaymas Mexico presents another host of problems. Aluminium, fiberglass or plywood/epoxy are all doable in Guaymas. I am leaning towards either fiberglass or plywood/epoxy. I like to build things and I know that I can build either a fiberglass or a plywood/epoxy tank. I just don't know which would be better. That is the reason for starting this thread. Thanks for your response.

 

Chuck,

The one issue I see with plywood would be if it not completely encased in epoxy you will get diesel absorbed into the wood. I have no idea what the effect of this would be, but I assume it would be pretty destructive. But I have nothing to justify that. I woul dbe concerned with the fuel dissolving the glue in the plywood, any chemical effect it might have, ect. But if the epoxy sheathing is complete I don't see an issue with it. The only difference between this and fiberglass would be the bulking material.

Take a look at http://duckworksmagazine.com/04/s/vintage/sbj/011/index.cfm where they build some wood tanks.

 

But who in their right mind would do that?
I have 145 gallon tanks built from gabboon ply-epoxy in my boat and they have 1/2 inch walls and 3/8th baffles.
54 gallon tanks could easily be done from 3/8th, probably even 1/4inch if baffled correctly.

 

I used 1/4" plywood for the 23 gallon holding tank that I made. It was coated inside and out with 6 oz fiberglass cloth. The corners had a fillet made of epoxy thickened with cabo sil and were re-enforced with fiberglass tape inside and out just like you would make a tape and seam dinghy. I followed the instructions in West Systems book on boat construction. I also put in a baffle. That tank came out so strong that you can stand on it. So I know that I can build a plywood/epoxy tank. But would a fiberglass tank be better. I don't know.

 

If you've done it before you should stick to the same method, less chance of surprises.

"That tank came out so strong that you can stand on it" I would hope so, the inertia of 23 gallons in a sea state is not to be taken lightly.

Two schools of thought from me. You can make a self contained tank, or you can make a tank that needs support (more like a bladder). The later is how many people make stainless tanks where it needs a frame to sit in.

If you wanted to make your tanks self contained from fibreglass you are going a core will save you a lot of product and deforming of the panels.