Who makes polyethylene fuel tanks?

You MUST leave 3% for poly tank expansion all directions.

You mean 4' long, not high?, but a 3% allowance is like 1.5", so you need to be a bit precise about maximums. A 48" length is only really 46.5" tank allowance. A foot deep is fairly shallow; does the include an allowance for filler neck? Have you considered putting the tanks under a console or are you steering from the side. A center console allows you to keep the filler above the sole and so you pick up 2" in depth. Just an idea. If the tank fills can be under anything; even a seat; you could run the plumbing up over stringers even as you must vent overboard and you get 2" deeper tanks.

 

I think you need to back up and snap some pics and think options for tank placements cuz I am not seeing good solutions in Moeller and your dims are giving me much smaller tank sizes for max customized aluminum tanks.

A space 48x36x12 allows the following tank.

46.5l
35"w
9.5"high

This is 66 gallons maximum. The 36" dimension will need span help underfoot, you have two inches to build or add a 2x8 flat.

Don't disconsider fuel tank under seats or revising things. Right now you are only looking at about 65 gallon tanks unless you already built in allowances which is just over 80 gallons max.

 

The 64 gallon tanks are what I used, but I had the vertical space. You might need to rethink your seating arrangements in the cabin so that a couple bench seats are fuel tanks or an aft bench all the way back is for the 150 gallon or you are gonna run into big money for really small tanks trying to eek out a few gallons.

 

Not to discount the ability of a "welder" to be able to weld the tanks, you should contact any one of the aluminum boat manufacturers in your area. The mounting of aluminum tanks in boats entails more than
setting them on a surface and bolting them in.

Their are at least two main areas to focus on:
1) The construction of the tank- material thickness, (I would recommend 3/16 at a bit of a higher cost of material and weight) baffles, internal stiffeners, pickup, filler + vent location(s) and sizing, sending units, return lines if applicable. Access to the tank when welding limits welding both sides of a weld bead but there is an easy solution around this.

2) Installation of the tanks-- if there is a return line, and you want to switch between tanks, a special 6 way valve is required, any supporting surface is required to have a proper glued rubber/ urethane/ plastic interface glued to the tank, straps to hold them down are the best fastening, grounding requirements, filler line paths, to be "legal" there are new requirements for venting, marine grade clamps, hose, fitting material, filtration,
pressure test ( keep under 3 pounds, if you end up going to an inexperienced marine tank "welder" PM me and I will check the ABYC pressure limit, too much pressure and baffles, stiffeners can let go, and turn a square tank into a slightly rounded tank as well as break welds)

I would recommend a 2 inch filler hose/filler. Note that many 2 inch fillers may have diesel marked on them but some will not be marked or be marked FUEL
It can a long time to pass 150 gallons of fuel through a 1 1/2 inch line if you have a lot of bends. Most marinas for diesel anyway will have small and large nozzles.


For a welder at a regular welding shop, there lies some risk that they may not get this right the first time and by the time that you figure out all the fitting, interface material, senders, vents, returns and source this at retail, you may be best to find an aluminum boat building company who would take on the build.

I would try Lifetimer or Eaglecraft on the island, though there appears to be quite a few aluminum boat builders in Vancouver, find one that has been around awhile. Any boat builders will have the necessary fittings, pick ups etc to perhaps save some money for you overall.

Note that some of the responders are from the US and maybe using US gallons in their posts.

What engines/fuel are you running? You are meticulous in your build so I expect that you have considered the strength in the area where the tanks are being mounted.

 

Thanks Barry. If I use custom aluminum I'll certainly use an experienced shop. It's literally a matter of life and death.

I'm going to use a pair of outboards. I'm looking at Honda BF115's at the moment. Not finalized. I have a few candidate locations. As I am regularly reminded, my interior framing is pretty heavy. Possibilities iclude between the transom and bulkhead 6, which is that monstrously heavy bulkhead with the door in it. Tanks in that center space, under the swim deck sole. Or vertically oriented tanks on the inside face of B6 either side of the door. Going forward there's F5 and F4, which are those heavy frames. Then B3 which is the forward end of the wheelhouse. 3rd option would be under the sole on either side, between F4 and Bulkhead 3. Under the captain's feet. You see the heavy 2x8 that runs from side to side under the door in B3? That defines the cabin sole fron F4 to B3. Lots of space under there.

 

Raise up the sole John, on the sides only. Be creative with your interior finishing to fit some tanks in there.

You don't need full head height to sit in a chair. Leverage it.

 

Some important items that I missed. I believe it is a requirement for tanks where there is a gravity component of fuel flow to the engine, ie engine inlet lower than the highest point in a full tank to require a
anti-siphon fitting at the top of the fuel tank. The concept being in the case of a fire where the fuel hose is burned through at a low spot, the fuel will not siphon into the fire. They are frequently omitted.

With the rather wide length configuration in your specific tanks, you may want to have the tank bottom have a slope in it to accumulate the fuel just below the fuel line pick up.
This will inhibit losing fuel pick up in heavy seas and a low fuel level. The down side of this is that any junk left in the tank will accumulate in this low spot, but most pickups will have a screen on the bottom.

Which then leads to being able to remove the pickup for screen cleaning. ( which might be never or certainly many years into the future) Stainless or aluminum fittings are the only fittings allowed under ABYC to be attached to a fuel tank, never brass or plastic.

If you screw a stainless fitting into an aluminum threaded collar that has been welded to the tank, more than likely if you remove it (like the pick up tube) the threads in the tank will gall making the tank fitting unusable again. We would weld an oversize aluminum collar onto the tank and install a SS stainless bushing then install the proper fuel line fitting/spud/barb etc into the bushing. So if a removal has to take place, we would use
one wrench to hold the lower bushing and one on the fitting would keep the aluminum collar from being damaged.

Same goes for aluminum fittings ie the fitting could gall the bushing. If it does, some careful work with a punch to collapse the bushing away from the collar will keep the collar intact to accept another bushing

If using stainless fittings, use only 316.

If I get some time today, I will send you a sketch of the way that we built our tanks. As our jet boats often would be in "firm" contact (high G's) with river bottoms at high speeds, we did have a tendency to perhaps overbuild the tanks but we believe that we had some better methods to provide a stronger weld bead area as well as being able to weld the inside of most of the seams.

 

I can certainly do that. And I might. I went to outboards because the internal volume sacrificed for a pair of engine boxes was too much. I am loath to make awkward intrusions. But that might be the way to go. I'm expecting preliminary quotes on aluminum tanks. If they turn out to be too much more than off-the-shelf tanks, I'll definitely build soles around the intrusions.

 

I did mean 48" high. Cover most of the wall either side of the door in the pic.

 

Here are better views of my choices. The 4H 3W 1D tanks against the inside faces of the big bulkhead either side of the aft cabin door. The bulkhead is 4'11" bottom to top. Fillers out through the bulkhead so I can fill the tanks outside, but with overhead cover.

Or under the soles forwarded, under all that junk I piled up when washing the hull last night. Dirty footprints...

 

Why not here?

 

Massive intrusion on living space. I mean maybe. But there are less intrusive compromises.

 

Unless you go up even more making storage space and a chart table.

 

I'm not convinced the far aft centre tank wouldn't work.
It's not ideal being so far aft of the CoG but it has so many advantages.
I've gone through numerous scenarios in my wee little brain and they all work pretty well.
John's going to be shocked.
But I thought he needed to explore his options.

Ad Hoc, you lurker, pipe up man.
All I have is steering wheel experience, and enough design experience to get me in trouble.
Most of what I have is feeling, your nemesis.
Well, he's a good friend of mine, I trust him, a lot.
He's gotten me through some shady times at sea.
Where do you think the tank(s) should go?

John, how high are those vertical 1" ply stringers in the aft hold?
What is the tank clearance height?
Hmmm, that's going to need to be ventilated.
Is there a factory tank available of suitable size?

 

Firstly John needs to establish the LCG of the lightship.
Only way he can do that is by listing all the items on the boat, and using a common datum...on small boats it is usually the transom...and establish the lever and hence moment, and then divide by the total weight, thus giving the LCG from the transom.
Then he needs a set of hydrostatics of his hull form.
From the LCG calculation, comparing against the LCB, from the hydrostatics, can you judge where the tank location is best suited.

Ideally, you do not want excess trimming from full to near empty tank, thus the only way to gauge where this is, is via the weights & LCG checks. Since when stationary, you do not want the boat trimming by the head. So firstly you need to get it at level trim - this is in the full load condition..and then again, check when in light ship condition.
The change in LCG from full load (i.e with the tank full too, to lightship LCG) may or may not be problematic. You may require moving some heavy items about to correct any excess trim. It may suit being slight aft, but in the absence of hard data, not so easy to state as such.
Then, looking at the hydrostatics and seeing the effects of the change in weight (displacement from lightship to full load) has on the location of the LCB.
From this, you can get a pretty good idea of where the ideal/preferred location will be.