Reality Check Please- Fuel Tank

[DogCavalry]

So this is what I'm thinking. What am I forgetting to think about?

 

[gonzo]

What is the tank mounted on and how is it secured?

 

[DogCavalry]

Tank is to be rigidly mounted between the sole and the hull, in front of the transom.

 

[DogCavalry]

My big question is whether or not placing all 4 penetrations on a heavy block that projects above the outside back deck sole presents issues I don't foresee. At the slight increase in complexity I have all fittings not merely accessible when required, but directly observable, while the Aluminum tank is in a completely weather proof environment.
The space for the tank is only 13"/33cm deep. If I put all the penetrations under the sole, I lose an unacceptable amount of tank capacity.

Dang, I miss @Mr Efficiency . He'd be on here already telling me that since nobody he's seen doing it, it's obviously a bad idea. Then he'd wring his hands, and tell us all it will all end in tears. Good times...

 

[jehardiman]

Mounting volatile tanks in a void/cofferdam is typical in shipbuilding. Make sure there is venting/sounding for the void so you know when the tanks starts to leak.

 

[baeckmo]

Good points there. I'd add that the tank should never, repeat never rest on its surface! It must have free-standing supports, fi L-profiles welded to its bottom, which rest on the support structure in the hull.

But I would prefer to have the tank standing free on the aft deck, then there is all the ventilation you can wish for, the connections are easily available; you can even arrange a seat on top of it.

Edit. ......and tanks should not be low and flat, that makes for trouble with sloshing and air in the fuel lines!

 

[DogCavalry]

I have certainly considered a tank mounted in the open, and may still go that way. Off the shelf tanks become a realistic option. But this is a 26' boat, with the space restrictions that implies. If not tanks or a tank back there, then what? It's a lot of volume to lose.

 

[bajansailor]

But I would prefer to have the tank standing free on the aft deck, then there is all the ventilation you can wish for, the connections are easily available; you can even arrange a seat on top of it.

+1 - I think this is a much better idea than having the tanks under the sole like on your Binliner or any other typical small speed boat.

Could you have a cube shaped tank up against the hull side on each side, with a seat or step over it? The seat or step could be hinged, for easy access to the connections on the top of the tank?
How much space do you really need in this aft cockpit? I am thinking enough for somebody to handle mooring lines safely and efficiently?

 

[BlueBell]

What does the design call for?
Oh right, there is no design, this is design-on-the-fly.

What have other boats like yours used for the same purpose under similar conditions?

I don't think at this point it really matters what you do.
Let necessity be your guide.
What do you need the tanks to do?
Make it so. If it doesn't work, you'll find out soon enough.

Why does it feel we've been through all this before?

Did we talk about an aircraft style bladder in one of your holds?

I thought availability and delay were issues, so, what's available?
Make it work.

Plywood Fuel Tanks https://www.boatdesign.net/threads/plywood-fuel-tanks.66598/

 

[fallguy]

Lotsa cabin on that boat. Offset the starboard helm station with a tank aft on the portside that doubles as a bench or boarding step. Seal it off from the living spaces. Done.

 

[fallguy]

What sender is planned? Do you have enough width for it? You will have sender wires running across the deck this way..

 

[gonzo]

If you have a fuel sender, it has to be accessible. I suppose it could be installed in the same location as the hose fittings.

 

[DogCavalry]

@BlueBell , the bladder tank is great idea, but pricey. $1200 CDN for a 50 gallon tank. I'd need 2. Ah, plus shipping. My boat mechanic here is trying to persuade me that ¼"/6mm stainless steel is the way to go. That seems excessive. But your previous idea about getting ethanol proof epoxy and building my own is very attractive. Whichever way I go I will definitely follow your suggestion about putting the filler and vent beside the door.