Convert Water Tank to Diesel?

[GulfportBarb]

We have a Beneteau 461 with 3 70 gal water tanks. We want to convert one to diesel. Our boat only carries 50 gal of diesel and we want to extend our range. Is it possible to use the existing (plastic) tank for diesel? I am guessing installing a switch / manifold to switch over tanks would be required, but what else? Obviously isolate the tank from the water system! Thanks

[Pizzazz]

Google it, lots of threads on sister forums. You have to make sure the tank material is compatible with diesel. Then you have a couple of options:

a) use the water tank for fuel storage and a transfer pump to refill your main fuel tank. This way you avoid complicated plumbing, a second Rocor filter, a fuel return line+T, etc. However be prepared for alternative transfer methods if your transfer pump fails.

b) put a fuel bladder in the water tank if you have access

c) replace water tank with a fuel optimized plastic tank

Lastly, if you need the range occasionally, just use a fuel bladder elsewhere on the boat.

[Captsteve53]

Peg Halls reply some years back to your question as:

Nope...won't work.

There's more than one type of polyethylene. Water and waste tanks are made of low density LINEAR PE, which petroleum will permeate in a heartbeat, even "melt" the plastic.

Plastic fuel tanks are made of high density CROSS LINKED PE, which is impervious to petroleum. They're also supposed to meet some other criteria for USCG certification...a minimum wall thickness, all fittings on top, no surface that can allow liquids to pool etc. but that may only apply to tank mfrs, not to individual owner one-offs.

I should add that some of the early waste tanks were cross linked PE...I had one on my '79 Trojan. So you might find one that could be converted, but the odds are slim.
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© 2017 Peggie Hall
Specializing in marine sanitation since 1987


And Vasco's reply same thread as:

A buddy converted one of his his blue roto-molded water tanks on a Beneteau 393 to diesel. A few years now and he's had no problems.
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Rick I

[NakedCity]

Using existing tank if possible or just using that space for extra fuel , a transfer system where you simply store the extra fuel to then transfer to your existing fuel tank is the only reasonable approach.

[TheThunderbird]

Change the tank.

Lovely SS

I turned from fuel to water, no problem (guest head, ahah..)

[Cheechako]

I added tanks to my catamaran. What I discovered is the original amount is more than enough and it was a waste of time. Just keep that in mind depending on your plans. Too much stored fuel just gets growth in it.

[GarthGregory]

My Beneteau 473 carries 900Ltr. of water and has 2 x identical roto moulded tanks of 235 Ltrs each ( all fittings are on top ), in the port / starboard quarter cabins - 1 x black ( diesel ) 1 x Blue ( water ) . they are made in France by Ocean Industrie ( I think ? ) - If I remove the water tank , I create a list issue . If the tank is okay and the piping was changed to fuel type , why not simply connect into a small T manifold with 2 x switches left and right ,for each tank - similar to a gas type open / closed switch - I would install a second Racor filter , as a an extra safety , in case of a blocked filter Algae etc - any comments Rgds Garth

[KP44]

Originally Posted by GarthGregory

My Beneteau 473 carries 900Ltr. of water and has 2 x identical roto moulded tanks of 235 Ltrs each ( all fittings are on top ), in the port / starboard quarter cabins - 1 x black ( diesel ) 1 x Blue ( water ) . they are made in France by Ocean Industrie ( I think ? ) - If I remove the water tank , I create a list issue . If the tank is okay and the piping was changed to fuel type , why not simply connect into a small T manifold with 2 x switches left and right ,for each tank - similar to a gas type open / closed switch - I would install a second Racor filter , as a an extra safety , in case of a blocked filter Algae etc - any comments Rgds Garth

Diesels also need a fuel return line that returns unused fuel to the tank. So it is a little more complex but easy enough.
But don't try to use a water tank for fuel. Take out the water tank and replace it with a same-size fuel tank. For diesel, I would use aluminum rather than stainless steel. Or a fuel-grade plastic.

[David M]

I agree that switching to a tank rated for fuel is the smarter idea. Otherwise, you do not know for sure if your water tanks material, scantlings and design is suitable. Why take the risk?

[jmschmidt]

Remove the water tank and have a plastic one made which meets the specs for diesel. It will come with all the attachment points you will need for venting, gage, fill, transfer, etc. I like a plastic tank (fuel or water) with a clear strip sight gage built in to check fuel level. Save the water tank, you might want to swap it back in sometime.

[David M]

You don't want to have it have it custom made.

Get a fuel tank made to an approved classification society or government agency. That could be ISO or USCG or something similar. If there is ever a fire caused by the fuel tank that is not certified as a fuel tank in any way, your insurance company would have a basis to deny your claim.