Converting water tank to fuel tank and vise versa

[Davie J.]

All the Amel boats have very large water tanks and much smaller fuel tanks. Although the fuel tanks are adequate, during a circumnavigation, fuel can be hard to find and the quality is sometimes questionable. With a water maker, i can make all the water I need and store it in a much smaller tank than has been standard equipment on most boats of the past. Would it make sense to switch the use of these tanks? If I did so, I could have in excess of 200 gallons of fuel.

[Calif.Ted]

You would have to replace the water tanks with fuel tanks.

[Davie J.]

I know that water tanks are generally stainless and fuel tanks are commonly black iron. But, why wouldn't a stainless tank handle diesel fuel? Could one clean up a black iron tank sufficiently to work as a water tank? Is there a rust problem and, if so, could just the water tank be replaced?
This is probably not worth the effort, but I've been self-isolating long enough to come up with crazy questions. After another week, who knows what conjecture might materialize...?

[Zanshin]

I opted to switch out a 500l water tank for a fuel one in my Jeanneau. My tanks are plastic, and the blue water tanks are not approved for fuel and had to be changed to a black colored one. I am not sure what material Amel uses, but suspect that if the water tank isn't the right material then switching contents is contraindicated. I do remember when looking into buying an Amel that the tank level is shown with a simple mechanical display and those materials would also need to be diesel-fuel resistant.

[a64pilot]

Three weeks from anywhere your water maker will break a part that you don’t have, and then you may wish for that big water tank.
I’d bet whatever the tank is made from it will be compatible, That neat float on a stick you May have to remove as it could leak.
But you can never go back, once it’s a fuel tank your not putting drinking water in it ever again.
If you really, really want more fuel, I bet there is a place on an Amel that you could fit a bladder, with a transfer pump.

[Valmika]

I have a 40 gal water tank right next to a 40 gal fuel tank in my aft cabin. That would make it easy to connect and fill. Water tank is plastic, fuel is aluminum I would like to do the same I think I would have to have an aluminum tank made and drop it in to the molded in water tank area. I'm not sure of the structural ramifications though. I have an 80 gallon water tank forward that I figure could get me where I need to go if watermaker stopped working.

[walidada]

I have a Classic Swan 47 -1977 now restored. She has One main tank 190 litres two Second tanks 90 litres 2 smaller tanks 60 litres. All stainless steel. I joined the two final tanks (60 litres) to the main fuel tank either side with a stainless tube welded to the join making into one tank on each side. increasing the tankage by 120 litres.
Never had any problems except to slow the filling so the two tanks on each side compensate.
Walidada SS 47 1977

I have an echoic water maker 82 litres per hour off the engine. Amazing reliability.

[jmschmidt]

As some have suggested, buy readymade plastic diesel tanks if they are a reasonably good fit in the space occupied by your existing metal ones. If not, have them custom made, they are not that expensive. Get exact dimensions by making cardboard mockups. Depending on location, you may need a small cheap electric fuel pump or two to transfer fuel to your regular tanks or directly to your engine. Save or sell the metal ones.

[NorthCoastJoe]

I am putting in a day tank for fuel, fuel transferred from the main tank will be filtered first. I will know that my fuel is clean going to the Racor before the engine. If my transfer filter is fouled it will not be a problem to change it while underway. Might be a little overkill. If you are worried about bad fuel this might be a way to go. My Downeaster can carry 200 gallons of water and I have no plans to change that.

[Davie J.]

Originally Posted by walidada

I have a Classic Swan 47 -1977 now restored. She has One main tank 190 litres two Second tanks 90 litres 2 smaller tanks 60 litres. All stainless steel. I joined the two final tanks (60 litres) to the main fuel tank either side with a stainless tube welded to the join making into one tank on each side. increasing the tankage by 120 litres.
Never had any problems except to slow the filling so the two tanks on each side compensate.
Walidada SS 47 1977

I have an echoic water maker 82 litres per hour off the engine. Amazing reliability.

I think you have an echotec water maker, correct?

[Lepke]

I don't believe you can clean a fuel tank enough to use for drinking water. In the military we had a water storage problem on a remote base. The college graduates decided 55 gallon drums that had previously held diesel could be cleaned and used for water. You could taste the diesel after several cleanings, many got sick. In the end, we let the officers drink it and got our own water sources. Or we drank other liquids.

[Kd9truck]

Imho , I need water to live, I would not convert a fresh water tank into a fuel tank, you can never as previously stated change back from fuel to water. I use my 100 gal FW tank in the bow as ballast in heavy seas that’s roughly 1,000. Lbs. I also have a method of capturing rain water and funneling it directly into both bow and stern tanks. I run the main head off of fresh water and never have Oder issues, I run my salt water head offshore, and only offshore.
The big question is, how many miles do you get out of your current setup?
I can get 400 nm out of my 57 gal of fuel tank, I do not know what off shore usage would be other than charging and running systems. Unless you plan on motoring when the wind is down how much do you need? How much fuel do you need to get your vessel out of harms way of the center of something ugly? Smaller vessels have smaller tanks because they are not intended/designed for longer voyages, no?
Gerri cans, a fuel drums, bladders, above deck or bellow, close to the center of the vessel as possible and supper secure.

[waterman46]

We had three water tanks on my Morgan 46. Overkill. Aft water tank, about 60 gallons, is aluminum. I converted it from water to diesel. This increased our upwind motoring range to 800nm. Been very thankful for that, since it avoids stopping for fuel on the Cabo to San Diego bash. You don't want to stop for fuel when you have less than 15knots headwind, as that is the time to put pedal to metal.

Our remaining water capacity is about 200 gallons. That easily lasts us 2 weeks with showers every day, more if we really want to rough it. Water maker broke down, still have not even missed it.

When converting existing tank to diesel, don't forget you will need to add the pick-up tube, and be sure to check the vent works OK. But the biggest headache was, we had to remove the tank and have the water outlet on the bottom, welded up.

And when using any diesel tank not part of the original installation, you need to remember to either drain/pump it into the main tank, or do not run fuel from the auxiliary tank directly to the engine until you have used up at least half of the main tank first. That is because the fuel return line from the engine will empty only into the original tank, so if was full already and you used the auxiliary tank, your main tank will overflow.

We used the existing electrical priming pump, with added valves and plumbing, to allow that pump to be used to empty the auxiliary tank into the main tank. Since the aux tank is higher than the main tank, once the flow gets going we can bypass the pump and let gravity do the work.

[AndyEss]

Originally Posted by a64pilot

Three weeks from anywhere your water maker will break a part that you don’t have, and then you may wish for that big water tank.
I’d bet whatever the tank is made from it will be compatible, That neat float on a stick you May have to remove as it could leak.
But you can never go back, once it’s a fuel tank your not putting drinking water in it ever again.
If you really, really want more fuel, I bet there is a place on an Amel that you could fit a bladder, with a transfer pump.

I’m with the Pilot on this one. You’re on a sailboat, you have transportation if the wind is blowing. You run out of freshwater (mechanical things do break) and you’ll be wishing you didn't kill that albatross.

We transported JetB in sling loaded bladders over the polar ice pack below our choppers. Good bladders can take a lot of abuse.