Converting Water Tank to Diesel

[Tspringer]

My boat has 225 gallons of water capacity and 100 gallons of diesel. I am installing a watermaker, so I can refill my water tanks. I cannot make diesel underway....

I have 2 fuel tanks, one deep in the bilge that holds about 75 gallons and another small tank under the cockpit floor that holds 25 gallons. The small tank drains to the large tank via a single line and a ball valve. The engine feeds from the large bilge tank.

I have a 70 gallon water tank under the starboard settee. The base of this tank is above the top of the large diesel tank in the bilge.

It would seem that the easy solution would be to change the deck fill and hoses plus the vent lines to diesel compatible and then run the tank drain line to the same line the current small tank drains into the large tank. This would give me the ability to drain another 70 gallons of diesel into the large tank.

Whats wrong with this plan? What do I need to do? Anyone done this before?



Terry

[Cheechako]

hmmm... I see no obvious problem... assuming it's a metal tank....

[Chief Engineer]

170 gallons of diesel fuel?

I suppose that you will be in out of the way places

What are you expecting as far as range?

[Tspringer]

Range? As far as I can get.

The tank is stainless steel.



Terry

[Cheechako]

I found that I had way too much fuel capacity... but I didnt sail around the world. Are you sure you need the fuel?

[GordMay]

Originally Posted by Tspringer

... The tank is stainless steel.
Terry

Only Low Carbon Stainless (316L or 317L), with minimum thickness of 0.0747" (14 gauge, or 1.9 mm), is suitable for diesel fuel tankage.

[Cheechako]

Yea, you're right, but one could say only 316 is acceptable for everything on a boat, but a helluva lot of tanks have been built out of 304....

[Tspringer]

Sheesh... yes I want the range and yes I am SURE I want more diesel capacity.

I am not positive on the type of stainless used on this tank. Is there any way to tell? The tank seems to be of a gauge that is sufficient.


Terry

[sv.Crake]

I have been thinking about the same thing on my boat. However, the water tank I am thinking about converting is a fiberglass tank. It is built into the hull so 2 sides of the tank are hull thickness. The back side of the tank seems pretty substantial.

I don't really want to drink out of this tank because it is glass, and, being original to the boat was made with polyester resin. Not sure on the health risks but it's probably not great.

So...

Would anyone see any issues with converting a fiberglass water tank to diesel.

[GordMay]

Originally Posted by Tspringer

... I am not positive on the type of stainless used on this tank. Is there any way to tell? The tank seems to be of a gauge that is sufficient.

There’s chemical spot test kits that can be used to identify the various grades of S/S.
Ie: Koslow ➥ Metal Test Kits - Reference Electrodes - Passivation Test Kits and alloy id by Koslow

Originally Posted by sv.Crake

... the water tank I am thinking about converting is a fiberglass tank. It is built into the hull so 2 sides of the tank are hull thickness. The back side of the tank seems pretty substantial ...
Would anyone see any issues with converting a fiberglass water tank to diesel.

Diesel; fuel tanks can be integral with a fibreglass hull, provided the core material (if applicable) is compatible with diesel (many are not), and will not permit fuel to migrate (many will). Due to the difficulty of ascertaining these properties of an old construction, I cannot recommend such a conversion.

[sv.Crake]

Thanks for the info.

My hull is a solid layup, as for the other (non hull) sides of the tank (the back and top) i believe these are also solid glass. There is a possibility that the back side of the tank is glass over plywood. I can figure this our pretty quickly.

Would this this work?

Just FYI, this boat is a 1978 Cheoy Lee Offshore 38. The tank is 100gal.

[Pblais]

Whats wrong with this plan? What do I need to do? Anyone done this before?

The idea is fine. The details may bring up issues. Our tanks are all the same type of tanks and the original waste tank never saw waste but has only seen diesel. All the tanks had pickup that pulled slightly off the bottom as a good fuel tank would. You need to also plumb the return similarly. Deck fill lines for fuel are NOT the same as for potable water so those must be switched for both tanks.

You'll never get a fuel tank clean if you can't open up all the baffles with good sized access ports. That is just a good thing to want on every tank on every boat. If you can do all that without ripping the tanks out then it might work. If you can rip out the tank then you could put new ones in their place and avoid the hassles of retrofitting them in place. I can pull all three of my tanks out and set them on the dock without doing any destruction.

[Tspringer]

Thanks Paul.

My tanks will not come out easy. The tank I am looking to convert would have no return. Basically, it would store diesel and when the main diesel tank ran low I would drain fuel form this tank into the main tank. The water tank I plan to convert sits higher than my main diesel tank, the water tank drain is higher than the top of the diesel tank. My plan is to install a valve and a line to simply allow fuel to drain into the main tank and pass through some sort of filter on the way. I will also replace the water tank vent with a fuel compatible vent and replace the deck fill and hose.


Terry

[Pblais]

The tank I am looking to convert would have no return. Basically, it would store diesel and when the main diesel tank ran low I would drain fuel form this tank into the main tank.

And you would make that happen how?

It's starting to sound a lot less possible without problems. As I said before all the stars need to be aligned for a deal like this to be practical and later desirable. Ideas put into practice tend to go afoul on the little bitty things. Gravity needs to be totally mastered.

[Tspringer]

Well... I assume that diesel like most fluids tends to flow downhill. Given that the bottom of the water tank I wish to convert is higher than the top of the main diesel tank it would drain into and that the converted tank would have adequate venting it would seem that routing the tank drain that is on the bottom rear of the tank to a valve and then to the diesel tank would suffice.

As I said, I have a 2nd diesel tank now that is mounted under the center cockpit floor and that is exactly how it operates. It drains to the main diesel tank via a fuel line with an in line ball valve. It has always worked fine.

Why would diesel not flow downhill?



Terry