Converting old propane truck into water truck for track?

Probably better off just getting a chassis and then buying a tank. 

You are going to have to do work on the propane tank to make it hold water and pump water out of it. It's not just holes in the bottom of the tank the are connected to the tubes you see. They use pressure to load and unload these thing using vapor, not a pump like what you are thinking. 

And do you really want to fill the whole tank with water before you start cutting on it? Because that is what you should do.

...Or just find a used water truck that someone is auctioning. 

Do propane trucks use a PTO? I don't know. 

Also I think you'll want some baffles in it, especially if you have any kind of terrain or plan on it being on the road.

Worked at a propane business for a few years.

Never thought of using a old propane truck for a water truck, but I guess it can be done.

Yes they have PTO that runs the pump. 

The tank has baffles inside that keep the movement of the liquid to a minimum. 
Be sure the tank is empty and no pressure before working on it.

Yeah they're definitely capable of doing what you want. I keen someone maybe 12-15 years ago who did this. Once he knew the tank was clear and empty. I forget exactly if he used an existing hole or used the existing bottom feed on it. But ran it to the largest harbor freight gas water pump and ran a hose to a pc of sealed pvc with some holes or slot in it and they worked it that way. It took a little more time then having it just shooting straight out the truck doing it by hand but it's the way he wanted to do it. All he really had to do was solid mount it to just drive and water if he wanted.

The market for a used water truck is crazy. A lot of construction, site development, road building companies use these. The propane trucks have a smaller market and can be had for much less. 

Seems like it'd be pretty easy to make something functional, just some big fittings to a trash pump and a pipe with some holes in it. The harder part would transitioning from functional to optimal, but you probably won't know how you feel about that until you experience functional first. 

I had looked at a few thinking about it back when I had 3 tracks, but ran across several fuel bodies at auctions and stocked up....lol

baffles are pretty dang important, especially if you have any slopes,

consider a 2000 gallon bulk tank. around 5' x 12' .

completely full, it is going to be around 8 tons of water. half full, 4 tons.

now once you start moving that truck around that water will eventually be slamming from one end of the tank to the other. If you are trying to ease down a hill with questionable traction, that 4 tons slamming back and forth is going to make it pretty interesting. 

There will be some side to side but a round tank makes it less. not sure how much a rounded propane tank body would dampen the front to back forces compared to flat bulkheads.

just saying baffles are often not thought of enough by people that do not have experience hauling liquids.

Water is about twice as heavy as liquid propane.   That's something to take into account against the suspension, brakes and running gear.

A quick search for the specific gravity of liquid propane is about 0.51
Specific gravity of water is defined as 1.0

 

I knew propane was not as heavy as water but had never really researched it enough to know water is twice as heavy.

So looks like you would have to be careful about getting too big of a tank.

About 2000 gallons was as much as i wanted to have on a single axle truck.

Using a water truck on a mx track is hard on them. A lot of flexing of the frame and if the tank is not mounted properly to accomodate it you will end up with cracks in the tank.

After ending up with a tandem axle truck i tried to go more with them because they are a lot more stable, ride better, and hold up better. I had one that was 3200 gallons. It was a big square tank that fit right on a volvo semi chassis.

Short enough to really turn, but really sure footed and not so big that it felt top heavy.

I dont know that i would worry about the pto.

I would think that an lp pump would not work well with water.

There is a real advantage to having a pump completely separate from the drivetrain for mx watering.

Air valve sprayers are awesome too. It has been awhile since i have built a truck, but i bought valves from a company, valew welding. Awesome design, easy to repair, and only require one airline that is used to keep the valve closed. The water pressure from the pump opened the valve when the air was released.

Did not intend to right a dissertation.