Some people use Pex, coiled up in a big coil in a cold frame (to trap heat) as their solar heater.
The volume of water inside a 3/4-inch hose to be 1.01 gallons. The volume inside a 5/8-inch hose is 0.57 gallon, and inside a 1/2-inch hose, there are 0.25 gallon of water.
The mathematical formula is (3.14)•d•d/4, where "d" is the inside diameter. Using this formula, the inside area of a 3/4-inch hose -- with a 1/4-inch wall thickness -- is 0.1962 square inches. Corresponding values for a 5/8-inch and 1/2-inch hose are 0.1104 and 0.0491 square inches, respectively.
First convert the inside area from square inches to square feet, using the conversion 1 square inch = 0.0069 square feet. You can then multiply the result by 100 feet to arrive at the total volume of water inside the 100-foot hose, in cubic feet. Multiply by 7.48 to convert to gallons.
If you used an Aqua Helix shower nozzle (0.5 gal per min at 40psi) you can get, at a 50-50 mix of hot and cold water, about 4 min of warm water for a 100 foot hose.
Not quite enough for a 6 minute 'Navy Shower'.
This is dependent on water temperatures and heat losses to the shower nozzle.
If hot water was only 140 deg F and cold water only 60 deg F, your shower time will be shorter.
A good 18 min shower (females - average) would take about 350 feet of heated hose.
A good 12 min shower (males - average) would take 300 feet of heated hose.
Rubber hose will take longer to allow heat transfer to the internal water too.
You could coil up some copper tubing (3/4" diameter, about 5 feet long) and place it into a rocket stove for hotter water, longer showers.
Cold inlet at the top and hot outlet at the bottom in a vertical configuration.
Coiling that size tubing will not be very easy.
3/4" refrigeration tubing is actually 3/4" O.D. whereas 3/4" plumbing pipe is 3/4" I.D.
The use of loose sand or perlite in the tube can resist kinks.
NON ASSUMPSIT. I am by no means an expert at anything. Just a lucky guesser.
We used a setup like this for showers at family reunions and it worked quite well. And there is something exhilarating about showering outside. I learned in a recent renewable energyclass about the the bacterial issues with this simple setup so it's not part of my long term plan. But yes, another vote for outdoor solar showers!